Upca Acoustics Jan 25,2007

8/14/2019 Upca Acoustics Jan 25,2007

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Acoustics is a sciencewhich deals with theproduction, control,

transmission, receptionand effects of sound inan enclosed space.

ARCHITECTURAL ACOUSTICS

1.0

Introduction

2.0 ThePhysics ofSound

3.0 SoundMeasurement

and Hearing

4.0 SoundReflection,Diffractionand Diffusion

5.0 SoundAbsorption

6.0 Behaviorof Sound inan EnclosedSpace

1.0 Introduction

1.1 Definition of Acoustics


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This involves the following

activities:

a. a study of the shape of

the room to control

echoes and to secure thebest distribution of sound;

c. a study of the shape,

design, and location of

and an estimation of the

amount of reflective

materials in the rooms

enclosure to project

sound to the audience;

e. a study of the amount

and location of absorptive

materials in a rooms

enclosure to cause sound

to die out in the optimumreverberation time.

RT = 0.16 V /A

Where V is room volume in

cubic meters

A is the total absorption in theroom in METRIC SABINS

RT is reverberation time in

seconds

a. the computation of the

rooms reverberation

time (RT)


1.0

Introduction



and Hearing


5.0 SoundAbsorption


1.2.1 Control of Sound in Room

1.2 Acoustical Concerns in Architecture


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This involves the following

activities:

a. the control of air-borne noise

through the insulation of

sound or the shutting-out ofunwanted sounds from the

outside. This requires a study

of the sound insulating values

of walls, partitions, doors and

windows and a study of the

ventilating systems to provide

a basis for the reduction of

the transfer of unwantedsound from one room to

another;

c. the control of structure-borne

noises through the isolation of

machines from the rooms or

the buildings structure.


1.0Introduction



and Hearing


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1.2.2 Noise Control

1.2 Acoustical Concerns in Architecture


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a) Prolonged Reverberation long

reverberation time (RT) due to

large amounts of highly reflective

surfaces and/or to large volume of

space which will take considerable

time for reflected sound to die out.

Effect of prolonged reverberation is

blurringwhich is harmful to both

speech and music.

Reverberation time is influenced

by:

Volume of the room Sound absorbing qualities of

the rooms surfaces Number of people and

furniture in the room

b) Echo distinct reflection of

original sound which results

when the path of reflected

sound is 20 m (65 ft) or more

than the path of direct sound. If

the difference is less than 20

m, the reflected sound will

reinforce the direct sound

which is desirable.

It is recommended that the

surfaces of the front part of an

auditorium must be highly

reflective to reinforce directsound and throw it to the rear

of the room. On the other hand,

the rear must be highly

absorptive so the delayed

direct sound will the absorbed

and not be reflected to the

front.


1.0Introduction



and Hearing


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1.3 Principal Acoustical Defects of Rooms


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c) Resonance is the reinforcement

of certain sound frequencies due

to sympathetic vibrations. This is

especially the case in enclosed

rooms with highly reflective

surfaces. The effect would be to

emphasize certain frequencies at

the expense of others, which isundesirable for balance desired in

rooms intended for music.

e) Undue Focusing of Sound is

caused by concave surfaces

which causes sound to

converge at certain points with

resulting loss of energy in other

parts of the room.

d) Flutter Echo a rapid but repetitive

succession of sounds caused by

highly reflective parallel surfaces

(wall to wall, or ceiling to floor).


1.0Introduction



and Hearing


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1.3 Principal Acoustical Defects of Rooms


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Sound is the human earsresponse to pressurefluctuations in the air causedby vibrating objects. Forexample, a tap on the wallproduces sound because thetap makes a wall vibrate. The

vibrating wall producespressure fluctuations in theair.


1.0Introduction



and Hearing


5.0 SoundAbsorption


2.0 The Physics of Sound

2.1 Sound and Wave Motion

Sound travels in space by a

phenomenon called wave motion.

Wave motion in air is similar to the

motion of a ripple produced by

dropping a pebble into a water pond.

2.2 Types of Sound

1. Speech

2. Music

3. Noise


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1.0Introduction



and Hearing


5.0 SoundAbsorption


2.3 Sound Frequency

the number of sound

ripples generated in unit

time.

2.3.1 Frequency (f)

The number of cycles that the air particles move back and

forth in one second in a sound wave is called the frequency

of the wave. Its unit is cycles per second (c/s) which is alsotermed Hertz (Hz) after the Austrian physicist Heinrich

Hertz (1857-94).


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Eight frequency bands, or octaves, are considered in room acoustics

with the following center frequencies: 63 Hz, 125 Hz, 250 Hz, 500 Hz,

1 kHz, 2 kHz, 4 kHz and 8 kHz.


