Introduction to Building Acoustics

8/16/2019 Introduction to Building Acoustics

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• Introduction to Building Acoustics

• Changes in the direction of sound travel• Sound Absorption and Absorption

Coefficient

• Reverberation and Sabine’s Formula • Factors affecting Acoustics of Building


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Introduction to Building Acoustics Bu i ld ing acous t i cs or archi tecturalacous t i cs deals with sound in the builtenvironment.

Structures with acoustic implications :• Airports• Churches• Theatres• Concert and opera halls• Educational structures, including class

rooms, lecture halls, libraries, musicpractice rooms etc.,


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Phenomena related to Changes in thedirection of sound travel

• Sound waves change their direction oftravel through four categories ofphenomena : reflection, refraction,diffraction and diffusion.

• These phenomena can occur when changesoccur in a sound wave’s medium of travel.

These physical principles are the same asthose that occur in the optical world withlight.


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(1) Reflection • When a sound wave encounters a sharp

discontinuity in the density of a medium,

some of its energy is reflected.• Reflective surfaces are typically smooth

and hard.

• A few common acoustic problemscaused by reflections are echoes androom resonance.


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• Echoes are caused by the limitations of ourhearing mechanisms in processing sounds.

• When the difference in arrival times between

two sounds is less than 60 milliseconds, wehear the combination of the two sounds as asingle sound.

• However, when this difference exceeds 60milliseconds, we hear the two distinct sounds.

• When these two sounds are generated fromthe same source, this effect is known as ECHO

ECHOES


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• Room reson ance occurs at spec i f ic f requencies inroo m s w here two ref lect ive wal ls are paral lel toeach o ther.

• In these cases , who le nu m ber mu l t ip les of s pec i f ichalf waveleng ths w i l l f i t between th e tw o w alls .• Since their su rfaces ref lect th e so un d, thei r mirro r

images bou nce off each wal l to se tup s tat ionary

press ure pat tern in a room .• This phenom enon i s ca lled a s ing le d im ens iona l (or

ax ial ) s tanding wave and i t i s the s imp les t form ofroom resonance .

ROOM RESONANCE


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(2) Refract io n • Just as light bends through a prism, the direction of

sound is altered when sound waves encounterchanges in medium conditions that are not extremeenough to cause reflection, but are enough to

change the speed of sound.• In addition to the speed of sound changing for

different materials or media, the speed of soundchanges with changes in temperature within the

same medium.• This variation in sound travel direction, caused by

variation in the speed of sound, is known asrefraction.


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(3) Diffract io n

• The pr in c ip le of d i ff rac t ion l im i ts theso un d redu c t ion effec t iveness o f anyop en-p lan off ice par t it ion or ou tdoo rn o ise b arrier.

• Sound w aves bend a roun d and o verthese typ es o f w al ls , ind epend ent o ftheir m ater ial , to im po se this l im it .


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(4) Diffu s io n

Wh en a so un d w ave ref lec ts o ff a

co nv ex o r un even su rface , i t s energyis s pread evenly ra ther than b eingl im ited to a disc rete ref lect io n. Thisphenom enon , kno w n as d i ffus ion .


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So u n d A b s o r p t io n

• The p roper ty o f a su r face by w hich s ou ndenergy i s con ver ted in to o ther fo rm o fenergy i s kn ow n as absorp t ion .

• In the p rocess o f absorp t ion sou nd energyis con ver ted in to h eat du e to f r ic t ion alres is tanc e ins ide the po res of th e m ater ia l.

• The f ib rou s and po rous m ater ial s absorbsou nd energy m ore , than o ther so l idmaterials .


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Sound Ab sorp t ion Coeff ic ien t

• The effec t iveness of a sur face in abso rb ing sou ndenergy i s expressed w i th the help of absorp t ioncoeff ic ient .

• The co eff ic ien t o f absorp t ion ̀ ’ of a materials isdef ined as the ra tio of so un d energy absorbed b yi t s su r face to tha t o f the to ta l soun d energyinc ident o n the su r face .

surfacetheonincident energy sound Total surfacethebyabsorbed energySound

=


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• A uni t a rea of open w indo w is se lec ted as thes tandard . Al l the soun d inc id ent on an o penw indow is fu l ly t rans m i t ted and non e i sreflected . Therefore, i t is c on sid ered as anideal abs orb er of so un d.

• Thu s the uni t of abs orpt ion i s th e op enw ind ow un i t (O.W.U.), w hich is n amed a“sabin” after the scientist who established the

unit .• A 1m 2 s ab in i s the am oun t o f sou nd absorbed

by on e sq uare m etre area of ful ly op enw i n d o w.


