soundCourse: B.Tech

Subject: Engineering PhysicsUnit: V

Chapter: 1

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Topics:

• INTRODUCTION• CLASSOFICATION• WEBER-FECHNER LAW• REVERBATION• FACTORS AFFECTING ACOUSTICS OF

BUILDINGS

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When matter vibrates or moves back and forth very quickly, sound is produced.

Example: When you hit a drum, parts of the drum will vibrate creating sound.

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• The sound that Produce pleasing effect on the ear is called Musical sound.

• Musical instruments make different sounds by plucking the strings.

• Example:-sound produce by instrument sitar, violin, flute, piano etc

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• The sound that Produce Jarring effect on the ear is called Noise sound.

• Noise sound make unpleasant to hear

• Example:-sound produce by flying airplane, road traffic, cracker etc

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Sound Absorption

Unit of ‘a’ Sabine also called OWU

Sound Energy absorbeda

Sound Energy incident

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Sound Absorption

• The property of a surface by which sound energy is converted into other form of energy is known as absorption.

• In the process of absorption sound energy is converted into heat due to frictional resistance inside the pores of the material.

• The fibrous and porous materials absorb sound energy more, than other solid materials.

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Absorption coefficient ‘A’

• a= sound energy absorbed/sound energy incident

• Unit of ‘a’ Sabine also called OWU• Sabine chose open window of 1 m2 as a

standard unit for absorption of sound• E.g., if sound absorbing material of 5 m2

absorbs the sound as 1 m2 then absorption coefficient is 1/5=0.20 Sabine or OWU

Reverberation & Reverberation time

• Prolongation or continuation of sound in hall through source is cut of is called …………….

• Sound goes through multiple reflection from walls, floor, ceiling, etc. before it becomes in audible.

• So sound lasts for some time after source has stopped.

• The time to inaudible sound in hall is……….• If there is no absorbing material in hall then

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0.167VT

as

Reverberation time

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Factors Affecting Acoustics of Buildings

(1) Reverberation Time

• Optimum value

Remedies• Providing windows & openings• Full capacity of audience• Heavy curtains with folds• Covering floor with carpets etc.

(2) Loudness

• There should be uniform distribution of loudness.Remedies• Using absorbents at high noise places• Low ceilings• Large sounding boards towards the audience• loudspeakers

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2) loudness

(3) Focusing

• Reflecting concave surfaces cause concentration of reflected sound, creating a sound of larger intensity at the focal point. These spots are known as sound foci.

• The spots of sound deficiency are known as dead spots. The sound intensity will be low at dead spots and inadequate hearing.

• Further, if there are highly reflecting parallel surfaces in the hall, the reflected and direct sound waves may form standing waves which leads to uneven distribution of sound in the hall.

Remedies• Curved surfaces can be avoided in hall

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3)focusing

(4) Echoes

• When the walls of the hall are parallel, hard and separated by about 34m distance, echoes are formed. Curved smooth surfaces of walls also produce echoes.

Remedies• This defect is avoided by selecting proper shape for the

auditorium. • Echoes may be avoided by covering the opposite walls

and high ceiling with absorptive material.

(5) Echelon effect

• If a hall has a flight of steps, with equal width, the sound waves reflected from them will consist of echoes with regular phase difference. These echoes combine to produce a musical note which will be heard along with the direct sound. This is called echelon effect. It makes the original sound unintelligible or confusing.

Remedies• The steps may be covered with proper sound

absorbing materials, for example with a carpet.

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5) echelon effect

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(6) Resonance Sound waves are capable of setting physical vibration in

surrounding objects, such as window panes, walls, enclosed air etc.

The frequency of the forced vibration may match some frequency of the sound produced and hence result in resonance phenomenon.

Due to the resonance, certain tones of the original music may get reinforced that may result in distortion of the original sound.

Remedies The vibrations of bodies may be suitably damped to

eliminate resonance due to them by proper maintenance and selection.

(7) Noise

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

• There are mainly three types of noises that are to be minimized.

• They are (i) air-borne noise, (ii) structure-borne noise and (iii) internal noise.

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7) Noise

• The noise that comes into building through air from distant sources is called air-borne noise.

• A part of it directly enters the hall through the open windows, doors or other openings while another part enters by transmission through walls and floors.

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.

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(i) Air-Borne Noise

• The noise which comes from impact sources on the structural extents of the building is known- as the structure-borne noise. It is directly transmitted to the building by vibrations in the structure. The common 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 vibration isolators between machines and their supports.

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

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(ii) Structure-Borne Noise

• Internal noise is the noise produced in the hall or office etc.

• They are produced by air conditioners, movement of people etc.

Remedies• The walls, floors and ceilings may be provided

with enough sound absorbing materials. • The gadgets or machinery should be placed on

sound absorbent material.

(iii) Internal Noise

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