1.0Introduction



and Hearing


5.0 SoundAbsorption


2.3 Sound Frequency

2.3.2 Frequency - Octaves


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1.0Introduction



and Hearing


5.0 SoundAbsorption


2.3 Sound Frequency

2.3.3 Frequency Range for Speech and Music


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1.0Introduction



and Hearing


5.0 SoundAbsorption


2.3 Sound Frequency

2.3.4 Pitch

PITCH is the frequency of sound wave perceived by thehuman ear. A high-pitched sound means that it has a highfrequency. The female voice is slightly higher pitched thanthe male voice.


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1.0Introduction



and Hearing


5.0 SoundAbsorption


2.4 Sound Velocity and Wavelength

2.4.1 Speed (c)

The speed of sound in air has been measured as 344 m/sec(1,130 ft/sec). This corresponds to 1,240 km/hr (770mi/hr) which is extremely small as compared to the speedof light (300,000 km/sec).

The speed of sound in air does not vary with the frequencyof sound or its loudness. Sounds at all audible frequencies,regardless of their loudness, travel at the same speed.In solids, the speed of sound (that is, the speed of travel ofvibrational energy) is considerably greater than in gases orin liquids.


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1.0Introduction



and Hearing


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2.4.2 Wavelength ()

The wavelength and the frequency of sound are related toeach other as shown in the equation below.

The greater the frequency of sound, the smaller its

wavelength. Thus, the wavelength of sound at 20 Hz is344/20 = 17.2 m (56.5 ft). At 20 kHz, the wavelength is1.72 cm (0.7 in).

c = f

c = speed in meters per time

f = frequency in cycles per time

= meters


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The wavelength of sound corresponding to the center frequencies areas shown below:

0.040.158,000

0.090.34,000

0.170.62,000

0.341.11,000

0.692.3500

1.384.5250

2.759.0125

5.4618.063

(m)(ft)

WAVELENGTHFREQUENCY


1.0Introduction



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2.4.2 Wavelength ()


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The physical quantity associated with the loudness of sound is itsintensity. It is defined as the amount of sound power falling on (orpassing through, or crossing) a unit area. Since the unit of power iswatt, the unit of sound intensity is watt per square meter (W/m2).

The sound intensity which is just audible, called the threshold ofaudibility, has been determined to be 10-12 W/m2 , and the intensitythat corresponds to the sensation of pain in the human ear isapproximately 10 W/m2. Thus, the ear responds to a very largerange of intensities since the loudest sound is 10,000,000,000,000times (1013 times) louder than the faintest sound.


1.0Introduction



and Hearing


5.0 SoundAbsorption


2.5 Sound Intensity and Loudness

2.5.1 Intensity (or Sound Pressure when measured by a microphone)



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LOUDNESS is ameasure of theintensity of soundand is expressed indecibels. It is a

quantity called thesound intensitylevel(IL) or, whenmeasured, thesound pressurelevel (SPL).

Table 2.5.2, shows

some of the typicalnoises in ourenvironment andtheir soundintensities andsound intensitylevels.


1.0Introduction



and Hearing


5.0 SoundAbsorption



2.5.2 Loudness and the Decibel Scale



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The dB scale also corresponds more directly to the ears perceptionof loudness. For instance, a change of 1 dB in sound intensity levelis hardly perceived by the human ear. That is why a soundintensity level is expressed in a whole number since expressing itas a decimal number indicates an unnecessary perception.

Table 2.5.3 below lists the ears perception of change in soundintensity levels.


1.0Introduction



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2.5.3 Ears Perception of Loudness

Substantial change10

Clearly noticeable5

Just perceptible3

Imperceptible1

Human PerceptionChange in Level

(dB)



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Sound levels from different noise sources cannot be addedarithmetically but must be added logarithmically. For instance, theresultant sound level of two noise sources, each producing a soundlevel of 80 dB, is not 160 dB. The combined sound level of thesetwo sources is 83 dB.

ADDING SOUND LEVELSAn approximate procedure is more commonly used as follows:Step 1: Determine the difference between the two sound levels tobe added.Step 2: Determine the amount to be added to the higher levelfrom Table 2.5.4A below. This gives the resultant sound level.


1.0Introduction



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2.5.4 Combining Sound Levels

010 or more

15 to 9

22 to 4

30 or 1

Decibels to be

added to higherlevel

Difference between

two levels to be added(dB)



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For example, let us determine the resultant sound level of 80 dBand 84 dB. The difference between the two levels is 4 dB.Therefore, from the Table, we will add 2 dB to the 84 dB level togive us the resultant sound level of 86 dB. Thus, 80 dB + 84 dB =86 dB. If the two sound levels are equal (a difference of zero), wewill add 3 dB to the sound level to obtain the resultant sound level.