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• The value of ̀ ’ depends on the nature ofthe m ater ia l as wel l as th e frequ ency ofso un d. It is a co m m on prac t ice to us e thevalue of ̀ ’ at 500 Hz in acoustic designs.

• If a material has the value of “

” as 0.5, itm eans tha t 50% of the inc ident so un denergy w i l l be abs orb ed per un i t area.

• If th e m aterial has a su rface area o f S

sq .m ., then the absorp t ion pro vided bythat m ater ia l i s

a = . S


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If there are different materials in a hall, then thetotal sound absorption by the differentmaterials is given by

A = a 1 + a 2 + a 3 + ……

A = 1S 1 + 2S 2 + 3S 3 + ……

or A =

where 1, 2, 3 ………. are absorptioncoefficients of materials with areas S 1, S 2, S 3,…….

n

nn S 1


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Reverberation • Sound produced in an enclosure does not die out

immediately after the source has ceased toproduce it.

• A sound produced in a hall undergoes multiplereflections from the walls, floor and ceiling beforeit becomes inaudible.

• A person in the hall continues to receive

successive reflections of progressivelydiminishing intensity.• This prolongation of sound before it decays to a

negligible intensity is called reverberat ion .


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Reverberation Time

• The time taken by the sound in a room tofall from its average intensity to inaudibility

level is called the reverberation time of theroom.

• Reverberation time is defined as the timeduring which the sound energy densityfalls from its steady state value to its one-millionth (10 -6) value after the source isshut off.


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If initial sound level is L i and the final level is

Lf and reference intensity value is I ,thenwe can writeLi = 10 log and L f = 10 log

Li – L

f = 10 log

As = 10 -6,Li – L f = 10 log 10 6 = 60 dBThus, the reverberat ion t ime i s th e per iod of

t im e in s econ ds , which i s r equ i red fo rsou nd energy to d imin i sh by 60 dB a f terthe sou nd s ou rce i s s topped .

I I i

I

I f

f

i

I

I

i

f

I I


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Sabine’s Formula for Reverberation Time

Prof.Wallace C.Sabine (1868-1919) determined thereverberation times of empty halls and furnishedhalls of different sizes and arrived at the followingconclusions.

• The reverberation time depends on the reflectingproperties of the walls, floor and ceiling of the hall.• The reverberation time depends directly upon the

physical volume V of the hall.• The reverberation time depends on the absorption

coefficient of various surfaces such as carpets,cushions, curtains etc present in the hall.• The reverberation time depends on the frequency

of the sound wave because absorption coefficientof most of the materials increases with frequency.


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Prof. Sabine summarized his results in the form of the followingequation.Reverberation Time, T

orT =

where K is a proportionality constant.It is found to have a value of 0.161 when the dimensions are

measured in metric units. Thus,

T =

This Equation is known as Sabine’s formula for reverberationtime.

A AbsorptionV Hall theof Volume

,

,

A

V K

A

V 161.0


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It may be rewritten as

T =

or T =

N

nn S

V

1

161.0

nn S S S S V

.......161.0

332211


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Optimum Reverberation Time

• Sabine determined the time ofreverberation for halls of various sizes.

• And from the results, he deduced thereverberat ion t ime tha t i s l ikely to be m os tsa ti s fac to ry fo r the purpo se fo r wh ich ahal l is b ui l t .

• Such satisfactory value is known as theoptimum reverberation time.


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Activity in Hall Optimum ReverberationTime (Sec)

Conference hallsCinema theatre Assembly hallsPublic lecture hallsMusic concert hallsChurches

1 to 1.51.31 to 1.51.5 to 21.5 to 21.8 to 3


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Factors Affecting Acoustics of Buildings

(1) Reverberation Time

• If a hall is to be acoustically satisfactory, it is essential

that it should have the right reverberation time.• The reverberation time should be neither too long nor too

short.• A very short reverberation time makes a room `dead’ . On

the other hand, a long reverberation time renders speechunintelligible .

• The optimum value for reverberation time depends on thepurpose for which a hall is designed.


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Remedies• The reverberation time can be controlled

by the suitable choice of building materialsand furnishing materials.

• Since open windows allow the soundenergy to flow out of the hall, there shouldbe a limited number of windows. They maybe opened or closed to obtain optimumreverberation time.


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Remedies• Cardboard sheets, perforated sheets, felt,

heavy curtains, thick carpets etc are usedto increase wall and floor surface

absorption. Therefore, the walls are to beprovided with absorptive materials to therequired extent and at suitable places.

• Heavy fold curtains may be used toincrease the absorption.

