Urban Acoustics: Getting Started on Urban Noise

by Sharon Hudson Berkeley, California © 2008

The purpose of this document is to provide basic information by which a lay person can understand noise, its role in the urban environment, and strategies for dealing with it. The technical and strategic framework provided by this document can help individuals and municipalities focus their efforts appropriately as they address urban noise concerns. It should be useful to all urban residents who wish to prevent or address noise problems, and to city planners and policy makers searching for ways to increase urban livability by reducing noise. It does not contain enough detail to solve difficult or complex noise problems that may require technical manuals or acoustical consultants. On the other hand, many noise problems are merely the result of ignorance or carelessness, which can be remedied going forward, often simply and with little expense. Although American policy makers have thus far been negligent in their duty to reduce noise, city planners, residents, and policy makers alike can be very optimistic about creating quieter cities through sensitive new construction and focused remedial action, assisted by effective noise regulations and enforcement. NOTE: This document can be downloaded without charge from www.sharonhudson.com/urban_planning/livable_cities.html . That document includes several appendices that are not included in the scribd version.

Table of Contents Noise in the Community 2 The Basics of Sound 8 Absorbing Sound 12 Isolating Sound 15 Mechanical Systems 18 Creating Spaces with Good Acoustics 20

Acoustics for speech and music 22 Eliminating noise from outdoor sources 25

Conclusion 28 Appendix Acoustical Terms 30 Sources 31

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Noise in the Community

“Calling noise a nuisance is like calling smog an inconvenience. Noise must be considered a hazard to the health of people everywhere." — Dr. William H. Stewart, former Surgeon General of the United States

Noise is defined as unwanted sound. The World Health Organization (WHO) Guidelines for Community Noise state:

“The perception of sounds in day-to-day life is of major importance for human well-being. Communication through speech, sounds from playing children, music, natural sounds in parklands, parks and gardens are all examples of sounds essential for satisfaction in every day life. Conversely, [the adverse effects of unwanted noise include]…temporary or long-term lowering of the physical, psychological or social functioning of humans or human organs…[including] noise-induced hearing impairment; interference with speech communication; disturbance of rest and sleep; psychophysiological, mental-health and performance effects; effects on residential behaviour and annoyance; as well as interference with intended activities.”

In addition to contributing to hypertension, heart disease, and other chronic illnesses, environmental noise reduces productivity at work, increases the rate of accidents, degrades performance at school, interferes with cognitive development, learning, and reading progress in children, intensifies the development of latent mental disorders, and negatively impacts mental well-being and social behaviors such as aggression and altruism. “Since 1973, the Department of Housing and Urban Development (HUD) has conducted an Annual Housing Survey for the Census Bureau in which noise has been consistently ranked as a leading cause of neighborhood dissatisfaction. In fact, nearly one-half of the respondents each year have felt that noise was a major neighborhood problem. In the 1975 survey, street noise was mentioned more often than all other unwanted neighborhood conditions. This survey has also shown that aircraft and traffic noise are leading factors in making people want to move from their neighborhoods. Approximately one-third of all the respondents who wished to move because of undesirable neighborhood conditions, did so because of noise” (EPA 1981). Recent surveys parallel these findings. The survey concludes that:

• Freedom from noise exposure is a component of neighborhood satisfaction, and quiet is highly valued.

• Exposure to (non-aircraft and non-highway) noise typical of many urban environments produces widespread annoyance, speech interference, and sleep disturbance.

• There is a strong relationship between exposure level and the proportion of a community highly annoyed by noise. However, lower levels of annoyance, and the number of complaints about noise, are poor predictors of the prevalence of annoyance, because many other factors influence annoyance and complaint levels.

• Population density is an important correlate of noise exposure. However, demographic factors alone are relatively poor predictors of noise annoyance.

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A new approach to noise is necessitated by the “smart growth” policies of increased densification, urban infill, and mixed land use. Higher residential density and larger buildings, mixed with increased business activity, pedestrian activity, evening and night activity, and public transit and traffic activity, increase the need for urban noise abatement. New and higher buildings will not only amplify noise in the urban center, but may also reflect freeway and other traffic noise into neighborhoods that have never experienced it before. Measures should be taken to anticipate and mitigate these effects. Noise also impacts our daily experience of nature by driving insects, songbirds, and other animals from the urban environment—even from wooded open space if it is near a freeway. Likewise, excessive or unpleasant noise reduces the amount of time people want to stay in an area, and may thereby reduce business and community activities. Finally, since noise remains the reason people most often give for wanting to leave their neighborhoods, noise reduction in cities is vital to any “smart growth” strategy to encourage urban living. Health experts now realize that noise is more than an annoyance; it is toxic just like other environmental pollutants, with significant health consequences. Living in a lively part of town, even by choice, does not reduce a person’s susceptibility to the physiological, psychological, and sociological impacts of noise. Studies show that there is no physiological adaptation to noise. Instead, long-term residency in a noisy area increases the need for areas of “escape” from the noisy environment. The various mechanisms people use to avoid noise stress and sensory overstimulation create social detachment, loss of community, and ultimately, higher crime rates. Thus noise pollution is both a public health and social equity issue, and permitting noise to pervade the less affluent parts of town is no more justifiable than allowing industrial pollutants to do so. Since low- and moderate-income people increasingly have no choice but to live in high-density and mixed use areas, social equity demands that we pay special attention to noise pollution in high-density areas. All people, businesses, industry, and organizations have an obligation to respect the health and comfort of others. The air is part of the shared commons, and offensive and excessive noise in public spaces is detrimental to the public health, welfare, and safety, and contrary to the public interest. Unwanted noise impinges on the right to the quiet enjoyment of one’s home. Less obvious are the downstream impacts of noise in destroying community cohesion and ultimately increasing policing budgets. And given the significant health impacts of noise, one must examine the policy of concentrating traffic on designated corridors and then lining the corridors with high-density housing. This not only subjects the most people to the most noise, it also subjects the less wealthy to the most unhealthy environment. It improves the well-being of wealthier neighborhoods, but at the cost of increased health impacts and social inequity. Solving any environmental noise problem involves four steps:

• Acknowledging and assigning priority to the problem. • Quantifying the problem using noise measurements or analytical means. • Creating regulations based on applicable criteria, goals, and noise limits. • Abating noise by enforcing regulations to eliminate, reduce, or control the noise.

Acknowledging the problem In the 1970s, federal and state efforts to examine and regulate the impacts of noise were initiated in many countries as part of the environmental movement. In the United States these programs were largely abandoned during the Reagan administration. However, such efforts continued in

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Europe, with the result that now noise consciousness and mitigation measures in Europe are about a generation ahead of those in the United States. While only recently have Americans started to upgrade noise from an “annoyance” to a serious public health problem, in EU documents “noise pollution” stands alongside “air pollution” as the two co-equal elements of “air quality.” American visitors to some European cities note their relative quiet despite their high population densities. Part of this quiet is due to reduced driving and the natural sound-damping qualities of their older buildings, but much is due to focused efforts to reduce noise levels through strict regulations, technological retrofitting, and acoustically sensitive new architecture. By 2007, every major city in the European Union was required to have mapped its traffic noise and found ways to reduce it (Discover 2005). American urbanites are not so lucky. Creating noise regulations is a very low priority in most American cities, and municipal police forces place noise and other “quality of life” infractions at the bottom of their enforcement priority lists. Therefore, call for change typically comes from the citizenry. However, some progress is being made. In some states, cities and counties must prepare Noise Elements as part of their general plans, although these elements tend to be cursory. In New York City “the first revisions in the city’s noise code in more than three decades” went into effect in July 2007, making the ordinance stronger, more proactive, and easier to enforce. Other American cities are strengthening their noise ordinances as they realize that noise regulations can be used to address accompanying nuisances such as alcohol abuse. However, government efforts are usually reactive and rarely acknowledge noise as an important issue in its own right that merits comprehensive planning. Quantifying the problem Urban noise is rarely and poorly addressed in public policy decisions in the United States. At best, in most places it is inadequately addressed as part of environmental assessments, or to conform to bare-boned federal or state building guidelines. Rarely, if ever, do American cities monitor noise levels, as they often do other kinds of pollutants—air, water, ground, food, etc.—just to be sure they are acceptable or not increasing. Even when they do measure noise, they often do so in ways that statistically mask intermittent and specific noise sources within averaged noise levels. Without adequate measurements, it is difficult for analysts and city policy makers to determine how noise is changing in the urban environment and what is causing the changes. This poses difficulty both in everyday noise enforcement, and in environmental assessments and other attempts at long-term planning. “Noise mapping” is one means by which urban noise can be quantified and analyzed; it is required in European cities but rare in the United States, except around airports and highways. A noise map uses noise readings and computer models to record, quantify, and predict area noise levels. “Noise maps…can assist in the assessment of proposed development on an individual site since the noise map will show the current noise level in the vicinity. Information provided by the noise map may suggest ways in which a proposal can be modified to provide benefits to the area, e.g., by screening an existing noise source….[T]he reduction methods used [need not be] large-scale and expensive” (DEFRA 1999). Creating regulations Most of the normal activities of life create sound. In the United States, the debate over noise boils down to “my right to do what I want” against “my right not to be damaged.” Society

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cannot function if an extreme position is taken on either side. Creating noise regulations requires balancing the legitimate needs and legal rights of sound creators and others who benefit from their activities, against the needs and rights of those subjected to unwanted and unhealthy sound. Rather than face this difficult task with a community discussion, coherent philosophy, effective code, or diligent enforcement, most municipalities hope that noise makers and their victims will achieve some tolerable truce without municipal involvement. Cities also usually rely on reactive, “complaint-driven” enforcement, which guarantees ineffectiveness, inequity, and the other evils of complaint-based systems. Not only do cities often refuse to enforce their own noise ordinances, they view the complainer and not the noise maker as the nuisance. There are two types of noise regulations: objective and subjective. Local noise ordinances usually include both, but each has its problems.