Thus, 80 dB + 80 dB = 83 dB.

From the Table, we observe that if two sound levels differ by 10 dBor more, we add nothing to the higher level to obtain the resultantlevel. In this case, the louder sound determines the overall soundlevel entirely. Thus, 80 dB + 90 dB = 90 dB.


1.0Introduction



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5.0 Sound

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The procedure can also be used to add a number of sound levels bysuccessively adding two levels, as shown in the example asfollows:


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The sum of a number of sound pressure levels

may be obtained by adding two levels at a time.

70 dB + 72 dB + 75 dB + 80 dB

74

78

82 dB THIS IS THE TOTALSOUND LEVEL



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However, if there are a number of sources of identical sound levels,their addition can be simplified through the use of the followingTable 2.5.4B.


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20100

1750

1320

1215

1010

98

87

86

75

64

53

32

Decibels to be added to higher

level

Difference between two levels to be

added (dB)



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SUBTRACTING SOUND LEVELSThe subtraction of sound levels can also be simplified through theuse of Table 2.5.4C below. For example, assume that the overallsound level in a space is 85 dB and we wish to eliminate a sourcewhose level is 80 db. What will be the resultant sound level in thespace? In other words, what is 85 dB 80 dB? From the table, 85

2 = 83 dB.


1.0Introduction



and Hearing


5.0 Sound

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010 or more

16 to 9

24 or 5

33

42

71

10 or more0

Decibels to be added to

higher level

Difference between two levels to be

added (dB)



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One of the ways in which acousticians classify noise sources isby the size of the source relative to the distance at which theeffect of the source is considered. According to thisclassification, a noise source is classified as:

apoint source, or a line source.


1.0Introduction



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2.6 Sound Attenuation by Distance



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At a distance greater than or equal to five times the largestdimension of the source, the source behaves as a point source.Thus, if the largest dimension of a sound source is 2 ft, it willbehave as a point source at a distance of 10 ft or greater from thesource.

More precisely, a point source is one which obeys theinverse square law. Assume that the acoustic power ofsource is W watts. Let us now draw an imaginary sphere ofradius R around the source with the source as the center.


1.0Introduction



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2.6.1 Point Source and Inverse Square Law




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Since the area of this sphere is 4R2 , the intensity of sound on thesurface of the sphere is given by:


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I = W / 4R2

The above expression gives sound intensity at distance R from thesource. Since quantities W and 4 are constants, we see from theexpression that sound intensity is inverselyproportional to thedistance squared. Thus, if the distance from the source, P, isdoubled, the intensity of the new point, R, is x its intensity atthe previous point. On the other hand, if the distance from thesource is halved, its intensity at this new point, Q, is 4 x the

intensity at the previous point.



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1.0Introduction



and Hearing


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Absorption




Sound intensities and sound intensity levels in a free field. The sound has been assumed to be

non-directional, that is, it radiates equally in all directions.

ANECHOIC SPACE

INTENSITY =4l

INTENSITY LEVEL =(lL+6) dB

Q

RP

2m

8m 4m

INTENSITY =0.251INTENSITY LEVEL =(lL-6) dB

INTENSITY =lINTENSITY LEVEL =(lL) dB



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1.0Introduction



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2.6.2 Line Source and Inverse Law


A Line Source is one which contains a large number of pointsources spread long a line. In practice, free flowing highwaytraffic behaves as a line source. While the sound spreads

spherically around a point source, it spreadscylindrically around a line source.

Considering the cylindrical spreading of sound, it can beshown that the sound intensity due to a line source isinversely proportional to the distance from the source, alaw called the inverse law.



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1.0Introduction



and Hearing


5.0 Sound

Absorption


2.7 Sound Fields

A space (or sound field) in which all sound comes directlyfrom the source (with complete absence of any reflectedsound) is called a free field, implying freedom fromreflections.

In practice, a free field is obtained in a room speciallyconstructed for this purpose, called the anechoic chamberwhere all walls, ceiling and the floor are covered withwedge-shaped sound absorbers.



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The ears property to integrate sounds was first discovered byHelmut Haas through experiments conducted on a large number oflisteners. A listener was set equidistant from two loudspeakers, A &B in an anechoic chamber (a chamber that is fully absorptive), sothat each loudspeaker subtended an angle of 45 deg at thelistener. A time delay mechanism was connected to loudspeaker B,so that the sound coming from B could be delayed with respect to

the sound coming from A.


1.0Introduction



and Hearing


5.0 Sound

Absorption


3.0 Sound Measurement and Hearing

3.1 The Haas Effect

Experimental set-`up for the Haas effect.