• Covering the floor with carpet alsoincrease the absorption.


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Remedies• Audience also contribute to absorption of

sound. The absorption coefficient of anindividual is about 0.45 sabins.

• In order to compensate for an increase inthe reverberation time due to anunexpected decrease in audience strength,upholstered seats are to be provided in thehall.

• Absorption due to an upholstered chair isequivalent to that of an individual.


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(2) Loudness

• Sufficient loudness at every point in thehall is an important factor for satisfactory

hearing.• Excessive absorption in the hall or lack of

reflecting surfaces near the sound sourcemay lead to decrease in the loudness ofthe sound.


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Remedies• A hard reflecting surface positioned near

the sound source improve the loudness.• Low ceilings are also of help in reflecting

the sound energy towards the audience.• Adjusting the absorptive material in the

hall will improve the situation.

• When the hall is large and audience more,loud speakers are to be installed to obtainthe desired level of loudness.


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(3) Focusing

• Reflecting concave surfaces cause concentrationof reflected sound, creating a sound of largerintensity at the focal point. These spots are knownas s o u n d f o c i .

• Such concentrations of sound intensity at somepoints lead to deficiency of reflected sound atother points.

• The spots of sound deficiency are known as deads p o t s . The sound intensity will be low at dead

spots and inadequate hearing.• Further, if there are highly reflecting parallelsurfaces in the hall, the reflected and direct soundwaves may form standing waves which leads touneven distribution of sound in the hall.


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Remedies

• The sound foci and dead spots may beeliminated if curvilinear interiors areavoided. If such surfaces are present, theyshould be covered by highly absorptivematerials.

• Suitable sound diffusers are to be installedin the hall to cause even distribution ofsound in the hall.

• A paraboloidal reflecting surface arrangedwith the speaker at its focus is helpful indirecting a uniform reflected beam ofsound in the hall.


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(4) Echoes• When the walls of the hall are parallel, hard and

separated by about 34m distance, echoes areformed. Curved smooth surfaces of walls alsoproduce echoes.

Remedies• This defect is avoided by selecting proper shapefor the auditorium. Use of splayed side wallsinstead of parallel walls greatly reduces theproblem and enhance the acoustical quality of thehall.

• Echoes may be avoided by covering the oppositewalls and high ceiling with absorptive material.


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(5) Echelon effect• If a hall has a flight of steps, with equal width, the

sound waves reflected from them will consist ofechoes with regular phase difference. Theseechoes combine to produce a musical note whichwill be heard along with the direct sound. This iscalled echelon effect . It makes the original soundunintelligible or confusing.

Remedies• It may be remedied by having steps of unequal

width.• The steps may be covered with proper sound

absorbing materials, for example with a carpet.


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(6) Resonance

• Sound waves are capable of setting physicalvibration in surrounding objects, such as windowpanes, walls, enclosed air etc. The vibrating objectsin turn produce sound waves. The frequency of theforced vibration may match some frequency of thesound produced and hence result in resonancephenomenon . Due to the resonance, certain tonesof the original music may get reinforced that mayresult in distortion of the original sound.

Remedies• The vibrations of bodies may be suitably damped to

eliminate resonance due to them by propermaintenance and selection.


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(7) Noise

• Noise is unwanted sound which masks thesatisfactory hearing of speech and music.

• There are mainly three types of noises thatare to be minimized.• They are (i) air-borne noise,

(ii) structure-borne noise and(iii) internal noise.


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• The noise that comes into building through air fromdistant sources is called air-bo rne noise .• A part of it directly enters the hall through the open

windows, doors or other openings while anotherpart enters by transmission through walls andfloors.

Remedies• The building may be located on quite sites away

from heavy traffic, market places, railway stations,airports etc.• They may be shaded from noise by interposing a

buffer zone of trees, gardens etc.

(i) Air-Borne Noise


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• The noise which comes from impact sources on thestructural extents of the building is knownas thes t ruc ture-born e noise . It is directly transmitted tothe building by vibrations in the structure. Thecommon sources of this type of noise are foot-steps, moving of furniture, operating machinery etc.

Remedies • The problem due to machinery and domestic

appliances can be overcome by placing vibrationisolators between machines and their supports.

• Cavity walls, compound walls may be used toincrease the noise transmission loss.

(ii) Structure-Borne Noise


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• Intern al no ise is the noise produced in thehall or office etc.• They are produced by air conditioners,

movement of people etc.Remedies

• The walls, floors and ceilings may beprovided with enough sound absorbing

materials.• The gadgets or machinery should be placed

on sound absorbent material.

(iii) Internal Noise

Documents

Introduction to Building Acoustics