building acoustics 2

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Science of sound dealing with wanted andunwanted sounds, assures optimum

conditions for producing and listing to

speech , music..

Hearing/Listeningcommunication channel

Next to vision

Unwanted scenes Vs Unwanted sounds


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Need To Study Acoustics

Optimum speech intelligibility Sound quality

Uniformity Loudness Richness

Clarity Minimum background noise Freedom from echoes


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lHistory of AcousticsAcoustics, one of the oldest branches of physics,originated with Pythagoras's studies of music over

2,500 years ago. Scientific milestones abound inthis field:

Galileo Galilei (1564-1642) discovered thegeneral principles of sympathetic vibrations,

or resonance, and the correspondencebetween the frequency of vibrations and thelength of a pendulum.

Leonhard Euler and Daniel Bernoulli's studiesof vibrating cords in the 18th centuryeventually led to the development of Fourieranalysis, one of the most important tools of

mathematics and mathematical physics.


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Sir Isaac Newton and Gottfried Wilhelm

Leibniz independently developed the theoryof calculus, which in turn allowed thederivation of the general wave equation bythe French mathematician and scientist Jean

Le Rond d'Alembert in the 1740s.Hermann von Helmholtz's On the

Sensations of Tone As a Physiological Basisfor the Theory of Music (1863) madesubstantial contributions to understanding themechanisms of hearing and to thepsychophysics of sound and music.


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John William Strutt's The Theory ofSound (1877/78), a monument of acousticalliterature, was the first treatise to examinequestions of vibrations, the resonance ofelastic solids and gases, and acoustical

propagation in material media.

Jean Baptiste-Joseph Fourier, a 19th-century French mathematician, establishedhis theory about the analysis of a complex

periodic wave into its spectral components.


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German physicist Georg Simon Ohmhypothesized that the human ear is sensitive

to these spectral components. His Law ofHearing stated that the ear is sensitive to theamplitudes, but not the phases, of theharmonics of a complex tone.

20th-century American physicist WallaceSabine initiated the science of modernarchitectural acoustics by finding ways tocorrect the acoustics of noisy rooms.

Hungarian-born American physicist Georg vonBksy validated Helmholtz's theory ofhearing with his Experiments in Hearing(1960), the classic of the modern theory of

the ear.


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Generation, Propagation,Transmission of sound

Sound results from vibrations in the mediumgas, liquid and solid.

e.g :tunnning fork, musical instruments etc

Speech is produced-complicated interaction ofthe lungs, vocal cords and passages in thethroat

The number of vibrations which occur in onesecond is called the frequency of the sound andis given the name Hertz


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Normal speech contains frequencies rangingfrom 20 Hz to 20,000 Hz, which is also therange of human hearing.

When the vibrations occur at frequencies less

than 20 Hz they cannot be heard and arecalled infrasonic

while ones above 20,000 Hz, which are alsoinaudible, are referred to as ultrasonic.


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Audible ranges of frequencies /intensity


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Acoustics involves three aspects

Production of the sound or vibration

Transmission of the sound through somemedium

Reception or detection of the sound


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Different levels of sound and theirsubjective effects

65 db Annoyance (psychological) 90 db Permanent hear loss(long

duration) 100 db Aural acuity (short duration)and

damage to Auditory Organs (long

duration) 120 db Pain 150 db instant loss of hearing


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Ears SensitivityAverage persons ears capacity - 20 20,000

Hz

(decreasing with age and subjective factors)

Sound Emitted- rate of energy flow(power) in watts

Lowest intensityThreshold of Audibility(10^-12w/sq.m)

Highest intensityThreshold of Pain(1w/sq.m)


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Decibel scale

Ear - Self defense mechanism Ears sensitivity decreases high intensity

sound Proportionate logarithm of intensity

N(db) = 20 Log I (measured intensity)

Io(threshold of audibility 10 w/sq.m)


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Behaviour of sound Enclosedspace

Reflected (r)

Absorbed(a)

Transmitted(t)If source I = 1,

r + a + t = 1


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Absorption Co-efficient

Absorption Co-efficient = a + t

(all thats not reflected : 1-r )

Absorption (A) = a x s

a = absorption co-efficient

s = Area of given surface


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Total Sound components: Direct component

Reverberant component

Sound types: Airborne

Structure borne


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Reverberation Time Reverberation time is defined as the time for

the sound to die away to a level 60 decibelsbelow its original level

Reverberation Time = RT 60 = time to drop60db

below originallevel

Sabines Formula:

RT=(0.16)V/Se in metres=(0.049)V/Se infeet's

V = Volume , Se = effective absorbingarea


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Reverberation for different

uses


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Acoustical Requirements :

Floors : Carpets,wooden,elevated floors

Ceiling : Wooden, Acoustical tiles(reflective)

Side Walls : Wooden (Reflective ),acousticalboards

Rear walls :Perforated acousticalboards(Absorptive ), soft boards with

upholstery


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Effects of Geometry andshapeSound is more pleasing if it is evenly dispersed, with no

prominent echoes, no significant "dead spots" or "livespots" in the auditorium. This even dispersion isusually achieved by avoiding any focusing surfacesand avoiding large flat areas which reflect sound intothe listing area. Sometimes it is desirable to add some

anti-focusing surfaces


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Focusing Surfaces


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Acoustical Properties ofBuilding Materials

Wood Reflective Gypsum Reflective , Perforated Gypsum Reflective cum

Absorption Concrete Reflective Glass Reflective Metal Reflective

Perforated gypsum board Absorptive Acoustical Tile, Glass wool Absorptive Fabric (carpet, seat upholstery, draperies)-

Absorptive


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Shilpa kala Vedika, Madhapur GREAT EMPHASIS IS LAID

ON ACOUSTICS TO CREATEA FOOLPROOF SOUNDSYSTEM FOR THEAUDITORIUM.