IMAGINARYSPEAKER

ANECHOICCHAMBER

DELAY MECHANISM

SOUNDGENERATOR

A

C

B



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Haas discovered the following:

When sounds coming from A and B arrived at the listeners earat the same time and were of equal loudness, the listenerperceived them as one sound coming from an imaginaryloudspeaker C located right in front of him. In other words,the ear integrated both sounds into one sound, and had theillusion of receiving the sound from a source equidistant

from the two sources. This is called the integration effect.

The integration effect occurs even if the sound from B isdelayed, provided that the delay is less than 40milliseconds, and the level of the sound from B is not morethan 10 dB above that from A.

Stated differently, if the delay between two sounds is up to 40milliseconds and if the delayed sound is no more than 10dB above the level of the earlier sound, the ear does twothings:


1.0Introduction



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3.1 The Haas Effect

(a) It perceives both sounds as one sound, addingtheir loudness, and

(b) It thinks that all the sound is coming from A the loudspeaker from which the sound came to the

listener first. This is called theprecedence effect.



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In an auditorium, the sound reaches a listener in two ways: firstthe direct sound coming from the speaker and subsequently, thereflected sounds coming from the surfaces of the room. Since thereflected sounds travel a longer path, they are delayed withrespect to the direct sound. The difference between the arrivaltimes of direct and reflected sounds is the delay time.

In a typical auditorium, a listener receives reflected soundsfrom various surfaces with different delay times and levels.A reflected sound is usually lower in loudness than its directsound. However, the sounds reflected from some curvedsurfaces (such as domes and vaults) can focus on a listener,increasing the level above that of the direct sound.


1.0Introduction



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3.2 Practical Significance of the Haas Effect



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Since domes and vaults are generally avoided in auditoriums, thereflected sound is usually of a lower level than the direct sound.Consequently, in the design of speech auditoriums, we generallyrestrict the initial time delay between reflected sound and directsound to less than 50 milliseconds, which is conservative,considering that in Haas experiment this time delay is for soundshaving the same level.


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Some of the various reflected sound paths, and

the direct sound path, to a listener in an auditorium.



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A delay of 50 milliseconds corresponds to a pathdifference of nearly 17 m (55 ft) between the directand reflected sounds. (344m/s x .050 seconds = 17.2m) Thus, in the design of speech auditoriums, we

require that the path length difference between areflected sound and direct sound at the listener shouldnot exceed 17 m. In practice, however, a round figureof 20 m (65 ft) is used.

A longer delay time is acceptable in halls meant formusic. Typically, a delay time not exceeding 80

milliseconds is the criterion for music spaces.


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Masking is a complex phenomena since it has bothneurological as well as sensory bases. That is, masking is notsimply the property of the ear but also of the brain. Forexample, we are often able to hear distant conversations ofparticular interest to us (or about us) in a noisy cocktail party.If these conversations were not of interest to us, we mightnormally not hear them.


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3.3 Sound Masking



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Studies on masking sounds have concluded the following:

A sound of a given frequency is more easily masked by asound of the same frequency. This means that the furtheraway the masking sound is in frequency from the frequency ofthe sound to be masked, the greater the sound level ofmasking sound required. For example, to fully mask a 65 dB,

400 Hz tone with another 400 Hz tone requires a level of 80dB. On the other hand, to completely mask a 65 dB, 1,000 Hztone by a 400 Hz tone, a level far in excess of 80 dB isrequired.

Low frequencies are generally more effective in maskinghigher frequencies than vice versa, particularly if they are

loud. Excessive low frequency noises must, therefore, beavoided since they constitute a serious source of interferencefor both speech and music.


1.0Introduction



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3.3 Sound Masking



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Because we have two ears, human hearing is binaural.Binaural hearing helps us locate a sound source in space,referred to as sound localization.

Studies indicate that the ears ability to perceive direction ofsound is due to:

(5) different arrival times of sound at the two ears (due to pathlength and acoustical shadow) and

(7) different sound levels (also due to path length differential).The ear has the ability of sound localization only in thehorizontal plane. In fact, the ear is able to localize the soundsource in the horizontal plane with an accuracy of 1 or 2degrees.


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3.4 Binaural Hearing



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If the sound source is located in the vertical plane --- thevertical plane passing through the center of the head andmidway between the ears --- there is no difference betweenthe arrival times of sound to the two ears. Consequently, theear cannot discriminate between the direction of the sounds inthe vertical plane.

This characteristic is used to advantage in establishing thelocation of loudspeakers in an auditorium for soundamplification. The loudspeakers, formed in clusters, arelocated in the center of the proscenium. This locates theactual talker, loudspeaker and listener in a vertical plane.Therefore, the ears are unable to distinguish between thedirections of sound coming from the talker and theloudspeaker.


