Aurar 501 Architectural Acoustics Unit 1

8/13/2019 Aurar 501 Architectural Acoustics Unit 1

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Need to study Architectural Acoustics

1. Different structure has to be consider separately like place, purpose, environment,economics, phycologyandsociologyof the group. Likely to use material as peravailability

and according tobasic rules and regulation of the acoustics.

2. Involvement ofgadgetsthat produces noise

3. Technological development develops acoustical problems. These problems has to be

solved by evolving effective techniques.

4. Need sufficientloud, without unwanted echo, flutters, sound focuses, dead spaces.

5. Needclarityand yetcontinuity.

6. Structures that demand acoustical solutions areWorkshop, Assembly Hall, Class Room,

Drama Theatre, Cinema hall, TV & Radio Stations etc.

7. Population explosionandincrease urbanization--- resultant of Noise Pollution.

8. Multi-storied buildingposes vibrational and sound flanking problems.

9. Study ofsound absorbent materials.

10. It should be solved and consider at the planning state and before undertaking actual

construction work of the proposed site.


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Sound, Vibration, Frequency, Wavelength and velocity of sound

1. Sound energy is created in our mouth as the tongue moves and creates vibrations in the air

present in the mouth.

2. The compression and rerefactions created in the air advance in the outer air in the form of a

longitudinal-wave.

3. These advancing vibrations strike our ear drum which in turn vibrates and conveys these

vibrations to the brain and then we name these vibrations as Sound.

4. For every vibration of the sound source, the wave moves forward by one wavelength ()

which is the distance between any two consecutive repeating points on a wave. Frequency (f)

is the number of cycle of vibration per second called Hertz (Hz). Velocity is the distance

moved in one second (v).

5. The velocity of the sound in the air increases with temperature and humidity. Sound travels

faster in liquids and solids than in air because the densities and elasticities of these materials

are greater and particles of such materials respond to vibrations as a faster rate.


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1. Sound Energy impinging on the ear drum causes it tovibrate.2. Vibrations transmitted to middle ear by three bones: Hammer, Anvil and Stirrup by level

action.

3. Middle ear - cavity contains air at Atoms Pressure. It is connected to throat byEustachian

tube. Sudden Pressure changes are manipulated by it.

4. Cochlea Fluid in it stirrup vibrations to brain.

5. External Ear.

Sensitivity of hearing depends upon

1. Sound Intensity

2. Pressure

3. Frequency range of sound4. Duration of exposure of sound

5. Age group

6. Psychology of the Individual


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Frequency ranges of Audible sound, Human-Ear and Sensitivity of hearing

1. Usually frequency range of audible sound is from20Hzto20 KHz.

2. To hear sound of 1000 Hz, minimum pressure change on the ear-drum is 3 x 10-5 N/m2.

3. The same sound of 1000 Hz becomes unbearable when the pressure change on the ear drum

is 30 N/m2.

4. Sound of 1000 Hz with Sound- Pressure- Level (SPL) of 2 x 10-5 N/m2 dyne/cm2 is the

reference (minimum) Loudness level and is called Loudness of Zero Phone.

5. Loudness depends upon both SPL and Frequency.

6. Generally intelligibility is provided by three octave bands

1. 700-1400 Hz

2. 1400-2800 Hz

3. 2800-5600 Hz

(Octave Band is the range of frequencies between any one frequency and double the frequency)

7. Our ear response to sound energy is logarithmic. If A is radiating 100W power and B is radiating

only 100 W power, A will sound only 60 times louder than B.


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Inverse Square Law, dB Scale for Sound Intensity, Pressure, Power and Loudness perception

The sound intensity from a point source of sound decreases in inverse proportion to the

square of the distance from the source.

i.e.I 1/d

OrIntensity = Power (Watts) / Spherical area

i.e.I = W / 4d

HenceI1 = W / 4d12 andI2 = W / 4d2

2

I1/ I2 = d22/d1

2

WhereS= Point source of sound

1= Surface Area, A at point D

2 = Spread of Sound Energy

3= Surface Area, 4A at point E

SD= Distance d1

SE= Double the distance, d2=2d1


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The range of values between the threshold of hearing and the threshold of pain is too large. The

ear feels different effects when listening to high intensities and low intensities.

As the ear response to sound is logarithmic, the decibel scale is convenient to use.

N = 10 log 10(I/ I0) = 10 log 10(W/W0) = 10 log 10(P/P0)2

Where

N = Number of decibels (dB)

I = Sound Intensity

I0 = Reference (Lowest) Intensity (10-12 Watts/m2)

W = Sound power of the source

W0= Reference power (10-12 Watts)

P = Sound Pressure

P0= Reference Pressure (2 x 10-5 N/m2)

Inverse Square Law, dB Scale for Sound Intensity, Pressure, Power and Loudness perception

Sound Power in

W

Level in

dB

Whisper 1 x 10-9 20-30

Conversation 1 x 10-5 60-70

Radio 0.1 100-110

Human Ear (Maximum) 130-140

Air Craft 100 140-150

Jet 105 170-180


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1. Calculate the change insound levelwhen the intensity of the sound is doubled.