1. Objective regulations rely on noise standards based on decibels, and noise levels determined by professionals with noise meters. Objective regulations let people know what to expect, are “fair” (the same for everyone) and are legally upholdable when the data is clear. However, the problems with objective standards are: a. It is difficult to determine the standard and to specify exactly how to apply it.

Deciding how, where, and when to take readings is not as simple as it sounds. b. Enforcing the metered standards requires expensive equipment, training, and

manpower, which often prevents enforcement entirely. c. It is impossible to write the code to cover all contingencies, and the “borders” of the

infractions are problematic: “[Objective regulations] can create a legal shield for people creating annoying noise that just happens to meet code” (Brooks 2003).

d. Objective regulations tend to reflect and favor property rights, not human rights. Their frequent emphasis on noise levels at property lines fails, technically, to reflect the irregular reflections and levels of urban noise, and politically, to protect the commons. In provisions and enforcement, property-based regulations also tend to favor the protection of property owners over tenants.

2. Subjective regulations are based on experiential criteria. They ban noises that, for

example, are audible from a certain distance, disturb “reasonable” people of “normal” sensitivities, endanger the “health or safety” of people or animals, damage property, or fall under the rubric of nuisance clauses. Subjective regulations have the advantage of being able, in theory and in practice, to protect more people in more places more of the time. But subjective regulations are “vague and nearly impossible to enforce efficiently, and often require court action” (Cavanaugh 1998). Subjective standards are hard to combine with stiff enforcement or penalties without legal challenges.

Subjective regulations work well when there is political will to enforce them, because they require relatively little administrative expense and give enforcers considerable freedom. Objective regulations are needed when there is little governmental will to enforce, and individuals are forced to demand remediation based on concrete legal standards and rights. Some subjective regulations have run into problems in court. Nonetheless, as noise awareness increases, and because of the expense and difficulty of using objective measures, it appears that there is a recent trend toward using more subjective measures if they can be legally upheld. Subjective standards reflect the reality that “the most important measuring devices are attached to either side of one’s head” (Brooks 2003).

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A particularly inadequate application of quantitative noise standards occurs in environmental assessments for development projects. If the environmental law does not define what constitutes a “substantial” increase in noise levels (as in the California Environmental Quality Act), it is left to developers’ consultants to decide how to measure and assess the impact of noise increases. Leaving aside the many means developers use to ignore, understate, or hide noise impacts, even honest and competent impact analyses are undermined by inappropriate measurements and lax standards for significant impact, methods which are now viewed as “standard” or “accepted” simply because of their continued use. For example, “averaged” ambient noise levels are almost exclusively used in environmental analyses. However, these averages understate the significant subjective impact of loud intermittent noises, and statistically hide increases in low-level but continuous sound sources by averaging them in with louder noncontinuous noises. Even worse, mitigation measures are often required only after the environmental noise level has reached the threshold of unacceptability. Meanwhile, anywhere that noise levels are still acceptable, new noise sources are allowed to increase decibel levels incrementally, without regard to the fact that these quickly add up. Thus, when it comes to noise, like traffic and other impacts, environmental assessments have become no more than permission slips for incremental damage, and schedules for when the environment will become unacceptable, not a means to ensure that a good or acceptable environment will be maintained. Noise pollution has also been ignored in the increasingly popular LEED environmental building standards. The core purpose of the U.S. Green Building Council, which created the LEED environmental building standards, is: "...to transform the way buildings and communities are designed, built and operated, enabling an environmentally and socially responsible, healthy, and prosperous environment that improves the quality of life." In addition to its energy and sustainability standards, the LEED program includes standards for items such as access to daylight and views, and protection from nocturnal light and numerous kinds of air pollution. But one unhealthy pollutant they currently overlook is noise—except in medical facilities, where it was only recently addressed. Abating noise Abating noise requires both enforcement and technical solutions. The former is the main problem. The noise-related policies and regulations of most U.S. cities are casual, haphazard, outdated, poorly appreciated, and rarely and reactively (not proactively) enforced. There is little will to enforce, given other police priorities and the lack of concern about urban noise on the part of authorities. But while it is the lack of enforcement that is most evident and annoying to the public, it is the attitude of policy makers and the public that most needs addressing. It is almost impossible to enforce a law unless the behavior demanded by the law reflects an accepted cultural value, so that compliance is almost entirely voluntary. Although the vast majority of Americans may voluntarily behave considerately with respect to personal noise, there are economic incentives for builders and businesses to avoid noise mitigation measures and to use the commons as a noise dump, and in the absence of noise awareness, regulation, or enforcement, most will do so. But a more important impediment to noise enforcement is that it is usually complaint-driven, so the main determinant of enforcement is the public’s determination to complain. But most Americans do not believe that noise is physiologically, psychologically, and sociologically important, or that they and others are entitled to live with comfortable and healthy noise levels (at least in the urban environment), or that this right should

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be monitored and maintained by government action. In this sense, noise sensitivity has not become a cultural value. Therefore a major component of noise abatement must be to instill an appreciation among the general public of the impacts of noise and the rights of themselves and others. Noise abatement is probably easier in cultures that are less individualistic, and where people have developed strategies over the centuries for living together in small spaces. But because of the health impacts of noise and the environmental urgency of encouraging urban living, America cannot wait for a slow, “natural” evolution of noise consciousness. Instead, governments must take actions to both speed up public awareness, and simultaneously to incorporate new noise standards into urban development and behavior. In fact, one can begin the process of abating noise—which is, after all, the main goal—without having extensively quantified the problem, if one simply recognizes that there is too much urban noise and that measures should be taken to reduce it. Although almost any kind of noise can be reduced by intelligent urban oversight, given existing built urban patterns, some kinds of noise cannot be eliminated, such as much traffic noise and noise reflected from existing buildings. The place to start reducing the total urban noise burden is by eliminating and mitigating noises that are unnecessary and relatively easy to address. These include most existing and new stationary noise sources (mostly mechanical systems, which could be addressed quickly); other new or continuing noise sources that could be mitigated through better design as new buildings are built, as new equipment is purchased, etc.; and some gratuitous behavioral sources, such as loud vehicles, stereos, and parties, which could be addressed at any time with political will. Regulations regarding acoustical building and infrastructure design, and mitigation of existing noise sources, including traffic, will take longer. In terms of technical solutions, there are three basic approaches to controlling noise: 1. By reducing or absorbing sound at the source. This is the best and usually cheapest place to eliminate noise. Install quiet machinery in the right place and surround it with sound-isolating materials. “Although a source of noise can be treated after installation, it’s generally twice as expensive and half as effective compared with designing proper noise control into the system before the noise source is installed” (CertainTeed 2003). 2. By blocking the path of sound. This means placing a sound-reducing barrier between the sound source and the receiver. This is difficult with low-frequency sounds because they bend around barriers and are not much reduced by the air. A heavy, air-tight barrier is required to isolate sound. 3. By protecting the ears of the hearer. A large part of protecting the hearer is to locate noisy activities in places where they will not bother people. Earplugs or ear protectors are the method of last resort, but may be necessary in some workplaces, or even for sleeping in very noisy environments.

“The knowledge of how to measure and control environmental noise is a professional expertise that is readily available throughout the country….We are well-prepared to make the 21st century a ‘quiet’ one. Yes, the invisible pollutant of environmental noise can be tamed” (Cavanaugh 1998).

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The Basics of Sound “Sound is a vibration in an elastic medium such as air, water, most building materials, and the earth” (Egan 1988). Sound waves are particles bumping each other to form waves, where the molecules of the medium (air, etc.) are either compressed or pulled apart, respectively called compression and rarefaction. Sound wavelengths audible to humans, and those reproduced by high-fidelity music systems, vary from about an inch long (16,000 Hz) to about 56 feet long (20 Hz). Frequency is the number of waves or cycles per second, expressed as hertz (Hz). Sound is absorbed by the air because when sound energy passes through air, the air molecules absorb energy when they bounce into each other. This occurs significantly at higher frequencies, between 2000 Hz and 10,000 Hz. Pitch is the subjective evaluation of frequency. Most sounds contain many frequencies, and a pure tone is a vibration at a single frequency. The wavelengths of human speech range from about 1.75 inches (at 8000 Hz, the upper range of consonants/sibilants) to 9 feet (at 125 Hz, the lower range of vowels). Most human speech is between 2000 and 4000 Hz, which is considered high frequency. In human speech, vowel sounds are around 500 Hz and consonants, especially the sibilants, are around 4000 Hz. The vowel sounds diminish slightly behind a speaker, but the consonant sounds drop off radically, so it is difficult to hear consonants when standing behind a speaker. The speed of traveling sound depends on the density and elasticity of the medium. Sound travels through air at 1130 feet/second (about 1 mile/5 secs), but it travels better and much faster through solids than through air. For example, it travels through steel pipes and ducts at 16,000 feet/second, from which it can be regenerated long distances away as airborne sound. Human sensitivity to sound is a function of both physiological and psychological factors, and varies from person to person. There are significant health impacts (beyond hearing damage) to environmental noise in general, but impulsive (sudden) sounds are the most damaging to hearing. People are generally most sensitive to high-frequency sounds like whining, hissing, or whistling, and less sensitive to low-frequency sounds, but the health impacts of low-frequency noise components are thought to be more severe than for most community noises. People perceive nighttime noise as being 10 dBA louder than the same level of noise during the day. Sound intensity is measured in decibels (dB). Sound intensity is proportional to the square of the sound pressure, which is perceived as sound volume and is measured by sound meters. The dB scale is logarithmic, and 130 dB is ten trillion times the intensity of zero dB. To reliably measure a sound source, the sound must be at least 10 dB over the background noise.

