12-Nov-01 Intro to Acoustics: Reverberation 2

Learning Outcomes• Explain Sound behaviour including

reflection, absorption, energy density, sound decay and reverberation.

• Learn how to design an acoustic room and its design considerations.

• Learn what is Electroacoustics.

History of Acoustical DesignThe recorded history of the acoustic design of

buildings seems to begin with the construction of amphitheatres by the ancient Greeks . (oratories and perform plays)

These were open air amphitheatres that housed up to 2000 people, all listening to a single orator or small group of actors.

There is a limit to the audibility of the human voice. Can you think of some of the techniques used by the ancient Greeks in the

construction of their amphitheatres? Did they work and, if so, how did they know how to make them work?

Sound behaviorSound waves propagate away from the source until they

encounter one of the room's boundaries - some of the energy will be absorbed, some transmitted and the rest reflected back into the room.

Sound arriving at a particular receiving point within a room can be considered in two distinct parts.

1. Sound that travels directly from the sound source to the receiving point itself. This is known as the direct sound field and is independent of room shape and materials, but dependant upon the distance between source and receiver.

2. After the arrival of the direct sound, reflections from room surfaces begin to arrive. These form the indirect sound field that is independent of the source/receiver distance but greatly dependant on room properties

The Growth and Decay of SoundThe sound intensity measured at a particular point

increases suddenly with the arrival of the direct sound and will continue to increase in a series of small increments as indirect reflections begin to contribute to the total sound level.

Eventually an equilibrium will be reached where the sound energy absorbed by the room surfaces is equal to the energy being radiated by the source.

This is because the absorption of most building materials is proportional to sound intensity, as the sound level increases, so too does the absorption.

The Growth and Decay of SoundIf the sound source is abruptly switched off, the

sound intensity at any point will not suddenly disappear, but will fade away gradually as the indirect sound field begins to die off and reflections get weaker.

The rate of this decay is a function of room shape and the amount/position of absorbent material.

The decay in absorbent rooms will not take very long at all,

whilst in large reflective rooms, this can take quite a long

time.

Reverberant Decay of sound in a small absorbent enclosure.

This gradual decay of sound energy is known as reverberation and, as a result of this proportional relationship between absorption and sound intensity, it is exponential as a function of time. If the sound pressure level (in dB) of a decaying reverberant field is graphed against time, one obtains a reverberation curve which is usually fairly straight, although the exact form depends upon many factors including the frequency spectrum of the sound and the shape of the room

Optimum Reverberation Times

Geometric AcousticsWave Theory and Normal ModesThe concept of a sound ray and the geometrical

study of sound ray paths play an important role in the design of large rooms and auditorium, enabling troublesome echoes and flutter effects to be detected and dealt with at the design stage.

A limitation of the geometrical approach is that usually only primary and possible secondary reflections can be studied before the sound ray being followed becomes 'lost' in the reverberant sound field and, in most enclosures, it is restricted to frequencies of 500 Hz and above.

Using Geometric Acoustics

Statistical methods are useful at the earliest stages of design, however, as more and more geometric information becomes available, why not use it.

As enginneers, we need to be able to determine not only how much absorber to use, but what type of absorber and where to put it. This is where the consideration of reflected sound rays can be quite useful.

Faults Attributable to GeometryPicket fence echoes - results from evenly

spaced reflection paths, such as the rows of raised seating in amphitheatres and the evenly spaced curves of compressed fiber fencing.

Depending on the number of steps and the path difference, such surfaces can produce a definite pinging sound when stuck by an impulsive sound source.

If d is the distance between successive steps, then the frequency of this ping is given by Fpfe = (c / (2d)) where c is the speed of sound in air.

Faults Attributable to GeometrySpurious echoes - Occur when a strong

reflection of the original signal can be clearly discerned by the listener.

This is simply a matter of looking at the internal envelope of the enclosure and checking for possible sound paths, which reflect off a sequence of large, highly reflective surfaces.

Faults Attributable to GeometryFlutter echo - Occurs when both the source and receiver are

between a pair of parallel, hard, surfaces. Some portion of the sound emitted by the source will be 'trapped' between the two reflective surfaces and will oscillate back and forth, being quite slow to decay. The listener will perceive this as a 'fluttering' noise. If the walls are a distance d apart, then the frequency of this flutter can be found in the same way as picket fence echo.