1.0Introduction



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3.4 Binaural Hearing



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Boundary elements of an enclosure have a profound influence onthe behavior of sound in an enclosure. When sound energy falls onthe boundary of an enclosure, such as a wall or a ceiling, part ofthe energy is reflected back into the enclosure, a part is absorbedwithin the material of the boundary and converted into heat, and apart is transmitted through the boundary element.

The reflected sound expressed as a fraction of the total soundenergy falling on a boundary element is called the reflectioncoefficientof the element, denoted by rho (). Thus:

= reflected sound energy incident sound energy


1.0Introduction



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4.0 Sound Reflection, Diffraction and Diffusion

4.1 The Boundary Phenomenon



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The fraction that is transmitted is called the transmissioncoefficient, and is denoted by tau () and the fraction absorbed isalpha (), called the absorption coefficient. Since the sum of thereflected, absorbed and transmitted amounts of energy must beequal to the incident energy, the following relationship must holdtrue:

+ + = 1.0

An open window, though not absorbing any sound, is considered aperfect acoustical absorber because all the sound falling on thewindow is transmitted outdoors. Thus, for an open window, =1.0.


1.0Introduction



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4.1 The Boundary Phenomenon



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Diffraction is the ability of sound to bend around an obstacle, sothat unlike light which travels in a straight line path, sound bendsand creates an acoustical shadow smaller than the optical shadow

of light.


1.0Introduction



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4.2 Sound Diffraction

Acoustical and optical shadows produced by asource.

SOURCE

ACOUSTICAL

SHADOWZ

ONE

OP

TICALSHADOWZ

ONE



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The degree of bending of sound around an obstacle is a function ofthe sounds wavelength or frequency. Low frequency (longwavelength) sounds bend by a greater amount than high frequency(short wavelength) sounds. Thus, the region of acoustical shadowbehind an obstacle is larger for a high frequency sound than for alow frequency sound.


1.0Introduction



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Diffraction of High Frequency Sound Diffraction of Low Frequency Sound

HIGHFREQUENCY

SOUNDSOURCE

LOWFREQUENCY

SOUNDSOURCE

OBSTACLE

ACOUSTICAL

SHADOW

OBSTACLE

ACOUSTICAL

SHADOW



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The diffraction effect is a function of the dimensions of the obstaclein relation to the wavelength of sound. Research indicate that for aplane (rectangular panel) to reflect most of the sound falling on it,both its dimensions must be at least 5 . Thus, the size of thepanel must be at least 3m x 3m (10 ft x 10 ft), if it is to be used asa reflector for a 500 Hz sound, since for a 500 Hz sound isapproximately 0.6m (2 ft). When the panel size is equal to inboth directions, most of the sound will bend around the panel withvery little sound reflected from it.


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Therefore, for a panel to function as an effective reflector, it isnecessary that both its dimensions be at least 5 . Sound reflectingpanels are commonly provided in a speech auditorium to throwreflected sounds toward the audience. Their size and stiffness aretwo important factors that determine their effectiveness asreflectors.


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As far as possible, both dimensions as a reflecting panel

should be at least five times the wavelength of sound to

be reflected.


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Acoustical shadow is useful in the design of barriers to protectbuildings and neighborhoods from traffic noise. Since lowfrequency sounds diffract substantially over the edges of anobstacle, a traffic noise barrier must be high enough so that thebarrier casts an acoustical shadow over critical areas of thebuildings to be protected. A traffic noise barrier must also belong and extend sufficiently beyond the end of theneighborhood, so that the buildings to be protected fall withinthe acoustical shadow zone.

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4.3 Acoustical Shadows

Sound diffraction by a

traffic noise barrier



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4.4.1 Passage of Sound Through Openings

4.4 Acoustical Transparency

Diffraction effect also occurs when sound travels through anopening. This is due to the bending of sound at the openingsedges.The amount of sound passing through an opening consists of twoparts:

- that contained within the optical zone, and

- that contained within the peripheraldiffracted zone. The diffracted zone is afunction of frequency, increasing as thefrequency decreases

Apart from the frequency, the size of the opening is also a

determinant of acoustical transparency.



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4.4.1 Passage of Sound Through Openings



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4.4.2 Acoustical Transparency of A Screen


The acoustical transparency of a screen is not merely a function ofits visual transparency and sound frequency, but also a function ofthe distribution of voids in the screen.

For a given visual transparency, small closely spaced voids provide

greater acoustical frequency than large voids spaced further apart.