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Sound source Composite chart from various sources.

Sounds are perceived as about twice as loud at night. Approximate decibel level

Threshold of hearing 0 dB (must have no background noise to hear this) 10 dB (seems nearly silent in everyday context) 20 dB Quiet home or yard; rustling leaves; quiet whisper @ 3 feet 30 dB Average whisper @ 3 feet 35 dB Soft stereo; 10 mph wind in trees; loud whisper @ 3 feet 40-45 dB Quiet conversation, office activities; heavy rainfall; microwave @ 3 feet 50 dB

Normal to loud conversation; TV listening volume 50-60 dB Hairdryer @ 3 feet; high setting 60-70 dB Busy city street traffic (not highway); heavy running tap water @ 3 feet; crinkling hard plastic wrapper @ 1 foot 70 dB

Vacuum cleaner @ 5 feet 75-80 dB Noisy cafeteria (long-term risk level for hearing loss) 80 dB Food blender @ 3 feet; slow delivery truck @ 40 feet 85 dB Lawn mower @ 5 feet 90 dB Stadium crowd noise; power saw; leaf blower; nearby thunder 100 dB

Chainsaw @ 3 feet 120 dB (threshold of pain) 130 dB Jet engine @ 75 feet 140 dB Shotgun blast 170 dB

Decibel readings are weighted (in letters A through E) to reflect sensitivities to different frequencies. The A weighting is intended to reflect human sensitivity to higher frequencies, and mostly neglects low-frequency sounds, B a little less, and C incorporates low frequencies. If the dBC level reading is 20 dB or more above the dBA reading, the measured sound energy is mostly low frequency (below 1000 Hz); if the difference is 10 dB or less, the sound is primarily mid and high frequency (above 1000 Hz). Community noise levels are commonly measured in dBA. However, single-number dB levels cannot accurately represent human perception, because the single number does not capture the differences in frequencies and quality of the sound. In addition, when measuring environmental noises with a low-frequency component, including mechanical noise and music with a loud or penetrating bass component, C-level readings are usually higher than A-weighted readings and more accurately reflect perceived volume and annoyance. Using the C-weightings is also recommended when measuring impulsive noises such as construction blasting, to capture the low frequencies that cause buildings to vibrate. The subjective loudness level of a sound is called its phon. A change in volume of 1 dB is imperceptible; a change of 3 dB is easily perceptible; a change of 6 dB is substantial; a change of 10 dB seems about twice or half as loud; a change of 20 dB is about a fourth or four times as loud. However, an increase in sound level at low frequencies is usually perceived as being much greater than an increase at high frequencies. A 3 to 5 dB increase in ambient noise levels is usually considered a “substantial” increase.

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When adding sound levels, the cumulative total is not the sum of the components; for example, if you add 50 dB to 40 dB, you get 50.4 dB, not 90 dB. As a rule of thumb when adding dB levels in free-field conditions: when the two levels differ by 0 or 1 dB, add 3 dB to the higher value; when they differ by 2 or 3 dB, add 2 dB to the higher value; when they differ by 4 to 8 dB, add 1 dB to the higher figure, and when they differ by 9 dB or more, use the higher figure. Sound volume drops off with distance; this is called attenuation. Without obstructions in open air (called free-field conditions), sound energy from point sources follow the inverse square law and drops off by 6 dB for each doubling of the distance from the source. Sound energy from continuous linear sources (such as trains or traffic) drops off by only 3 dB for each doubling of the distance. Sound from an area source (a crowd, large mechanical equipment surfaces) drops off by 3 dB for each doubling of the distance close to the source (about 1/3 the dimension of the equipment), and about 6 dB for each doubling of the distance farther away than that. The inverse square law does not apply within short distances very near the source, and is also inaccurate at long distances, where sound fall-off is usually stabilized by reverberation. Reverberation time or decay time (also called persistence) is the time it takes for sound to decay by 60 dB (to one millionth of its original volume). In complex spaces with reflections, reverberations, etc., sound does not decay in a straight line. For example, if there is an absorbent living room next to a ‘live’ stairwell, the sound may die within the living room but then a reverberation may be bounced back into the living room from the stairwell. Clarity is the ratio of early (direct) sound energy to late or reverberant sound energy. Sound reflection occurs similarly to light reflection, but because sound activates air molecules in different directions around the wave, the reflection is imperfect. The angle of incidence equals the angle of reflection if the dimension (length or width) of the reflecting surface is larger than about 2 to 4 (preferably 4) times the wavelength of the impinging sound. Thus large reflectors of many square feet are usually required for effective directed sound reflection. Flat reflectors can help focus sound where you want it. Concave sound-reflecting surfaces focus the reflected sound onto hot spots, which is usually undesirable. Convex reflectors are the best for diffusing sound across a wide range of frequencies. Diffusion of sound occurs when reflections in different directions from varied surfaces randomly redistributes sound, making sound appear to come from many or all directions at once. Diffraction and flanking are the bending or flowing of a sound wave around an object or through an opening. Sound does this in part because sound is transmitted by the molecules of the medium jostling each other, which they do in all directions, not simply in a straight line. Sounds “bend” around objects that are smaller than their wavelengths, so lower-frequency sounds bend more than higher-frequency sounds. Like water or air, sound can travel through any irregular opening, such as air ducts, wall outlets, joist spaces, lighting fixtures, etc. Absorption is the ability of materials to convert sound energy into some other form of energy, or energy that is inaudible or not bothersome. It is measured as the energy of the incident sound minus the energy of the reflected sound. Transmission is the sound energy that penetrates or travels through materials. The sound may either pass directly through a porous material, or it may cause vibrations within the material that then emanate out on the other side.

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Sound waves hitting each other from opposite directions can cancel each other out; this is called noise cancellation. This technology uses microphones and processors to determine the frequencies of the sound, and then recreates those frequencies, amplifies them, and sends them back from the opposite direction in real time. It only works with directional sound sources, not diffused sound. The potential application of noise cancellation is limited and the technology is currently in its infancy, but is very effective in certain circumstances. Isolation is using a barrier to contain sound, at the source when possible, otherwise at the receiver. Isolation works best when the barrier is close to either the source or to the receiver. Isolation may work by either reflecting sound back into the source area, and/or by absorbing sound. Commonly an absorbent material in front of a heavy barrier “traps” sound, preventing reflection, while a barrier behind prevents transmission. An acoustic shadow is the area of reduced sound behind a barrier. Sound barriers can create an acoustic shadow that is proportional to the barrier size, and a transitional zone of partial shading. Noise is defined as unwanted sound. Your favorite symphony becomes noise if you are forced to listen to it when you don’t want to. Obviously, this subjectivity—likes and dislikes—adds to the difficulty of agreeing on community standards and writing noise ordinances. Ambient noise is the composite “normal” or “existing” level of noise at a location. Many people call this “background” noise. In common use, this term is often applied to background noise exclusive of any nearby recognizable noise source, such as voices, traffic, or someone hammering. Other times, the term refers to the average noise level in an area over a significant time period, including all noise sources weighted by duration, and sometimes weighted to “penalize” nighttime noise (see the appendix Acoustical Terms for more information). The latter “averaged” ambient level is most often used in community noise measurements and environmental analyses, despite its inadequacies. Noise annoyance “is a function of perceived loudness, duration, and psychological association” (Brooks 2003). Low volume continuous sound is annoying and stressful, but abrupt, intermittent, pulsing, or fluctuating sound, or sound that carries information, such as music or speech, is most distracting to concentration. When people feel that the noise is necessary or serves a beneficial purpose, and when they are in control or know what to expect, they are less bothered by noise. Annoyance is not a good indicator of physiological harm: noise can be physiologically harmful without being annoying, and vice versa. Likewise, annoyance levels and complaints are usually a lagging indicator of community harm. The best ways to eliminate noise are by preventing it, by absorbing it, or by isolating it; usually absorption and isolation are combined to create what is popularly called soundproofing. When sound hits a surface, the original sound energy is reflected, absorbed, and/or transmitted (some sound also flanks around the barrier). For acoustical engineering purposes, each of these three components of the original energy (of a building material, for example) is measured under controlled test conditions, though results in field conditions are rarely as good as those obtained in the lab.