THE PERFORATEDALUMINUM PANELS ONTHE SIDES ARE IMPORTEDFROM ITALY.

ACOUSTICAL MATERIALGLASS WOOL BEING USEDIN THE CHECKERED

FALSE CEILING OFENTRANCE LOBBY


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Characteristics of sound

Frequency-no.of vibrations per second,measured in cycles per sec or Hertz

Intensity or Loudness-measurement of quantityof sound energy, measured as below(intensityphysical quantity, loudness depends on humanear)

Measurement of sound- very sophisticated someasured as logarithm of intensity of sound i.edecibels.


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Outdoor Noise Levels

Air Traffic 100-110 db

Rail traffic 90-110 db Heavy road traffic 80-90 db Medium road traffic 70 80 db

Light road traffic 60 70 db


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Acceptable indoor Noise levels

Radio and T.V studios 25- 30db Music room 30- 35 db

Hospitals 35-50db Apartments, hotels, homes35-40db Conference rooms, small offices, libraries35-

40db Class rooms40-45 db Public offices, banks45-50db

Restaurants 50-55db


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Optimum reverberation time and audience factorsfor acoustical design

Type of building Reverberationtime

Audience factor

Cinema theatres 1.3-1.5s 2/3

Churches 1.8-3s 2/3

Conference rooms 1-1.5s 1/3

Music concert Hall 1.6-2s Full

Assembly hall 1-1.5s Quorum(probable

size)Lecture hall 1.5-2s 1/3

Large halls 2-3s full


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General principles in acoustical design

Site selection and planningVolume, i.e size and height

Shape Treatment of interior surfaces Reverberation time

Seat, Seating arrangements, audience Sound absorption


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Auditorium AcousticsAcoustic separation is necessary at the entrance of theauditorium and also between projection room and

auditoria. At entrances, this is achieved with lobbies andsound reducing door sets.

Sound Propagation in an Auditorium

Sound waves travel at about 345 meters/second, so thatthe sound coming directly from a source within anauditorium will generally reach a listener after a time ofanywhere from 0.01 to 0.2 seconds.


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Shortly after the arrival of the direct sound, aseries of semi-distinct reflections from various

reflecting surfaces (walls and ceiling) will reach

the listener. These early reflections typically will

occur within about 50 - 80 milliseconds.

The reflections which reach the listener afterthe early reflections are typically of lower

amplitude and very closely spaced in time.

These reflections merge into what is called thereverberant sound or late reflections.


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If the source emits a continuous sound, thereverberant sound builds up until it reaches an

equilibrium level. When the sound stops, the

sound level decreases at a more or less constant

rate until it reaches inaudibility.

For impulsive sounds, the reverberant sound

begins to decay immediately.


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Direct sound and earlyReflections Early reflections are the single

most overlooked opportunityfor developing good soundingspeech in todays auditoriumsand sanctuaries, live theaters,lecture halls and evenclassrooms.

These most desirablereflections can be mechanicallyinduced by appropriatepositioning and shaping ofsound reflecting surfaces. Theycan be electronically emulatedusing a time delayed

distributed sound system.


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Direct , Early Reflections , late

reflections


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Good sounding auditoriums dont misuse theirsound systems to create overly loud sounds.

They use sound systems to generate acomfortable loudness for the direct sound andthen compliment this with a bevy of earlyreflections, immediately followed by a distinct

absence of late reflections and finally backfilledwith a groundswell of distant soundingreverberation.

Early reflections cant be distinguished from thedirect sound and thats why they are the onlyreflections that actually add to clarity of speech.


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Echoes

Echoes can be fun but they also ruin theunderstanding of speech Early reflections are those that bounce off nearby objects.

But when the object is located some distance away, thesituation changes, you can hear the reflection off of it and we

call this acoustic event an echo. An echo can be great fun at times but when its time to pay

attention to someone talking, it also makes listening verydifficult. The echo is a good example of a late reflection

because it is a reflection that can be distinguished as beingseparate from the direct signal.

A good, clear and bright sounding auditorium providesample opportunity for many early reflections to reach eachseat in the hall.


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Types of noises/ methods tocontrol

External noises Distance Screening

Planning Positioning of opening Noise insulating building

envelope

Internal noises Reduction at source Enclosing and isolating source by

absorbent screens

Planning :separating noisy spacesfrom quiet ones Placing of noise equipment

massive part of the building. Covering surfaces resilient

materials Noise in space- by absorbent

surfaces Airborne-airtight and insulating

construction, Structure bornediscontinuity.


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Kala Akademi, panaji,Goa.Ar.Charlea Correa

Visual construction of Acoustically

transparent materials- Auditorium

Interior

building acoustics 2

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