Problems


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Problems

2. Calculatetotal SILcaused by combination of levels of95dB and 90 dB.Given I0 =Reference (Lowest) Intensity (10-12 Watts/m2)


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Problems

3. A sound produced at one end of along tubeis heard twice at the other end in an

interval of2 sec. The velocity of sound in metal is5130 m/s. and that in air is343 m/s.

Find thelengthof the tube.


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Problems

4.90%of the sound of level20 dBis absorbed by the wall and the rest is reflected

back.Find the level of reflected sound.


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UNIT 1

Sound Engineering

Introduction to architectural acoustics - Characteristic and measurement of sound,

frequency, intensity, decibel scale, auditory range, effects of sound on humans,loudness.


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Acoustics and acoustical environment

1. Archaeoacoustics


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1. Basic principles in acoustic quality ?

The acoustic environment affects our experience

Silence not always a good acoustic environment

The significance of nature and the cultural environment

Different claims and expectations


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2. How does the acoustic environment affect natural and cultural environments?

Requirements for indicators and inventory method

Basis in municipal comprehensive planning reporting acoustic qualities in

different areas and thus also being able to utilise and develop the areas more

actively.

Indicators of sound/noise put into practical form important parts of the Swedish

Government's environmental objectives A Balanced Marine Environment,

Flourishing Coastal Areas and Archipelagos, A Magnificent MountainEnvironment, Flourishing Lakes and Streams and A Good Built

Environment.

The indicators are also necessary in order to be able to determine interim targets for

natural and cultural environments intransport-policy objectives.

Indicators and an inventory method forsound and noise and natural and cultural

environments are required in theplanning of infrastructure and activities that

produce noise.


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Indicators usable in practiceNoise sources

Road traffic Rail traffic

Air traffic (airports) Shipping and boating

Cross-country vehicles Firing ranges and shooting galleries Industrial activity Motor sport

Wind power

Assessing the acoustic quality of an area

Determine the occurrenceandaudibilityof different sounds of significance in theenvironment under consideration for impact in the total acoustic picture (acoustic

landscape).

Determine whichsoundsareexperiencedaspositive. Estimate the conscious impact

of the sound and try to also includeunconscious impact.

Determine whichsoundsareexperiencedasnegative. Estimate the conscious impact

of the sound and try to also includeunconscious impact.

Assess the overall acoustic quality by weighing together the impacts ofpositiveand

negativesounds. Classify the quality on some scale, which includes at least the grade of

good quality.


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Behaviour of sound in an enclosed space.


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Reflection

1. This occurs when thewavelength

of asound wave

issmaller

than the surface ofanobstacle.

2. In the case of anenclosed space, the sound waves hit every side of the enclosure

continuously until thesound energyreduces tozero.

3. The amount of wavesreflecteddepends on thesmoothness, size, and softnessof

thematerialsof enclosure.4. The angle ofincidenceof soundraysisequalto that of thereflected raysonly if

the surface of the reflector is flat.

5. But when it is curved, the angles aredifferent.


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Absorption

1. When sound waves hit the surface of an obstacle, some of its energy isreflected

while some arelost throughitstransferto themoleculesof thebarrier.

2. The lost sound energy is said to have beenabsorbedby thebarrier.

3. The thickness and nature of the material as regards its softness and hardness

influences the amount of sound energy absorbed.


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Refraction

1. This is the bending of sound when it travels from one medium into another

medium.

2. The difference in the composition of the twodifferent media bendsthe sound i.e.

the angle of incidence changes into an angle ofrefractionas it travels into the new

medium.


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Diffraction

1. When the wavelength of a sound wave is smaller or equal to the size of the

obstacle, the sound rays tend to bend round the edge of the obstacle thereby

turning the edge to a sound source.


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Transmission

1. In this phenomenon, sound wave is carried bymoleculesof the obstacle through

vibrationandre-emittedat the other side irrespective of the medium.

2. It can bestructure borne,air borneorimpact sound.


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Reverberation and Echo

Reverberation:

1. This is the persistenceof sound in an enclosed space as a result of continuous

reflection or scatteringof sound after the source has stopped.

2. It is one the mostprominent behavioursof sound in an enclosure.

3. It occurs when sound waves hits a surface and are reflected toward another

surfacewhich alsoreflectsit.4. Some of the sound is absorbed with this continuousreflection which gradually

reduces the energy of the sound to zero.

5. The phenomenon can affect the audibility of sound in an enclosure, especially if the

reverberation time, which is the time taken for the sound pressure level to diminishto60 dBbelow its initial value is considerably long.

Echo:

1. This occurs when the reverberation time is long enough to cause a distinct

repetitionof the direct sound.

2. This condition is an advanced form of reverberation where the sound is heard

clearly and repeatedlyafter some time until it fades.