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Absorbing Sound Environmental noise, inside and outside, usually consists of a combination of direct noise and reverberated noise. In the lab, absorption is measured by how long it takes a sound to die down within rooms containing different materials; this is the reverberant noise reduction (NR). Different room treatments result in different amounts of noise reduction. However, reducing reverberant noise does not affect the noise that is received directly from the source. Therefore, sound-absorbing treatment will impact a room user more if most of the noise is reverberant and not free-field (direct) exposure. In other words, acoustical ceiling tiles will do nothing to reduce the direct sound from the computer in front of you. However, if you can remove yourself from the direct line of sound (for example, by moving a bed or desk out of line-of-sight with a noisy window), then acoustical/absorbent room treatment can reduce your remaining (reverberant) noise exposure. Sound absorbing panels and furnishings have only limited ability to reduce the overall noise level in a room. Each doubling of absorption reduces the total dB level by only 3 dB. A reduction of 10 dB in NR is the upper limit in most room situations, and 3 to 6 dB is the practical limit. The basic types of sound absorbers used by architects and builders are porous materials, vibrating (resonant) panels, and volume resonators. Porous materials absorb sound when sound energy encounters friction and turns into heat energy. This is usually achieved by porosity and other characteristics of the absorbent material. Acoustical tiles absorb sound when the sound waves force air into little holes, where friction turns the sound energy into heat. The remaining sound energy either bounces off or passes through. For absorbing materials, thickness is very important, but weight or density is not: increasing the density of an absorber does not increase its TL. Lightweight absorbers can work just as well as heavy absorbers. Porous fibrous materials are called fuzz. A simple test of the sound-absorbing ability of fuzz is that if the material is thick but you can blow through it with moderate pressure, it should be a good absorber. Airspace behind the porous material helps any excess sound not reflect back into the originating space, while a reflector behind the absorber will contain the sound energy within the porous material. Generally, when it comes to concrete, brick, stone, wood, and other such building and paving materials, the rougher the surface, the more sound absorbent. Rough concrete is much more absorbent than smooth stone or wood, for example. Unpainted coarse concrete block absorbs about 1/3 of the sound hitting it and is relatively good at absorbing low-frequency sound. It is important not to paint sound-absorbent materials because it fills in the pores; instead, stain it or tint it. “When painted, concrete reflects nearly all the sound that hits it…When concrete is left unpainted, or merely stained, it is moderately sound-absorptive” (Brooks 2003). Glass reflects

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almost all the sound that hits it. The effectiveness of a sound-absorbing material is measured by its absorption coefficient, which is the fraction of the sound energy that the material absorbs (or does not reflect), from 0 to 1 (100%). A brick wall has about 98% reflected sound and 2% absorbed/transmitted sound, so its coefficient is 0.02. The absorbency rating is based on the decay time of the sound hitting the material. To achieve the coefficient one must replicate the testing conditions, which may not be the same as real-world conditions. In addition, materials have different sound coefficients for different frequencies. Thickness has a significant effect on sound absorption of porous materials: the thicker the better.

Caution! In room acoustics, the absorption coefficient is all the energy that is not reflected back into the space: that is, it includes both absorption and transmission. Within a closed space, the transmitted sound does not disappear but acts like reflected sound, and therefore the reflected and absorbed sound add up to 100%. An open window has 0% reflected sound and 100% transmitted sound—sound transmitted to the other side of the window. But within the originating space the open window reflects no sound and therefore it has a “perfect” absorption coefficient of 1.0, although most people do not think of an open window as “absorbing” sound.

The noise reduction coefficient (NRC) and the sound absorption average (SAA) are single numbers that combine the absorption coefficients between 250 and 2000 Hz (NRC) or 200 and 2500 Hz (SAA). SAA and NRC values are usually almost the same. Highly absorbent materials have NRC and SAA values close to 1; non-absorbent materials have values close to 0. A material is usually considered sound absorbing if it has a NRC value greater than 0.35 to 0.5; it is reflective if it is below 0.2. However, the NRC and SAA may not reflect the coefficient at the frequencies you are interested in. For example, to evaluate absorptive materials for rooms where unwanted speech noise must be absorbed, the material must absorb the speech frequencies. Most common sound-absorbing materials like draperies, carpets, and fiber or acoustic panels, absorb much more high-frequency sound than low-frequency sound, although gypsum board walls absorb low and high frequencies at about the same rate. Padded chairs and human bodies are a major sound absorber in auditoriums, classrooms, and other congregation spaces, and they too absorb more high-frequency than low-frequency sound. The efficiency of absorption can be increased by proper placement of sound-absorbing materials; this is called the area effect. Efficiency is increased as the perimeter (edge) area of the panels increases relative to their total area, because of extra absorbency around the edges. For this reason a checkerboard pattern, while using only half as many tiles, has nearly the same sound absorbency as a solidly covered surface. For maximum efficiency, use a spaced pattern (there are different spaced patterns for different kinds of materials and spaces), and leave exposed as many edges and sides of the absorbers as possible. Sound-transparent (transpondent) facings can be used over sound-absorbent materials, usually for aesthetics or protection. Facings can also be used to modify absorption and the frequencies absorbed. Vibrating panels absorb sound by converting sound energy into vibrational energy, which can

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be dissipated by internal damping and radiation. Permitting paneling such as wallboard to vibrate reduces transmitted and reflected sound. An airspace behind the panel absorbs the energy; putting fuzz in the airspace greatly increases the absorption. To control the frequencies of the sound absorbed, one can vary the spacing between supports, the thickness of the panel, and the thickness of the airspace. Low-frequency absorption especially increases with airspace depth increase. The portion of the sound energy that is not absorbed by a vibrating panel will be transferred through the partition and re-radiated as airborne sound on the other side. Volume resonators absorb sound by trapping it. They reduce sound energy using vibrating panels, friction, and interreflections within a cavity from which sound cannot easily escape. These specialized resonators are used to absorb sounds at specific frequencies, such as low-frequency (120 Hz) electrical equipment hum. Wood slat panels, slotted concrete masonry units, and to some extent heavy folded draperies, also operate on this principle.

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Isolating Sound Isolating sound means containing it or keeping it from entering another space. When sound is effectively isolated, it is not transmitted, but is either absorbed or reflected back into the originating space. Sound is isolated through a combination of absorption and reflection, and the more effectively the sound is isolated, the less sound is transmitted. Transmission loss (TL) is the measurement of how much sound energy is reduced by transmission through materials. In the lab, transmission loss is measured between rooms divided by a wall of the material being tested. The ratio of the sound energy impinging on one side of a material to that radiated from the other side is used to calculate the TL, which is given in decibels. The TL usually increases with the frequency of the incident sound; it is difficult to keep low-frequency sounds from passing through materials.

Caution! Sound transmission loss is not the same as sound absorption. Sound absorbing materials control sound within spaces by being porous and allowing sound to pass through them to create molecular interactions. Porous sound absorbers (fuzz) absorb sound but do not prevent its transmission; they are very poor sound isolators. Such materials may only reduce transmitted sound by a couple of decibels. They need a solid, sound-reflecting surface behind them to be effective in reducing sound transmission. Conversely, a material that provides good sound transmission loss is usually a non-porous, good reflector of sound. However, the TL cannot tell you whether a material absorbs or reflects sound. If you are on the other side of a thick brick wall from a sound source, you don’t hear much sound, but the TL does not reveal whether the impinging sound has been reflected or absorbed.

The TL of materials is given a rating called the sound transmission class rating (STC), which conflates the TL at different frequencies between 125 Hz and 4000 Hz (speech range) into a single number, expressed in decibels of sound reduction. The higher the TL rating, the better the material is at reducing sound transmission. This rating, however, does not capture different TLs at different frequencies. Both dBA and STC ratings underrepresent low-frequency sounds, and can be misleading if the sound has a large low-frequency component, especially since low frequencies also travel better through walls. STC ratings fail to evaluate performance below 125 Hz, where music and mechanical equipment noise may be high. One must take extra precautions in these cases and not rely on the STC tables. In practice, the noise reduction (NR) between two rooms depends on the area of the wall between the rooms, the transmission loss of the wall, the amount of leakage, the absorption in the receiving room, and the relative sizes of the two rooms. The NR is greater when the sound is transmitted from a small, live room into a large, dead room than vise versa. In the first case, the NR is greater than the TL; in the second case, the NR is less than the TL. The TL in real life is rarely as good as when measured in the lab; it will usually be at least 3 to 5

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dB lower in the field than in tests. When TL is measured in the lab, the receiving room is larger than the transmitting room. In addition, when sound impinges from one direction perpendicular to a surface (such as from traffic), the effective TL is lower than when the sound comes from all directions. Glass is usually the material in a real-world wall with the lowest TL. If glass covers only 1/8 of the surface of a brick wall, the TL of the wall will decrease by 21 dB compared to the solid wall. This is why much attention is paid to reducing noise transmission through windows. However, it is the initial “leak” through the low-TL material that causes most of the problem, and increasing the glass to 100% of the wall decreases the wall’s total TL by only 9 more dB, to 30 dB of TL. If the all-brick wall has a TL of 50 dB, the all-glass wall will have a TL of 20 dB. Sound waves can vibrate building elements such as walls, floors, and ceilings. The fundamental principle of sound isolation, called the mass law, is that, in general, the heavier (in mass or weight) the material, the less the sound transmission and the better the isolation. For every doubling of weight/mass/thickness of concrete between 3” and 24”, for example, the transmitted sound will be reduced by 4 to 6 dB. The mass law applies to transmitting/isolating materials, not absorbing materials, which are not affected by density. Some building materials have natural frequencies at which they reverberate, greatly increasing their sound transmission at those frequencies. This is called the coincidence effect, and contributes to structure-borne sound. Like structural elements, piping and plumbing fixtures reverberate and can transmit sound around a building, and should be isolated from vibration. But when properly isolated, reverberation can be used to dissipate sound energy and reduce sound transmission. For example, a less stiff material (such as grooved plywood) acts as a vibrating panel and has much better TL performance than a stiff material (ungrooved plywood)—up to 15 dB better at mid and high frequencies. The more internal friction (damping) the material has, the better the transmission loss. Materials with internal damping (such as plywood) “thud” when struck; those without internal damping (such as steel) “ring” when struck. Lead has good damping and the best TL performance per material weight. Sheet metal air conditioning ducts have poor damping and are poor sound isolators. Impact noises (such as walking, dropping things) on the floor are transmitted directly downward and also horizontally through the building structure, to emerge elsewhere. The impact insulation class (IIC) rating of floor and ceiling materials indicates how well they reduce structure-borne impact sound. There are many methods to construct floors and ceilings to minimize the noise from impact sources, and building and housing codes require that this be done. Usually sound reflection and absorption are combined to isolate sound: Maximum transmission loss in walls comes from double-wall construction, with isolating airspaces and/or soundproofing fuzz between the walls to reduce the reflection of sound inside the partition, and without leaks. The greater the distance between the walls, the greater the TL; in fact, with double-pane glass windows, noise reduction increases noticeably only with airspace of 1 to 4 inches. However, increasing the glass thickness or using laminated glass (with a “limp” plastic layer between two glass panels) is just as good as smaller airspaces.