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Common used screens:

Several manufacturers make fabric covered sound absorbingpanels. Typically, these consist of rigid fiberglass boards held inwood or metal frame and wrapped with perforated fire resistantfabrics.

Perforated plywood, hardwood or metal panels are also used ascovering materials.

Metal panels are particularly effective in dusty environments, sincethe panels can be taken down, washed and put back in place.



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1.0Introduction


3.0 SoundMeasurementand Hearing


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4.5 Diffuse and Specular Reflections

To be a good sound reflector, a building element must besufficiently large in relation to the wavelength of sound and alsosufficiently stiff of heavy weight construction.

Sound reflection from a large, heavy and a nonporous surface canbe either:

Specular reflectionDiffuse reflection



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4.5.1 Specular Reflection


Specular reflection is a mirror type reflection, similar to thereflection of light from a mirror. In specular reflection, the incidentsound beam is reflected off the reflecting surface as per Snells law.According to this law, the reflected beam makes the same anglewith (the normal to) the reflecting surface as the incident beam.In other words, the angle of incidence (i) is equal to the angle ofreflection (r).



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4.5.2 Diffuse Reflection


In diffuse reflection, the incident sound is reflected equally in alldirections (uniform scattering). Diffuse sound reflection is similarto the reflection of light by a matt surface or frosted glass.



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4.6 Sound Diffusion

If room boundaries consist of sufficiently large surfaceirregularities, the sound field in such room will be diffused. Aperfectly diffuse sound field is defined as one in which soundarrives at the listener from all possible directions in equal strength.

Sound diffusion is one of the important acoustical requirements for

rooms used for musical performances. A room with a few largespecularly reflecting surfaces, and which does not contain adequatesurface irregularities to diffuse sound, produces harsh reflectionsknown as acoustic glare an undesirable effect for music. On theother hand, with adequate diffusion in the room, the listenerreceives sound from various directions and has the feeling of beingenveloped by music a desirable sensation for music.



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4.6 Sound Diffusion



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4.6.1 Effect of Room Geometry and Size on Sound Diffusion

4.6 Sound Diffusion

Sound diffusion is a function of room geometry.

Rectangular rooms with flat parallel walls have poordiffusion.

Even a slight splay (1 in 20) in one of the walls in an

otherwise rectangular room improves diffusion. The more the room deviates from rectangularity, orthe more irregular the room shape, the greater sounddiffusion in the room.



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4.6 Sound Diffusion



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4.6 Sound Diffusion

Size of the room is another factor that affects diffusion.

Diffusion is more easily obtained in a large room thanin a small room

Because of its small size, it is difficult to achieve

diffusion in a music recording studio or a control roomunless special sound diffusers are used on roomsurfaces.



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4.6.2 Effect of Sound Absorption on Sound Diffusion

4.6 Sound Diffusion

Reflective room surfaces increase diffusion in the room. The morereflective the surfaces, the greater the diffusion. Conversely, theprovision of sound absorption decreases diffusion.

Although sound absorption reduces diffusion, the alternateapplication of sound absorbing patches improves diffusion. Thesize of the patches must be of the order of the wavelength ofsound. Therefore, to produce diffusion over a wide band offrequencies, patches must be of various sizes. Note, however, thatalternate application of absorbing patches to obtain diffusionshould be used only in spaces where sound absorption is otherwiserequired.



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4.6.3 Interior Ornamentation

4.6 Sound Diffusion

Pilasters, piers, balconies, exposed beams, coffered ceilings, andany other surface ornamentation that scatters sound increasediffusion.

Sufficient diffusion, provided by extensive ornamentation andprotruding balconies is considered to be one of the reasons for thegood acoustics of some symphony halls.



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4.6.4 Diffusion and Convex Reflectors

4.6 Sound Diffusion

Convex reflective surfaces also increase diffusion. They do so byscattering sound. A concave surface, on the other hand, tends tofocus sound. Focusing is the opposite of diffusion since focusingtends to concentrate sound into one direction and location, starvingother locations of adequate sound. Thus, a dome or similarconcave surface provides poor acoustics for an auditorium.



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4.7 Sound Diffusers

A sound diffuser is a surface element that produces diffusereflection.

Any reflective surface with irregularities of size comparable to thewavelength of sound will work as a diffuser. The greater therandomness in surface irregularities and sizes, the better thediffuser.



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Absorption


4.7.1 Quadratic Residue Diffuser

4.7 Sound Diffusers

The diffusers made from surface modulations have two majorlimitations:

The surface protrusions and recesses have tobe large to provide good diffusion at lowfrequencies.

There is no objective method of determiningthe extent of scattering produced by suchdiffusers.