Dead spots- These can occur at positions, which are far from reflecting surfaces, and which receive sound only after it has passed over an absorbent surface. For example, at the rear of a gently raked theatre or cinema where the sound must pass over the audience and ceiling reflections are blocked by a balcony.

The Placement of Reflectors and AbsorbersBy analyzing the paths of sound rays, it is easy to

determine which areas require reinforcement (in the form of a reflector) and which require damping (in the form of absorber).

Consider someone speaking at the rate of up to 8 syllables per second. Each syllable takes about 125 ms. Therefore, if clear reflections of the first syllable arrive mid-way through the second (or even the third) the speech may not be easily discernible by the listener.

Objective MeasuresFor many years the reverberation time was the only real objective measure of the acoustic performance of an auditorium. For many architects, even today, it still is. However, there are many more aspects to sound behavior in rooms.

Early Decay TimeClarity and DefinitionSpatial ImpressionSpeech Intelligibility

Objective MeasuresEarly Decay TimesThe reverberation time, as discussed earlier, refers to the

time taken for the reverberant component of an enclosure to fall by 60 dB after the source is abruptly switched off. In an ideal enclosure this decay is exponential, resulting in a straight line when graphed against Sound Level. Studies of actual auditoria, however, show that this is not always the case.

Research (Kuttruff 1973) has shown that it is the initial portion of the sound decay curve process, which is responsible for our subjective impression of reverberation as the later portion is usually masked by new sounds. To account for this, the Early Decay Time (EDT) is used. This is measured in the same way as the normal reverberation time but over only the first 10 - 15 dB of decay, depending on the work being referenced.

Clarity and DefinitionClarity and Definition - refer to the ease with which

individual sounds can be distinguished from within a general audible stream.

This stream of sound may take many forms; a conversation, a passage of music, a shouted warning, the whirring of machinery, whatever.

The degree of clarity is, of course, greatly dependant on the particular sounds involved, however, from an architectural point of view, it refers to the ratio between the amount of early to late arriving sound energy.

Spatial ImpressionSpatial Impression refers to a feeling of being enveloped

within the music, surrounded by it not just 'looking in at it'. This impression is primarily a function of interaural cross correlation, or the relative contribution of lateral reflections.

Simply put, spatial impression is determined by the subtle differences in signal received by each ear. If all of the sound energy comes from straight in front of or behind you, the signal at each ear will be the same.

If the sound bounces around the auditorium and approaches from the sides, the signals at each ear will be quite different due to diffraction around the head and slight time delays.

Speech IntelligibilityIn terms of individual communication, speech is

probably the most important and efficient means, even in today's multi-media society.

The intelligibility of speech refers to the accuracy with which a normal listener can understand a spoken word or phrase.

Given the fact that some of the information communicated through speech is contained within contextual, visual and gestural cues, it is still possible to understand meaning even if only a fraction of the discrete speech units are heard correctly.

Designing Auditoria

These are serious requirements and it must be remembered that, when an audience enters an auditorium, they have every right to expect comfort, safety, pleasant surroundings, good illumination, proper viewing and good sound." L.L. Doelle, Environmental Acoustics

Outline of Acoustic Requirements for Good Sound

There should be adequate loudness in every part of the auditorium, especially in remote seats.

The sound energy should be uniformly distributed within the room.

Optimum reverberation characteristics should be provided in the auditorium to facilitate whatever function is required.

The room should be free from acoustical defects (distinct echoes, flutter echoes, picket fence echo, sound shadowing, room resonance, sound concentrations and excessive reverberation).

Background noise and vibration should be sufficiently excluded in order not to interfere in any way with the function of the enclosure.

Adequate LoudnessThe auditorium should be shaped so that the audience is as close to

the sound source as possible. In larger auditoria the use of a balcony brings more seats closer to the sound source.

The sound source should be raised as much as is feasible in order to secure a free flow of direct sound to every listener.

The floor on which the audience sits should be properly raked as

sound is more readily absorbed when it travels at grazing incidence over the audience.

As a general rule, however, the gradient along aisles of sloped auditoria should not be more than 1:8 in the interests of safety. The audience floor of theatres for live performance, especially open or arena stages should be stepped.

Adequate LoudnessThe sound source should be closely and abundantly surrounded by large sound-reflective surfaces in order to increase the sound energy received by the audience.