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For the best TL performance, use double-wall construction, with wide layer separation, and light metal studs or channels to “resiliently” support the walls and reduce the stiffness of the barrier. Use screws, not continuous adhesives, to attach wall boards to metal studs, so the boards can vibrate. For wood studs, use flexible adhesives, not rigid screws. The ideal TL construction would be heavy and airtight, but limp. Flanking is the ability of sound to go around barriers or through holes. Like water and air, sound can travel through any opening, such as air ducts, outlets, joist spaces, lighting fixtures, etc. Therefore, construction must be carefully overseen in order to be sure all flanking paths are blocked. Seal all cracks and leaks, including around doors, avoid back-to-back wall outlets, etc. Where it is impossible to create airtight seals, use baffles or stuff possible leak paths with absorbent material. Calking properly can increase the TL of a wall by more than 20 dB. Tiny leaks have major impacts. A one-square-inch hole in a 100-square-foot gypsum board partition, for example, can transmit as much sound as the entire rest of the partition. A leak of 1% of the surface area can decrease TL by 15-50 dB. No matter how good the material, generally speaking, one cannot have a TL of over 20 dB with a 1% leak. Therefore, when revelers leave a window or door open only a few inches, thinking their party noise is 90% contained, it actually reduces exterior noise by relatively little. Unfortunately too, because sound leaks like water and bends around corners, it is impossible to allow in a breeze while keeping out sound. Doors or windows that are louvered to allow in air will be useless as sound barriers. Drapes that are thick enough to reduce sound will block perceptible air flow. Cross-talk is the transmission of sound from one room to another through air ducts. Various kinds of mufflers, sound traps, deflectors, and other means are used to minimize cross-talk and other duct noise problems.

Example of sound enhancement due to reflection, and sound isolation combining absorption and reflection (based on Egan 1988)

Let’s say you have a small 70 dB noise source, like a doorbell. With no isolation, if the sound source is near one reflective surface, the sound level will be increased by about 3 dB; if it is in a corner with two reflective surfaces, the increase will be 6 dB. The buildup of the sound in a reverberant room can be over 8 dB, to 78 dB. Surrounding the doorbell with fuzz alone would reduce the sound by only 3 dB. To isolate the sound, a thick solid material (such as plywood) with an airtight seal is necessary. This treatment can reduce the transmitted sound by 20 dB, which would make the bell sound about ¼ as loud outside the box. In fact, the plywood box increases the dB level inside the box to 78 dB due to reverberation, but the plywood reduces transmission by 28 dB, yielding a net 20 dB reduction outside the box to 50 dB. If you line the plywood-box isolator with sufficient fuzz, the noise level is further reduced because the reverberation is reduced within the box. You can get another 6 dB reduction inside the box, and a volume outside the box of only 43 dB.

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Mechanical Systems Noisy mechanical equipment is located in basements, on rooftops, in kitchens, and often in parking garages and loading docks, and sometimes high-volume air flow is vented directly onto public sidewalks. Occasionally electrical transformers are not properly shielded and cause continuous humming. Equipment like vending machines, refrigerators, and residential dryer vents and air conditioners can add noise to high-traffic public spaces. These stationary noise sources are often continuous and increase “background” noise in commercial, mixed use, industrial, and even high-density residential areas. They differ from traffic and pedestrian noise, which are tied to human activity patterns and tend to die down or even disappear in some areas at night. “The mechanical rooms in large buildings can be a major source of noise, sometimes exceeding 100 dBA” (Egan 1988). Mechanical equipment in buildings creates both airborne and structure-borne sound. Builders should purchase the quietest equipment available, and then reduce structure-borne sound through proper location and mounting, and reduce airborne sound through absorption and isolation near the source. Mitigating noise from stationary building sources is relatively easy and cheap; failing to do so is usually a matter of will and attention, not money or technology. For example, “it is currently possible to build inaudible HVAC systems, though [unfortunately] this is rarely done,” even in rooms designed specifically for speaking (Brooks 2003). Structure-borne sound and vibration travel through structural elements of buildings. “Remote location of noisy machinery is essential because of structure-borne vibration” (Brooks 2003). “The hum of transformers or the vibration of ventilation machinery can travel long distances through a building structure and will seriously degrade listening conditions” elsewhere in the building (Brooks 2003). Vibration can radiate as sound from a solid surface. “Imagine what happens if you place a strongly vibrating air-conditioning unit on the roof of an auditorium. The entire roof acts to radiate noise into the seats below” (Brooks 2003). Structure-borne sound cannot be controlled by the insulation techniques that work for airborne sound, such as using low-density infill materials. The types of mechanical equipment that require special acoustical treatments and mountings include fans, refrigeration units, compressors, boilers, cooling towers, water pipes, and transformers. The general rules to reduce noise from mechanical equipment are these: Locate mechanical equipment at grade or in basements, on or near the heaviest and most solid structural elements of the building. Use sound absorptive materials all around the equipment and the equipment room, and separate the sound-sensitive areas of the building from the mechanical room with buffer spaces like closets or halls. Correct orientation of the equipment, and absorption and isolation methods, are also a must. Do not locate fans near walls, where they can create turbulence between the wall and the fan. Rooftop equipment subject to wind loading, and all equipment subject to earthquake, may require special mounts. Noisy and vibrating machinery should be structurally isolated from the building elements with resilient mounts (steel springs, rubber, neoprene pads, etc.). The resilient mounts must provide

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a certain amount of static deflection, and the mounting method is based on the lowest frequency of the equipment. The more the machine vibrates freely in the air, a relatively poor transmitting medium, the less sound will be transmitted to and through any structure it is fastened to. The load of the equipment should be evenly distributed on the mounts, and rigid connections to the structure (such as tight bolts) must be avoided. If the structural element that the equipment is fastened to is lightweight or vibrates, even more care must be taken. Equipment may also be adjusted or modified to make it quieter. A fan or motor may be operated at lower speed, for example. Keep the equipment in good repair by lubricating moving parts, tensioning drive belts, and tightening loose and vibrating screws or bolts. Noise from air-distribution systems (HVAC) comes from air turbulence and vibration of machinery and other system parts. It has the following general characteristics: Noise from turbulence occurs at mid frequencies (250 to 2000 Hz). Fan rumble is low frequency (<250 Hz), and air outlet noise (hiss) occurs a high frequencies (>1000 Hz). Very low-frequency rumble can be caused by turbulence within ducts, and duct-borne noise travels independently of the direction of the air flow in the duct. To reduce HVAC noise originating from air turbulence, air-distribution systems should have minimum air flow resistance. A good guideline is “lower and slower”: lower volumes of air moved through the system with fans and blowers operating at a slower speed. Obstructions and turns should not occur near fans. Elbows should have a large radius and be lined with absorptive materials. Changes in duct size should be tapered, not sudden; branch-offs should be angled. Terminal devices should not be in critical spaces, and removal of dampers and grilles can reduce noise from turbulence by over 10 dB. Reduce noise further with fiberglass-lined ducts, plenums for supply and return, and other noise isolation methods. When it comes to remedial action, sophisticated problem analysis is often required for the most effective solutions. “Most noisy equipment has several noise sources, all of which must be considered.…At minimum, octave band noise levels from the equipment should be obtained. You cannot solve a noise problem by knowing only the overall noise level generated by the equipment” (CertainTeed 2003). On the other hand, once the noise sources are understood, simple actions like mounting and maintaining equipment properly, relatively low-tech isolation methods, and strategically placed sound absorbers can yield significant noise reductions. Remember that C-weighted, rather than A-weighted, decibel readings are usually used to evaluate equipment sounds. C-weighted readings give more emphasis to low-frequency sounds and are likely to be higher than A-weighted readings.

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Creating Spaces with Good Acoustics When sounds are unwanted by the listener, they are considered noise: unpleasant, bothersome, distracting, or physiologically or psychologically harmful. Noise annoyance is determined by the information content, predictability, necessity, and controllability of the noise. The most annoying noises contain information, and are unpredictable (irregular), unnecessary, and uncontrollable by the recipient (CABA 2006). To solve any acoustical problem you must know the specific sources of the sound. The components of sound in rooms and other spaces are these: 1. Direct sound comes from sources inside and outside the space. Indoors, free-field conditions only exist very close (under 5 feet) to the source; after that, the sound enters the reverberant field of noise bouncing off reflective surfaces. Treating the ceiling and walls of a room with sound absorbing materials can reduce sound levels, but not from direct sound near the source. 2. Reflected sound is heard after sound bounces off all reflective surfaces. An echo is a distinct sound reflection, and echoes are not heard in reverberant spaces. The buildup of sound is due to repeated reflections or diffusion of sound from enclosing surfaces. 3. Reverberation is the persistence of sound in a space; it is created by the buildup of reflected and diffused sounds and is minimized by sound absorption. Reverberant rooms are called “live” and absorbent rooms are “dead.” Reverberation reinforces all the sounds in a room. The factors affecting acoustics in a space, in order of importance, are 1. Size. Large spaces (including outdoor spaces) are difficult to manage acoustically, even for single-purpose spaces (e.g., for music, theater, conversation), and it is even more difficult to manage large multi-purpose spaces. 2. Background noise. Most everyday spaces need to have their noise (unwanted sound) reduced. This is done by (1) isolating the space from noise sources and (2) reducing reflection and reverberation. In the indoor environment, eliminate HVAC noise (unless it provides desirable, controlled background noise), vending machines, and refrigerators. 3. Space geometry. Rooms for speaking and music (both performance and practice) must be the right shape and size. Certain room proportions can cause greater resonance when all surfaces are reflective, as in bathrooms. But since most rooms have some absorbent surfaces, this is usually unimportant in housing room design. 4. Room or space surfaces. Correct use of absorbing and reflecting surfaces can eliminate noise and reinforce the desired sounds, such as speech or music, in the space. Unfortunately, although size, shape, and background noise (all above) are usually the more important acoustical factors, acoustical remediations usually rely on room furnishing and surface treatments, because room users have the most control over these. In the United States, there are federal noise standards for highway construction and traffic noise impacts. State, county, and municipal ordinances govern other types of activities and maximum allowable external and internal noise levels for various land uses. These standards and regulations vary from place to place. Regulations and guidelines also come from other sources