A diffuser that overcomes the above limitations is called aquadratic residue diffuser. A quadratic residue diffuser consists ofan array of linear slits (or wells) of constant width.



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Absorption


4.8 Source-Image Relationship in Specular Reflection



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4.8.1 Higher-order Images

4.8 Source-Image Relationship in Specular Reflection

If there is a set of two reflectors in a space, an image produced byone reflector works as the source for the other reflector, producingan image-of-an-image.



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4.9 Flutter Echo

If there are two parallel reflectors, we will obtain an infinitenumber of images of the source since each image works as asource for the other reflector. This may be confirmed by standingbetween two parallel mirrors; an infinite number of images of theself will be seen.

The above fact implies that if a sound source is located betweentwo parallel reflecting walls, a listener will receive reflected soundfrom an infinite number of images.



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4.9 Flutter Echo

Since the speed of sound is 344 m per second, the time gapbetween each successive reflected sound will be 87 milliseconds.This, according to the Haas effect, will produce echoes. Sincethese echoes recur after a regular interval of 87 milliseconds, theyproduce a flutter effect flutter echo.

Flutter echo is an acoustical defect and must be avoided inauditoriums and other assembly spaces. It affects speechintelligibility and produces tonal coloration of music.

Therefore, two parallel reflective walls should be avoided in anauditorium. Splaying one or both walls of the room by a little as 5degrees will usually eliminate the flutter effect. Also, treating one

of the parallel walls with sound diffusers or sound absorbingmaterials will eliminate flutter.



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The standard method of rating the effectiveness of asound absorbing material is by its absorptioncoefficient. The absorption coefficient of a materialvaries with the angle of incidence of sound theangle at which the sound strikes the surface of thematerial. However, in most rooms, the sound

strikes its surfaces from all angles with almost equalprobability. Therefore, we are usually interested inthe random incidence absorption coefficient.

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5.0 Sound

Absorption


5.0 Sound Absorption

5.1 Rating of Sound Absorbing Materials


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a.Porous absorbers

Almost any material whose surface is porous may beconsidered a porous absorber. The porosity of the materialmay be either due to the fibrous composition, or due tovoids between granules or particles of the material.

Fiberglass and rigid fiberboards are common porousabsorbers.

A sound wave falling on a porous absorber causes the airin the voids of the material to vibrate back and forth. Asthe air vibrates in the voids, the vibrational energy of theair is converted into heat due to friction between air

particles and void walls.

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5.0 Sound

Absorption



5.2 Types of Sound Absorbing Materials



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a.Porous absorbers

Porous absorbers are commonly used in low-height officepartitions, and as wall- and ceiling- mounted panels.

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a.Panel or membrane absorbers

A solid unperforated panel installed against a hardsubstrate with an intervening air space acts as a panel ormembrane absorber. When a sound wave falls on such apanel, it sets the panel into vibration. Since the panel is

never fully elastic, it loses some energy by damping.Damping is a measure of the resistance of a vibratorysystem to sustain vibrations.

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a.Panel or membrane absorbers

Examples of these absorbers are interior drywall, windows,wood panels and flooring, suspended reflectors and others.However, the panel absorber is not a sound absorbingmaterial in the same sense as a porous absorber. It is

seldom added to building interiors to control noise.

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a.Volume absorbers

Volume absorbers are also known as cavity absorbers,cavity resonators and Helmhotz resonator. This absorberconsists of a volume of air connected to the generalatmosphere through a small volume of air called the neck.

A volume absorber is similar to an open bottle where thevolume of air in the bottle is connected to the outsideatmosphere through the air in the bottles neck.

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a.Volume absorbers

The most common application of a volume absorber is theuse of acoustical blocks for noise control in manufacturingplants, school gymnasiums, a/c rooms, auditoriums andthe like.

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Absorption






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The primary reason for the difference between thesound produced inside an enclosed space and thatproduced outdoors is that the sound produced insidea room bounces back and forth from room surfaces,while a sound produced outdoors travels freely awayfrom the source.

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Absorption


6.0 Behavior of Sound in an Enclosed Space



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A sound impulse is a short burst of sound such as thatgenerated by pricking a balloon or by a hand clap in alarge room. These sounds do not die instantaneously,instead, persists for a while, then, decreases in level overtime.

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6.1 Impulse Response of a Room



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a)The Reverberation Phenomenon

The persistence of sound in a room after it is turned off isrelated to the amount of absorption in the room. In fact,as soon as a sound is produced, it travels in space invarious directions and hits room surfaces, from which it isreflected and rereflected. At each reflection, some energy

is lost by absorption, and eventually all the sound isdepleted.

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a)Directional Distribution of Reflections

The directional distribution of sound is particularlyimportant in halls meant for music. Sound coming frommany different directions creates a sense of volume orenvelopment in the room an important requirement forthe appreciation of music.