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Factors that affect the behaviour of sound in an enclosed space.

The way in which sound behaves in an enclosed space depends on many factors which

include:

Reduction in its intensity of sound

This can results due to thedistancebetween itssource and the receiver.

Absorption of direct sound by the audience

The listeners of the sound absorb some of the sound in the process ofhearing.

Absorption of direct and reflected sound by surfaces

Thewalls, ceiling and floorof the enclosureabsorbsandreflectsound waves

thereby controlling the way the sounds behave.Reflection of sounds from right-angled corners

Sound incident to a right-angled corner of room will be reflected back towards

source if surfaces are acoustically reflective. This can in turn produce echoes

especially inlarge spaces.

Dispersion of the sides of an enclosure

Reflections can be controlled by making one surface dispersive i.e. not at

right angle to each. This would have affected the reflection of the sound thereby

affecting its behaviour.


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Factors that affect the behaviour of sound in an enclosed space.

Edge diffraction of sound

Edge diffractionresults in the curvature of part of a sound wave around the

edge of a barrier. This causes the obstacles to scatter the sound waves making it

behave like a source of sound.

Sound shadowAny barrier interrupting a sound wave will create a shadow, synonymous to

light rays. However, because of edge diffraction some sound will creep into this but

such penetration is frequency dependent - high frequencies are less diffracted than

low frequencies. Such problems can occur in auditorium with balconies.Primary reflection

This depends on the angle of incidence which is equal to the angle of

reflection. Also, the nature of sound reflector is important.

Panel resonance

Sound waves can propagate "through" a solid material by panel vibration. The

sound does not actually penetrate the material but rather causes this to vibrate and act as

a sound source itself. The panel will be vibrated by both direct and reflected sound

waves.


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Reverberation and Reverberation time calculation.

Sabine equation.

Dead and Live Rooms

Reverberation :Several repetitive sound reflections from different surfaces in a room

reduce the sound energy. Even when the sound source is stopped, some sound energy is

retained in the room for some time. This process of Sound energy retention is called

Reverberation.

Reverberation Time(R.T.)

It is the time required for a sound of 60dB level to become inaudible i.e. to come down

to zero dB level.According to Sabine it is time for sound intensity to come down to 10-6 of its original

intensity.

Factors affecting R.T. :1. loudness of original sound

2. Absorption by bounding surfaces, people, furnishings etc.

3. Volume of the room


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Sabines equation

RT= 0.16V/SHere

RT = Time in sec

V = Volume of the room in M3

S = Total absorption in the room in M2 i.e.

S = (1S1+ 2S2+ .) + a1, a2.

Where 1= absorption coefficient of a surface area S1 etc. and

a1= absorption by empty chairs etc.

RT= 0.05 V/Swhen V is in Cu. ft. and S in Sq. ft.


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1. Speech

2. Studios3. Cinema Theatre

4. Chamber Music

5. School Auditorium

6. Music Hall7. Church Music

A i i R T l l i


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1. Uniform distributionof sound energy in the room.

2. Decay besmooth,without fluctuations.

3. Theabsorptive treatmentnot to be considered at a few places in the room i.e.

diffusion and random scatteringof sound in the room.

4. No domes and curved surfaces.

5. Temperature and humidity to remain constant.

6. If the total absorption in the room is much greater than the volume of the room; asin the recording studio etc.,the Sabine's formula need correction.

7. With angle reflectors on dias and parabolic shape of the room, the RT will be low.

8. given, is usually for random, diffused incidence of sound on the absorptive

material of about6m2 panel. How ever this specification is not realised in actualrooms.

9. The audience member members are expected to be seated,one behind the other.

In the half filled room, this condition may not be realized.

Assumptions in R.T. calculation:

P RT


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1. Every member of the audience should receive direct sound from the source and thatshould be followed by the sound from the first reflection only.

2. Smooth parallel walls be avoided to avoid resonance effects.

3. Few items in the room should be for sound reflections and remaining items forsound absorption and diffusion.

Proper RT

D d d Li R


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1. For live room to get fullness and warmth of music, RT should be in the range of1.5 sec to 1.8 sec.

2. For low frequency sound, RT be little higher and higher frequencies RT be shorter.

3. If the RT is larger at high frequencies then music will be harsh or Rasping.4. If the RT is larger at low frequencies then it will be :Boomy.

5. Similarly if RT is too low it will be Shrill.

6. Live room have large volume and longer RT at mid and high frequencies. Large

number of absorbents makes the room dead.

7. Sound reflection from concave surfaces having un-naturally high intensity are

termedHot-Spots. This situation is at the cost ofDead-Spotswhere hearing

conditions are poor. (Ex. Balcony)

Dead and Live Room


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Partha Sarathi Mishra

Asst. Professor

School of Architecture

GITAM University

[email protected]

THANK YOU

Documents

Aurar 501 Architectural Acoustics Unit 1