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such as workplace health and safety standards and health organizations. For example, the World Health Organization recommends that outdoor noise levels be low enough that “at night…the recommended level of 30 dBA inside bedrooms for steady state continuous noise can be met with the windows open” (Kittredge 1999). The Department of Housing and Urban Development (HUD) has guidelines limiting building in areas with excessive outdoor noise, such as near freeways or heavy industry. The guidelines require noise reductions that usually involve replacing natural ventilation with air conditioning systems, which unfortunately decreases quality of life and increases energy use. Or, if noise barriers are required by local or environmental regulations to protect residents from outdoor noise, these barriers usually consist of high, opaque walls that often have negative impacts, both aesthetic and sociological. Conversely, buildings designed to foster a sense of community within healthy neighborhoods enable residents to address noise problems more effectively. The Federal Housing Administration (FHA) sets standards for housing construction. It recommends certain maximum noise levels in rooms for different uses, and minimum requirements for noise isolation between rooms and dwellings, with greater isolation for quieter rooms. For example, there must be greater isolation between a (noisier) source kitchen or living room and a (quieter) receiving bedroom than between two rooms of similar acceptable noise levels. Crowded living conditions become problematic, therefore, if residents begin to do more types of activities in their bedrooms, above the bedrooms in other units. Knowing what the space will be used for is important when specifying what level of ambient noise is acceptable. Guidelines differ, but here are some suggested acceptable ambient noise levels derived from various sources:

Type of Room - Occupancy dB(A) Concert and opera halls, recording studios, theaters, private bedrooms 25 - 30

Very quiet Live theaters, conference and lecture rooms, cathedrals and large churches, libraries, private living rooms, hotel bedrooms

30 - 35

Quiet Board rooms, conference and lecture rooms, private living rooms with windows open 35 - 40

Somewhat quiet

Public rooms in hotels, small offices, classrooms, courtrooms 40 - 45

Moderately noisy

Larger offices, bathrooms, reception areas, lobbies, corridors, department stores 40 - 50

Noisy Institutional kitchens, laundry rooms, computer rooms, supermarkets, busy offices 45 - 55

Very noisy Noisy commercial kitchens, industrial shops 55 - 60 High-density life means that more residential noise sources of all kinds—more people, more voices, more cell phones, more stereos, more activities, more barking dogs, more cars, more car alarms—are crowded into smaller areas, with more noise-reflective buildings and other surfaces. To maintain a livable environment, acoustical planning should accompany any significant increase in residential density. Local zoning laws were originally devised to separate residential living from the nuisances and

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contaminants of commerce and industry, including noise. Here is a noise description from a 2008 environmental impact report on a California shopping center:

“Sources of on-site noise from operations/activity at the shopping center would include patrons to the 24-hour businesses such as fast food establishments and the anchor tenant. Additional sources would be from truck deliveries, loading dock activity, heating and ventilation units, and air-conditioning and other mechanical equipment. Noise sources typically associated with commercial delivery activities include truck engines, back-up horns, beepers and signals, truck-mounted refrigeration/generator units, forklifts, handcarts, roll-up doors, PA systems, and voices from drivers and store employees. Roof-mounted heating/cooling systems are proposed on all or most of the stores within the shopping center…”

These commercial noises also occur with institutional uses such as schools, group living quarters, hospitals, socially active churches, etc., and in significant amounts they are incompatible with nearby residential use. To protect residential livability, the current promotion of mixed use development must be addressed with as much caution as enthusiasm. Urban noise not mitigated at or near the source either becomes a permanent urban blight, or must be addressed by isolating the receiver. Both ignoring noise and trying to isolate it at the receiving end have negative health, sociological, and urban design consequences. Prevention is key. Acoustics for speech and music Although “dead” (highly absorptive) rooms are good for homes and for sleeping, they are not good for either distance speech or for music. However, the reflections and reverberations of any “live” space must be carefully controlled. Good acoustics for speech are not the same as good acoustics for music; thus single-purpose spaces are the easiest to design, but they are rare. Good multi-purpose performance spaces therefore usually have sound absorbing and reflecting features that can be arranged to fit the performance, musical or spoken, and sometimes the audience size. Sound is heard after bouncing off all reflective surfaces, but within a room, “early reflections” are heard as reinforcing the direct sound, while “late reflections” are heard as persistent noise in the room, or even echoes. For speech performances, you want intense early reflection into the audience from the stage only, no late reflections from anywhere, minimal reverberation, and no background noise. Concert listeners, on the other hand, vary in their liking for reverberated or “rich” sound versus “clean” unreverberated sound. Speech intelligibility is mostly affected by reflections, reverberation, and other background noise. Persistent reverberation interferes with speech. The larger the room, the longer the reverberation time, but each time you double the amount of absorption in the room, you reduce reverberation time by one half. You can likewise use absorption to control echoes, and to control the apparent location of the sound source. Acoustical ceiling tiles are appropriate for spaces where you want to absorb noise, but do not necessarily create good acoustics for distance listening. Bodies also absorb sound and reduce reverberation time.

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Generally, reverberation times for classrooms and other small-scale speech venues should be under 1 second (or 0.5 sec for children, the elderly, and the hearing impaired), while reverberation times in large spaces for music can exceed 2 seconds; mixed use spaces range from 1 to 2 seconds. For the best speech comprehension, keep reverberation times under 0.3 seconds. Public speakers can and should slow their speech down to accommodate reverberant rooms. The speech intelligibility index (SII) is a weighted speech-to-noise ratio indicating how well speech can be understood in the presence of noise. An SII of 1 indicates perfect intelligibility, and 0 indicates no intelligibility. People are less sensitive to interference with low-frequency sound (below about 125 Hz) than to higher-frequency sound; however, the hearing-impaired need more clarity at those lower frequencies. Sound diffusion is usually considered a good thing for creating “rich” listening, eliminating echoes and direct reflections, and reducing the annoyance of background noise. Diffusion comes from sound waves scattering when hitting complex surfaces, and is created in traditional spaces by architectural ornamentation or surface irregularities. Modern architectural design often lacks these natural acoustical forms. “The interiors of listening spaces should be full of playful, complex, sound-diffusing forms” (Brooks 2003). The shape and scale of the space is also important. For both music and speech, a long and skinny room is better than a short and wide one. Place the wanted sound source (speaker, musician) at the short end of a long room. Certain room shapes (concave, etc.) should be avoided because they bounce noise badly. Auditoriums and other spaces designed for distance listening usually have sound reflective panels placed to reflect sound into the audience from the podium. Reverberation is enhanced by open space above the listener, which is characteristic of concert halls. Reflectors can be used to direct sound into the audience from behind or above the speaker. When working with reflected sound, for good speech and fair music listening, use the following formula: (feet between source and reflector plus feet between reflector and listener) minus (direct distance from source to listener in feet) should be greater than 34 feet. Below 23 feet would be excellent for both speech and music. Flutter echo is caused by the repetitive interreflection of sound between two surfaces, usually parallel or concave; it is usually heard as high-frequency ringing or buzzing. Flutter may be heard at specific locations in a room. Prevent flutter by avoiding parallel surfaces (use a tilt greater than 5 degrees), good sound absorption on the surfaces, breaking up surfaces to diffuse the sound, and avoiding concave walls and ceilings. Background noise is damaging to listening activity, although people are usually able to “tune it out” and focus on the sound they are listening for. In rooms designed for listening, the worst direct noise sources are usually the HVAC systems; when necessary in speaking venues, turn the HVAC off so people can hear the speaker. When background noise is low, people tend to be quieter. But when room acoustics are poor and/or background noise is high, people talk more loudly, initiating a vicious circle, or resort to the microphone, which often makes things even worse because amplification systems require their own acoustical expertise. However, background noise is useful when it contributes to sound isolation by masking distracting intruding noise, such as intelligible speech. Acceptable masking noise levels vary