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a)Impulse Diagram

In an impulse diagram, the vertical axis represents thesound level and the horizontal axis represents the times ofarrival of impulses. Each impulse is or a lower level thanthe one preceding it, showing a gradual decrease in soundlevel over time. This progressive decrease is a

consequence of two factors:

Increasingly higher-order images are weaker inpower

They are farther away from the listener

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Absorption




1.5 GLOSSARY OF IMPORTANT ACOUSTICAL TERMS


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absorption coefficient the fraction of the incident sound

energy absorbed by a surface.

anechoic chamber a sealed room in which all the surfaces

are designed to completely absorb all sound produced in the

room.

attenuation a reduction in sound level. Sound attenuation in

air-conditioning is specified in terms of dB per meter.

background noise ambient noise

break-in noise transfer of noise from a space surrounding

the duct into the duct through duct walls.

break-out noise transfer of noise from the interior of a duct

through duct walls into a space outside the duct.

dead room a room containing an unusually large amount of

sound absorption..

decibel (dB) a unit of measurement for sound pressure level,

sound intensity level or sound power level.



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diffraction a change in the direction of propagation of sound

as a result of bending caused by a barrier in the path of a sound

wave.

diffuse sound (field) a sound field in which the sound comes

in equal intensity from all directions.

direct sound the sound that arrives at a receiver along a direct

line from the source without reflection from any surface.

echo a sound that has been reflected with sufficient timedelay.

environmental noise exterior background noise in a

neighborhood (ie. traffic, aircraft).

fidelity faithful reproduction of a sound source.

flutter echo a rapid but repetitive succession of sound from a

sound source usually occurring as a result of multiple

reflections in a space with hard, flat and parallel walls.

frequency the number of full cycles per second measured.



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impact noise noise caused by the collision of two objects.

infrasonic a sound that is below the human audiblefrequency, below 20 Hz.

insulation see isolation

intermittent sound a sound which is discontinuous or

fluctuates to such an extent that at times its sound pressure

level falls below a measurable level.

inverse square law a law which states that the sound

intensity in a free field varies inversely with the square of the

distance from the source.

isolation a lack of acoustical connection.

leak a small opening in a barrier that allows airborne sound

to pass through.

live room a room containing an unusually small amount of

sound absorption.



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loudness an auditory sensation that depends on sound

pressure level and the frequency of sound.

masking the increase in the threshold of audibility of a soundthat is required so that the sound can be heard in the presence

of another sound.

noise isolation class (NIC) a single number rating derived from

the measured value of noise reduction between two rooms.

noise reduction (NR) the reduction in sound pressure level of

noise.

noise reduction coefficient (NRC) a single number rating

derived from measured values of sound absorption coefficients

of a material at 250, 500, 1000 and 2000 Hz.

outdoor-indoor transmission class (OITC) a weighted single

number rating of the sound reduction effectiveness of a partition

that separates an indoor space from the outside.

pitch a listeners perception of the frequency of a pure tone.



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reflection coefficient a measure of the sound reflective

property of a surface.

resonance the relatively large amplitude of vibration

produced when the frequency of the source of sound is equal

to the natural frequency of a room.

reverberant sound field a sound field created by repeated

reflections of sound from the boundaries in an enclosed space.

reverberation the continuation of sound in an enclosed

space after the initial source has been terminated.

reverberation time (RT) the time it takes for sound intensity

to decay by 1 millionth of its steady state value after the sound

source has been terminated.

sabin a unit of measure of sound absorption.

scattering an irregular diffraction of sound in many

directions.



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sound insulation the ability of a barrier to prevent sound from

reaching a receiver.

sound intensity (SI) the average rate of sound energy flowthrough a unit area in a given direction.

sound intensity level (SIL) a quantity expressed in decibels

of airborne sound.

sound lock a small space that works as a buffer between asource room and a receiving room.

sound pressure fluctuating pressure of sound superimposed

on the static air pressure.

sound pressure level see sound intensity level

sound transmission class (STC) a single number rating of

the sound insulation rating of a partition.

structure-borne sound sound propagated through a solid

structure.



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transmission coefficient the ratio of transmitted soundenergy to incident sound energy

transmission loss (TL) is the measure of sound insulation of

a partition.

wavelength distance between two adjacent compressions or

rarefactions in a sound wave.

white noise a noise whose energy is uniform over a wide

range of frequencies. This is analogous to the term white

light, which consists of almost equal amount of light of

different wavelength (colors). A white noise sounds hissy.


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Upca Acoustics Jan 25,2007

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Transcript of Upca Acoustics Jan 25,2007