It must be remembered that the dimensions of the reflecting surfaces must be comparable with the sound waves to be reflected.

In addition, the reflectors should be positioned in such a way that the time-delay between the direct and reflected sound is as short as possible, preferably not exceeding 30 msec and definitely not more that 80 msec.

The floor area and volume of the auditorium should be kept at a reasonable minimum, thus shortening the sound paths.

Recommended Volume-per-seat values for various auditoria

Type of Auditorium   

  Minimum    Optimum    Maximum 

Rooms for Speech

2.3 3.1 4.3

Concert Halls  6.2 7.8 10.8

Opera Houses 4.5 5.7 7.4

Catholic Churches 

5.7 8.5 12.0

Other Churches

5.1 7.2 9.1

Multipurpose Halls

5.1 7.1 8.5

Cinemas 2.8 3.5 5.6

Elimination of DefectsThe basic defects attributable to room geometry have been touched in a previous lecture and consist of echoes, sound concentrations, sound shadowing, distortions, coupled spaces and room resonance.

1. ECHOES

These are probably the most serious and most common defect. They occur when sound is reflected off a boundary with sufficient magnitude and delay to be perceived as another sound, distinct from the direct sound. As a rule, if the delay is greater than 1/25 sec (14m) for speech and 1/12 sec (34m) for music then that reflection will be a problem.

Solution: Either alter the geometry of the offending  surface or apply absorber or diffusion.

2. SOUND CONCENTRATION

Sometime referred to as 'hot-spots', these are caused by focused reflections off concave surfaces. The intensity of the sound at the focus point is unnaturally high and always occurs at the expense of other listening areas.

Solution: Treat with absorber or diffusers, better still, redesign it to focus the sound outside or above the enclosure.

3. SOUND SHADOWING

Most noticeable under a balcony, it is basically the situation where a significant portion of the reflected sound is blocked by a protrusion that itself doesn't contribute to the reflected component. In general, avoid balconies with a depth exceeding twice their height as they will cause problems for the rear-most seats beneath them.

Solution: Redesign the protruding surface to provide reflected sound to the affected seats or get rid of the protrusion.

4. DISTORTIONS

These occur as a result of wildly varying absorption coefficients at different frequencies. This applies an undesirable change in the quality and tone coloration (of frequency distortions) to sound within the enclosure.

Solution: Balance the absorption coefficients of acoustical finishes over the whole audible range.

Elimination of Defects

5. COUPLED SPACES

When an auditorium is connected to an adjacent space, which has a substantially different RT, the two rooms will form a coupled space. As long as the airflow is unrestricted between the two spaces, the decay of the most reverberant space will be noticeable within the least reverberant. This will be particularly disturbing to those closest to the interconnection.

Solution: Add some form of acoustic separation (a screen or a door) or match the RT of both rooms.

6. ROOM RESONANCE

Room resonance is similar to distortions in that it causes an undesirable tone coloration, however, room resonance results from particularly emphasized standing waves, usually within smaller rooms. This is a significant concern when designing control rooms and recording studios.

Solution: Apply subtle changes in overall shape of the room or find out which surfaces are contributing and use large sound diffusers.

Elimination of Defects

Conflicting Requirements for Speech and Music

SpeechThe acoustics of a space designed for speech must primarily ensure

definition and intelligibility, remembering, of course, that understanding in the speech communication process depends as much upon gesture and facial movements as it does on vocal projection.

The audience's expectations regarding the actual quality of the speech signal is not too critical, as long as the speaker's voice and accent are recognizable and the vocal information is understandable.

Music Music audiences, on the other hand, have inherited quite a developed

expectation of particular sound qualities for various styles and eras of music. Whilst definition is a prerequisite for speech, excessive clarity in music gives

the subjective impression of brittleness or dryness. In addition, it accentuates unwanted bowing or fret noise, making the musicians job even more difficult.

Electro-acoustics The reasons for using sound amplification equipment within an architectural contextTo increase the sound level when a sound source is too

weak to be heard. To provide additional sound to audiences beyond the

intended range of the source. To project sound back to the stage for the benefit of the

performers. To alter the Reverberation Time or other impression of an

auditoria. To reduce the relative effects of background noise. To provide paging, information or warning facilities. To reproduce electronic or recorded material.