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with room use, and with the nature of the masking noise. Although it is often desirable to have masking background noise in offices and restaurants—which may have to be controlled more than generated—deliberately creating background noise in residential spaces, such as with “white noise” generators, is rare and means that other impinging noises are unacceptable to the hearer. When creating background noise, remember that pure sounds seem louder than impure ones, and should be reduced by up to 10 dB relative to impure tones. Frequencies below 125 Hz and above 4000 Hz must be reduced by 4 to 5 dB; otherwise, the noise will be unpleasant—rumbly or hissy. Restaurant acoustics: Restaurants abound with noisy activities, so overall sound levels almost always need to be reduced. This should be done through acoustical treatments of walls and ceilings; very high ceilings are also good. Where possible, install padded chairs and carpeting, and tablecloths to reduce the sound of clanking silverware and dishes. However, sound levels can probably not be reduced satisfactorily by sound absorption methods alone, so sound prevention is key. Kitchen noise should be isolated, HVAC systems should be quiet, and staff should “set the tone” by quiet behavior. Table sizes should be small enough that patrons can talk to each other without shouting. Classroom acoustics: In classrooms, all speakers must be able to hear each other; thus a classroom must be different than an office or an auditorium, where speaking and listening are both one-way. To understand speech, children, the elderly, and the hearing-impaired all require better acoustical conditions than adults. Older school classrooms were ventilated with open windows in quiet neighborhoods. The main noise problems in modern classrooms are HVAC and other mechanical systems, noise from outside (other classrooms, etc.), and noise bouncing around hard surfaces inside the classroom. Office acoustics: “Employees name freedom from noise as one of the most important factors affecting their ability to work effectively. Noise has a negative effect on focus, task performance, comfort, stress levels and, indirectly, on organizational productivity” (CABA 2006). Memory, problem solving, and creativity in office environments are not impacted as much by overall noise level as by unpredictable, intermittent, and distracting noises. Long-term physiological impacts of noise are especially important in work environments, because workers spend long periods with very limited ability to avoid noise or minimize its impacts. The precise acoustical design of an office depends on the office layout and use. The considerable reflected noise in an open-floor office plan is reduced by high ceilings and acoustical ceiling tiles. But in an open office, you don’t necessarily want zero background noise, because a certain level of background noise creates privacy for conversation and eliminates distractions. Offices usually require some speech privacy. Intelligible speech is distracting, so the goal of speech privacy is to render speech unintelligible from one space, enclosure, or room to another, which is done by adjusting the signal-to-noise (speech-to-background noise) ratio. The background (ambient) noise level should be high enough to mask speech, but should not be uncomfortable or unpleasant; it should be neutral and unobtrusive. Carefully controlled air flow may provide background noise, but fan noise and rumble from turbulence or machinery are not desirable.

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Eliminating noise from outdoor sources Some studies have shown an increase in urban noise of about a decibel per year over the past few decades, meaning that the perceived urban noise level may have increased about fourfold in twenty years. In recent years, nighttime noise levels have also been increasing relative to daytime levels, in large part due to nighttime trucking and delivery activity. The overall acoustical problem in urban environments is that buildings reflect and amplify noise. Clusters of buildings (especially parallel ones) create urban sound boxes and resonating chambers, often around noise-generating activities like streets, parks, courtyards, etc. Free-field fall off is very limited, reflections increase sound rather than reduce it, and the sound paths from source to receiver are complex. Taller buildings create more noise problems than shorter ones, due to mechanical equipment, greater reflection due both to size and common building materials like glass, and lack of acoustical protection of upper floors, in addition to population density factors. In cities it is often very difficult to retroactively ascertain the source of a sound, especially when there is no baseline information or monitoring of new sound sources. Outdoor barriers can isolate unreflected sound, more or less along “lines of sight,” although sound, especially low-frequency sound like engine roar, can bend around obstacles. However, sound in urban areas is unpredictable due to ubiquitous reflections, diffusion, and absorption. Fall-off with distance does not occur as in free-field conditions, and noise readings taken at ground level often do not reflect noise at upper stories. “If there are barriers or grass-covered surfaces between the noise source and a high-rise building, sound levels at upper floors may be much higher than those at ground level” (Egan 1988). Traffic is the greatest source of noise annoyance in most urban areas. Most Americans are exposed to disturbing levels of traffic noise, which increases with population and building density. Because it approximates a linear source, free-field traffic noise is assigned a decay rate of 4.5 dB per distance doubled, instead of 6 dB. Traffic noise is affected by roadbed material (asphalt is 5 dB quieter than concrete, for example), traffic volume and speed (higher speeds are noisier), wet or dry pavement, number of trucks, grade, etc. Elevated highways produce more far-reaching noise than ground-level highways, because the line of sight is not blocked, although nearby their noise may be less than grade-level roads. Below-grade roadbeds (at least a 12-foot depression) are the best for attenuating road noise. Earth berms or acoustic walls are common, but they are ineffective beyond about 300 feet of the barrier and for upper floors of buildings. Where feasible, tunnels for urban highways eliminate noise and have other urban design benefits. The reduction of traffic volume is very difficult, because American driving patterns are dictated by economic and sociocultural factors, and the ability to address them by manipulating urban form is limited. For example, “just in time” business practices have greatly increased the use of trucks relative to trains; the number of trucks on freeways doubled between 1990 and 2000, and each 18-wheeler emits noise equivalent to 28 passenger cars. The two-wage-earner family limits our ability to decrease commuting. Mass transit use is limited more by lifestyle and employment factors than by transit quality or urban form. Therefore, although cities might reduce driving by providing local opportunities to satisfy daily needs, and urban driving may gradually decrease with economic and lifestyle changes, reducing miles driven is a very long term project. Nonetheless, without reducing miles driven, existing urban traffic noise can still be reduced

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through technological and behavioral changes. Individual vehicles have been made much quieter over the decades by automatic transmissions, quieter engines and tires, and so forth—remember, horse-drawn carriages clattering over stone streets were hardly silent! Today, passenger car traffic noise is primarily from tire noise (originating at road level), not engine noise, while engine and other mechanical noise (originating up to 8 feet off the ground) is the primary noise source for trucks. SUVs and light trucks make more than twice as much noise as smaller passenger vehicles. Urban traffic noise and its annoyance could be significantly reduced by manipulation of the following factors: vehicle sizes, traffic speed, amount of stopping and starting, road surfacing, more regulation of trucking, acoustical building façade treatments near roads, careful positioning of new residential density to avoid noise from traffic arterials, and enforcement of laws relating to construction and delivery vehicles, idling trucks, and loud vehicles (such as vehicles with loud mufflers, stereos, or car alarms). In addition to traffic, “stationary, or fixed, sources of environmental sound abound in and around practically all communities, large and small. Industrial plants, outdoor rooftop air-conditioning equipment, electrical transformers, power plants, waste processing plants, and yes, even outdoor concert amphitheaters can and do produce unwanted sound. This noise interferes with the enjoyment of residential property and with sleep, and detracts from the general physiological and psychological well-being of the community” (Cavanaugh 1998). Buildings can be designed to reduce noise. The primary goal in the urban environment is to reduce noise everywhere, and most acoustical building treatments will benefit both those inside and those outside the building. To protect the outside urban environment, building designers can minimize mechanical and venting noise, use absorptive exterior finishes, use surface decoration to diffuse noise, and avoid surfaces and shapes that reflect noise into public and private spaces. Buildings can be built so that noisy activities are contained within rooms or courtyards or behind barriers, and not projected into surrounding spaces. To protect the building inhabitants, in addition to soundproofing of floors, walls, and windows, buildings can be built up on podiums, or with recessed floors or atriums. They can be designed with architectural barriers between doors and windows and exterior noise sources. Openings for ventilation can be placed in noise shadows (such as near the floor behind a balcony wall). Windows may be protected by balconies or overhangs (preferably sound-absorbent); railings or fences should be solid and absorbent instead of transparent. Deep-niche windows create sound reflections within the niche, so the niche should be treated with sound-absorbing materials. The common habit of lining up parallel buildings increases noise reflections from the streets between them or perpendicular to them; angling the walls may help reduce this problem. Courtyards should not face the street, because there can be a noise buildup from multiple reflections at the back of a courtyard of up to 10 dB, and parallel buildings perpendicular to a street (barracks style) can have the same sound buildup at the rear. Courtyards with parallel walls also cause flutter echoes. Greenery is a very desirable urban amenity, but while a dense urban canopy can reduce noise over a broad area, trees and bushes are ineffective as localized noise barriers; they scatter sound but do little to absorb or isolate it. All their sound attenuation comes from their leaves (so trunks at ground level and bare winter branches don’t help much), and it takes a dense planting 100 feet thick to reduce noise levels by 7 to 11 dB. A single row of trees has no effect, and plantings may even reduce the effectiveness of sound barriers by scattering sound over or around them.

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However, trees can turn direct sound into diffused sound. Sometimes people are less bothered if the sound is multidirectional or they cannot see an annoying noise source, but other times people perceive sound as louder when they cannot see the source. Trees may also cause wind turbulence, perhaps increasing sound, or perhaps decreasing annoyance by masking unpleasant sound with a more pleasant one. Grass is an excellent sound absorber (the thicker the better), and grass, fresh snow, and other ground covers reduce noise levels for sounds traveling near the ground of reflecting off the ground. Since most passenger car and pedestrian noise originates near ground level, if grass lawns are replaced (e.g., to save water), the replacement plantings or other materials should also be sound absorbent, or urban and suburban noise levels may increase. Earth berms covered with sound-absorbent vegetation like grass are fairly good sound isolators—the steeper, the better. However, earth berms alone are not as good as walls for isolating freeway noise. And if you put a sound-reflective surface on top of a berm, like a sidewalk or bike path, the effectiveness of the berm is reduced. Wind, temperature, and water impact noise outdoors in complex ways. Wind generally creates an upwind “shadow” at the level of the noise source (not above) and a downwind enhancement of noise at ground level. Wind at right angles to the noise path has little or no impact. In urban settings, “wind tunnels” and wind turbulence created by buildings increase noise as well as other kinds of discomfort. Air temperature layers may cause sound to travel long distances at night—up to ten times as far. On a clear, calm summer day, sound bends upward from the ground; on a clear, calm night, it bends downward, causing up to a 10 dB difference in sound level between day and night for sound sources up to 1000 feet away. Water causes sound to refract toward its surface, amplifying the sound, and a surface of water reflects almost all sound hitting it. Dripping water can create unwanted and amplified noise if eaves, gutters, and drains are improperly positioned. Urban noise accumulated gradually through inattention, and can be decreased gradually through attention. Yet because of its ubiquity and seeming inevitability, when faced with urban noise, the culture that can send men to the moon and robots to Mars suddenly loses its ability to analyze a relatively simple problem and locate the areas for potential interventions. Contrary to popular belief, urban noise does not come out of nowhere; it is not “just there”; it did not come from God as a brilliant afterthought on the eighth day. Simply stated, every noise has (1) a physical source, usually created by human beings, and (2) one or more paths by which it travels. Here lie the solutions to urban noise. Some urban noise sources require socioeconomic, cultural, or political solutions, which take time. But many others—especially the stationary sources—are easy to address technologically. Only a few noise sources, like freeway noise, may not be soluble at the source by local action. The urban noise path, on the other hand, is often long, devious, and hard to trace, which argues—again—for mitigation at the source. Any noise eliminated at the source is a downstream benefit for dozens, hundreds, or even thousands of people along its route. The paths of urban noises usually involve many reflections, each of which is a potential opportunity for absorption. Some acoustical materials can absorb almost all the sound hitting them, and new materials are being developed all the time. Yet many modern buildings reflect almost all the sound hitting them directly into the “earspace” of urban residents. If unnecessary noise sources were eliminated, if building facades and other urban structures were designed to be absorptive rather than reflective, and if new structures incorporated a few other simple acoustical principles, American cities would be much quieter places, even at higher population densities.

