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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)
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
1.2.1 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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 SoundAbsorption
6.0 Behaviorof Sound inan EnclosedSpace
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 SoundAbsorption
6.0 Behaviorof Sound inan EnclosedSpace
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).
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 SoundAbsorption
6.0 Behaviorof Sound inan EnclosedSpace
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 SoundAbsorption
6.0 Behaviorof Sound inan EnclosedSpace
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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ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 SoundAbsorption
6.0 Behaviorof Sound inan EnclosedSpace
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 SoundAbsorption
6.0 Behaviorof Sound inan EnclosedSpace
2.3 Sound Frequency
2.3.2 Frequency - Octaves
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ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 SoundAbsorption
6.0 Behaviorof Sound inan EnclosedSpace
2.3 Sound Frequency
2.3.3 Frequency Range for Speech and Music
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ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 SoundAbsorption
6.0 Behaviorof Sound inan EnclosedSpace
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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ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 SoundAbsorption
6.0 Behaviorof Sound inan EnclosedSpace
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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ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 SoundAbsorption
6.0 Behaviorof Sound inan EnclosedSpace
2.4 Sound Velocity and Wavelength
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
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 SoundAbsorption
6.0 Behaviorof Sound inan EnclosedSpace
2.4 Sound Velocity and Wavelength
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 SoundAbsorption
6.0 Behaviorof Sound inan EnclosedSpace
2.5 Sound Intensity and Loudness
2.5.1 Intensity (or Sound Pressure when measured by a microphone)
ARCHITECTURAL ACOUSTICS
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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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 SoundAbsorption
6.0 Behaviorof Sound inan EnclosedSpace
2.5 Sound Intensity and Loudness
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 SoundAbsorption
6.0 Behaviorof Sound inan EnclosedSpace
2.5 Sound Intensity and Loudness
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 SoundAbsorption
6.0 Behaviorof Sound inan EnclosedSpace
2.5 Sound Intensity and Loudness
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
2.5 Sound Intensity and Loudness
2.5.4 Combining Sound Levels
ARCHITECTURAL ACOUSTICS
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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:
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
2.5 Sound Intensity and Loudness
2.5.4 Combining Sound Levels
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
2.5 Sound Intensity and Loudness
2.5.4 Combining Sound Levels
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
2.5 Sound Intensity and Loudness
2.5.4 Combining Sound Levels
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
2.6 Sound Attenuation by Distance
ARCHITECTURAL ACOUSTICS
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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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
2.6.1 Point Source and Inverse Square Law
2.6 Sound Attenuation by Distance
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Since the area of this sphere is 4R2 , the intensity of sound on thesurface of the sphere is given by:
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
2.6.1 Point Source and Inverse Square Law
2.6 Sound Attenuation by Distance
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.
ARCHITECTURAL ACOUSTICS
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ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
2.6.1 Point Source and Inverse Square Law
2.6 Sound Attenuation by Distance
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
ARCHITECTURAL ACOUSTICS
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ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
2.6.2 Line Source and Inverse Law
2.6 Sound Attenuation by Distance
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.
ARCHITECTURAL ACOUSTICS
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ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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.
ARCHITECTURAL ACOUSTICS
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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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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:
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
3.2 Practical Significance of the Haas Effect
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
3.2 Practical Significance of the Haas Effect
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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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
4.2 Sound Diffraction
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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
4.2 Sound Diffraction
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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.
ARCHITECTURAL ACOUSTICS
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
4.2 Sound Diffraction
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.
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
4.3 Acoustical Shadows
Sound diffraction by a
traffic noise barrier
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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
4.4.1 Passage of Sound Through Openings
4.4 Acoustical Transparency
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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
4.4.2 Acoustical Transparency of A Screen
4.4 Acoustical Transparency
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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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
4.4.2 Acoustical Transparency of A Screen
4.4 Acoustical Transparency
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
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
4.4.2 Acoustical Transparency of A Screen
4.4 Acoustical Transparency
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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurement
and Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
4.4.2 Acoustical Transparency of A Screen
4.4 Acoustical Transparency
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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
4.5.1 Specular Reflection
4.5 Diffuse and Specular Reflections
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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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
4.5.2 Diffuse Reflection
4.5 Diffuse and Specular Reflections
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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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
4.6 Sound Diffusion
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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
4.6.1 Effect of Room Geometry and Size on Sound Diffusion
4.6 Sound Diffusion
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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
4.6.1 Effect of Room Geometry and Size on Sound Diffusion
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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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
4.8 Source-Image Relationship in Specular Reflection
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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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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1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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.
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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.
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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.
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
5.0 Sound Absorption
5.2 Types of Sound Absorbing Materials
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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.
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
5.0 Sound Absorption
5.2 Types of Sound Absorbing Materials
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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.
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
5.0 Sound Absorption
5.2 Types of Sound Absorbing Materials
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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.
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
5.0 Sound Absorption
5.2 Types of Sound Absorbing Materials
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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.
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
5.0 Sound Absorption
5.2 Types of Sound Absorbing Materials
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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.
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
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.
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
6.0 Behavior of Sound in an Enclosed Space
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.
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
6.0 Behavior of Sound in an Enclosed Space
6.1 Impulse Response of a Room
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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.
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
6.0 Behavior of Sound in an Enclosed Space
6.1 Impulse Response of a Room
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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
1.0Introduction
2.0 ThePhysics ofSound
3.0 SoundMeasurementand Hearing
4.0 SoundReflection,Diffractionand Diffusion
5.0 Sound
Absorption
6.0 Behaviorof Sound inan EnclosedSpace
6.0 Behavior of Sound in an Enclosed Space
6.1 Impulse Response of a Room
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.
1.5 GLOSSARY OF IMPORTANT ACOUSTICAL TERMS
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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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THANK YOU! THE END