Speaker Placement

There are essentially three types of loudspeaker system;

1. A centrally located system. Also known as a high level system, this is

essentially a single cluster of loudspeakers located near the source. Such a system gives maximum realism as the amplified sound, whilst increasing loudness and clarity, is still associated with the original source.

A centrally located system.

2. A distributed system.Basically a number of loudspeakers spaced throughout the auditorium. This is also known as a low level system as each individual speaker operates at a low amplification level to service only a small part of the whole audience

Whilst it is preferable to use a centrally located system, there are many situations in which it must be used, for example;

Where the ceiling height is too low for the installation of a central system.

Where not all of the audience have a direct sightline with the central loudspeaker.

When the amplified sound is used to overcome high background noise levels.

Where the serviced space may be divided into several smaller spaces.

In large halls where the source position may vary significantly.

Whilst realism cannot be expected from a distributed loudspeaker system, it does provide high intelligibility where the room is not too reverberant.

3. A stereophonic system

Two or more loudspeaker clusters at strategic positions within the auditorium. Such systems are used when there are a number of different sources to be amplified or the source is quite mobile. By using two or more microphones, each connected to their own cluster of speakers, the spatial relationship between the sources is preserved in the amplified sound. This is achieved because the sound is amplified at intensities proportional to the distance between the source and the microphone and the ear perceives the resultant directional cues.

A stereophonic system

One of the main points to consider when placing speakers is the fact that their directionality is frequency dependant.

As discussed in previous lectures, low frequency sounds are pretty much omni-directional, being able to diffract around obstacles (including the speaker cabinet) quite readily.

High frequencies, however, are highly directional with only limited diffraction capacity.

The speech band (the frequencies in which we are most often interested) occupies the mid-frequencies.

This means that they only partially diffract around the speaker cabinet.

As a result, no matter where the speakers are placed, some members of the audience will receive significant low frequency energy but little higher frequency energy. This can make the speech sound muddy and even more difficult to understand.

As a result of these problems, line or column speakers are often preferred over conventional radial or multicellular horn speakers.

These consist of 6-10 loudspeakers mounted next to each other to form a column. Such loudspeakers act to concentrate the sound energy into a beam, which has a wide angular spread in the horizontal plane and a narrow spread in the vertical plane .

This minimizes the amount of sound energy radiated away from the audience, which often causes further reflection problems.

THE HAAS EFFECT

The Haas effect refers to a phenomenon where the sound that arrives at a listener first determines the perceived direction of the source.

This is pretty reasonable if we consider the normal physical situation wherein the direct sound travels in a straight line between source and receiver whilst reflected sound must take a more complex route.

To accommodate this, we need to place a delay on loudspeakers close to the audience. This has to be such that the direct sound arrives first, very closely followed by the loudspeaker output.

Case Study 1: Lecture RoomThe following case study is based on an actual consulting job. It

is intended solely to inform students of the processes involved in acoustic design and the interaction that can occur between the acoustic consultant and the architect.

This study presents results from an acoustic analysis carried out on the proposed design for a 200 seat lecture facility. The proposed building is an earth-covered structure designed to take maximum advantage of thermal mass, passive design and ESD principles. It features a natural ventilation tower, which draw air through an intake plenum beneath the seating to take advantage of ground temperature cooling in summer and heating in winter.

The preference of the architect is for a slightly 'live' facility that is suitable for both unassisted speech and music production. The requirements for speech and music are slightly different so some compromises will be required.

Case Study 1: Lecture Room

3D view of internal envelope model

The following case study is based on an actual consulting job. It is intended solely to inform students of the processes involved in acoustic design and the interaction that can occur between the acoustic consultant and the architect.

Increasingly, modern auditorium are multi-functional requiring that the acoustics be suitable for a range of purposes. The budget rarely allows for variable acoustic treatments so the design process becomes one of compromise.

This study presents the acoustic design and performance analysis of proposed additions to a gymnasium building. The redeveloped facility is to serve as both a gymnasium as well as the main ceremonial assembly hall for a medium size educational institution. It will also be used to house a new pipe organ.

Case Study 2: Assembly Hall

Case Study: Assembly Hall

What are your recommendations for each case?

Individual Case Analysis study to be submitted next meeting

THANK YOU FOR YOUR KIND ATTENTION