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Conclusion

It is unfortunate that for the past generation the United States has been desultory, even negligent, in addressing noise pollution. Factors that have enabled Americans to ignore the increasing urban noise levels include our individualistic (rather than communal) national philosophy; the economic ability of many Americans to flee noisy cities for the suburbs; cultural acceptance of social inequities; and higher political regard for the property rights of polluters than for those of victims of pollution. The inclination and ability to flee urban centers has diminished our need and will to create livable cities. However, environmental degradation and global warming now necessitate the urgent creation of cities that attract people because they offer a healthy and sustainable quality of life, for people of all ages and a variety of lifestyle preferences. There are three ways Americans can begin to give noise pollution the attention it deserves: we can recognize the health impacts of noise, we can reclaim our right to the commons, and we can realize that noise is a proxy for other environmental degradation. The health impacts of environmental noise abound in the medical literature and are not discussed in this document. They are gaining scrutiny as rising health care costs lead health professionals to focus on creating and maintaining a healthy population, rather than just on curing disease. The direct correlation between health and social class in America is likewise leading to increased study of long-term environmental stressors, of which noise is one. It is important to note that the physiological impacts of noise can be very indirect. For example, maintaining a rigid body posture in response to unpleasant noise can cause musculo-skeletal disorders over time. In light of noise’s physiological and psychological impacts, new noise sources are being addressed before their impacts become too widespread, and old ones are being reassessed. For example, researchers are concerned that continuous exposure to noise from wind (energy) farms may have significant health impacts, and European workplace noise standards now require that orchestra members be protected from exposure to the music’s louder passages and percussion instruments. The second way Americans can elevate their appreciation of noise pollution, and reclaim our right to a livable environment in other ways as well, is to reassert our right to the commons—our commons. Rural and suburban life to a great extent isolate people and allow them to dissociate themselves from the concept and reality of the commons, but urban life, with its close quarters and limited private space, makes shared common spaces particularly important. In practice one cannot rely on the private property right to the quiet enjoyment of one’s home to defend oneself against invasive noise and other pollution. The power to address all kinds of pollution is enhanced by recognizing that the air, the water, the climate, some land, energy reserves, natural resources, and other elements of the environment are shared resources that belong to everyone. Such concepts as the disputed commons (negotiation of who uses the commons), the abused commons (overuse leading to degradation), the polluted commons (usually applied to the natural environment), and the privatized commons (taking of the commons by private parties) are widely recognized by historians and others. To these one can add the idea of “the forsaken commons.” The forsaken commons is largely a feature of decaying urban areas. It is a cycle of initial degradation of the commons, which is ignored by authorities, followed by revulsion, alienation, and disengagement of those who use the commons, followed by their neglect and irresponsibility, permitting further degradation. This cycle occurs with crime, blight, and

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schools, as well as noise. Viewing this process in a spatial context—that is, what kind of spaces we make, who uses them, and how and why—reveals the important role that city planners and developers play in creating and maintaining the health of the urban commons. In the case of noise, the forsaken commons begins when the annoying noise of human activity (including traffic, mechanical and venting noise, loud pedestrians, invasive music, etc.) starts to degrade the commons. At first this is ignored because cities have more important things to worry about. But at some point those who formerly enjoyed the commons no longer enjoy it; they retreat behind their headphones, they close their windows, they sleep with earplugs. They install opaque fencing, fans, air conditioning, thick drapes. Developers adapt to the degraded commons by designing buildings that shun street life, in which users need not interact with the outside environment. Because in an urban setting public spaces are the major arena for communal activities and for access to nature (through parks), the psychosocial impacts of losing use of the public space in cities are especially great. Recognizing the stress-reducing effects of access to nature, quietude, greenery, and natural darkness, some researchers have dubbed the psychological effects of lack of access to nature “nature deficit disorder.” As residents disengage from the public space, they care less and less about increased noise and further degradations. They further reduce their oversight of the commons—which, incidentally, also increases crime and policing expenses. By abdicating responsibility for the maintenance of the commons, the residents collude in their own dispossession of it. New residents may not even remember a time when the commons was pleasant and people felt entitled to enjoy it. Sociologists call this gradual adaptation to a worsening environment “creeping normalcy.” When the civil population has finally abandoned the commons, the abusers of the commons, whether deliberate or accidental, have free rein, and the commons become even noisier and less attractive, and residents more isolated. This is the vicious cycle of the forsaken commons. Finally, noise awareness will increase when people realize that silence is “green.” Those who are concerned about global warming should understand that noise is the echo of our carbon footprint. That is, almost any sound that is heard in the urban environment correlates, as cause or consequence or both, or symbolically, with energy use and environmental damage. As described above, over certain comfortable levels, the noise made by human activity in the commons is a signifier of social deterioration as well as the future environmental cost of fewer people willing to live in high-density areas. Another obvious signifier of environmental abuse is increased traffic noise. Truck noise, for example, is the sound of the consumption and transport of goods, primary causes of energy use and greenhouse gas emissions. Reducing consumerism and increasing local manufacture would simultaneously reduce noise and global warming. Likewise, a quiet building is more likely to be an environmentally friendly one. A huge HVAC system operating nonstop in a temperate climate, where buildings were previously ventilated with open windows, is neither “green” nor pleasant, and also contributes to “sick building syndrome.” Loud equipment is also often old equipment, and usually energy inefficient. Conversely, what is quiet is often energy-efficient, such as insulated walls and windows, good weatherproofing, natural ventilation, and new, energy-efficient vehicles and equipment. But the most important correlation between quiet and environmentalism is subtle and intangible, and it is this: Any city that is actively reducing urban noise is probably dedicated to creating the kind of city that more people will want to live in. And a quiet city is a healthy city, both for human beings and for the planet.

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Appendix: ACOUSTICAL TERMS frequently found in environmental assessments

AMBIENT NOISE LEVEL – The composite of noise from all sources near and far. The ambient noise level constitutes the normal or existing level of environmental noise at a given location (i.e., the background noise level). A-WEIGHTED SOUND LEVEL (dBA) – A single number in decibels representing the sound level in a manner representative of the ear’s response, that is, with the effects of the low and high frequencies reduced relative to the medium frequencies. COMMUNITY NOISE EQUIVALENT LEVEL (CNEL) – The average equivalent sound level during a 24-hour day, with weighting factors equivalent to penalties of five decibels for evening periods from 7:00 PM to 10:00 PM, and ten decibels for nighttime periods from 10:00 PM to 7:00 AM, to reflect increased nighttime noise sensitivity. Usually A-weighted. DAY-NIGHT AVERAGE SOUND LEVEL (DNL or Ldn) – The average equivalent sound level during a 24-hour day, with the addition of 10 decibels to levels obtained between 10:00 PM and 7:00 AM. Usually A-weighted. DECIBEL (dB) – A unit for describing the amplitude of sound, equal to 20 times the logarithm to the base 10 of the ratio of the pressure of the sound measured to the reference pressure, which is 20 micropascals (20 micronewtons per square meter). EQUIVALENT ENERGY LEVEL (Leq) – The sound level corresponding to a steady state sound level containing the same total energy as a time-varying signal over a given sample period. Leq is typically computed over 1, 8 and 24-hour sample periods. Lmax or Lmin – The maximum or minimum A-weighted noise level recorded during a noise event or time period. Ln – The sound level exceeded “n” percent of the time during a sample interval. For example, L10 equals the level exceeded 10 percent of the time. NOISE – Any sound or signal that is undesirable because it interferes with speech and hearing, or is intense enough to damage hearing, or is otherwise annoying. NOISE CONTOURS – Lines drawn on a map that connect points of equal noise exposure (Ldn or CNEL) values. They are usually drawn in 5-dB intervals. SOUND EXPOSURE LEVEL (SEL) – The level of noise accumulated during a single noise event over a duration of one second. Specifically, it is the level of time-integrated A-weighted squared sound pressure for a stated time interval or event, based on the reference pressure of 20 micronewtons per square meter and reference duration of one second. SOUND (NOISE) LEVEL – The weighted sound pressure level obtained by the use of a sound level meter having a standard frequency filter for attenuating or accentuating part of the sound spectrum.

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(EEA); State of Environment report No 1/1999. Environmental Noise: The Invisible Pollutant by William J. Cavanaugh and Gregory C. Tocci. E2SC,

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Various additional online sources

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