Engineering Acoustics Lecture 11

7/28/2019 Engineering Acoustics Lecture 11

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Chapter 6 . . .

Room Acoustics


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Sound Energy Distribution in Rooms

The acoustic energy density of the diffused sound field

at the steady state,E0 = Ed + Es

Ed Energy density due to direct sound field

Es Acoustic energy density due to diffused field

The sound level at a distance r,

a) For a non-directional sound source

b) For a directional sound source with a directivity factor Q

}R

4

r4

1

{log10LL c2wr

}R

4

r4

Q

{log10LL c2wr


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Sound Energy Distribution in Rooms . . .

Where,

The sound power level of the source,

Rc Room Constant

the average sound absorption coefficient of all

surfaces in the room

S total surface area

}10

W{log10L

12-w

-1

SRc


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Example

A small machine is situated on the non-absorbingfloor in the centre of a room of dimensions 12m x

5m x 4m. The sound power level of the source is

85dB. Calculate the sound level at a distance of3m from the machine if the reverberation time is

0.8 seconds.


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Answer

2

2

3

48mAA

240x0.160.8

A

Vx0.16T

formula,sSabineusingBy

m256

4)x54x125x(12x2areasurfaceTotal

m240

4x5x12Volume

0.1875

256

48tcoefficienabsorptionAverage


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Answer

dB74.3

74.31510.68585

}59.07

4

34

2{log1085L

3rand2QdB,58L

}R

4

r4

Q{log10LwL

2

w

c

2

2m59.07

0.1875-148

-1SRcconstantRoom


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Chapter 7

Noise Control Engineering


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Noise control

Noise control is a method for reducing unwanted

sound.

Noise chain

The noise reduction measures may be applied to

either one or more of the above links.

Noise SourceTransmission

pathReceiver


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Noise control

a) Control at the source

Useful control actions are,i. Elimination of noisy equipment and works

methods


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Noise control

ii. Substitution of quieter machinery and methods

iii. Modification


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Noise control

Blocking the transmission Path

Noise control is achieved by isolating the noise

source by its,

Position

e.g. Airports are isolated from residential area

Mountings

e.g. Spring Mounting, Rubber Mounting

Enclosures

e.g. Compressor enclosure


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Noise control

Protecting the Receiver

The following control actions can be taken to

achieve noise control at the receiver.

Control of exposure time

Provide personnel hearing protection

e.g. ear plugs, ear muffs

Job rotation

Provision of quiet working areas for time whennot working on the noisiest processes


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Interference of Waves

Overlapping of waves. This is governed by the

Principle of Superposition which states that theresultant displacement of two or more waves is

given by the algebraic sum of the individual

displacements of the overlapping waves.

Constructive interference

Destructive interference


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Noise control

Types of noise control:

There are two types of noise control.

1. Active noise control

2. Passive noise control


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Active Noise control

It uses the principle of destructive interference

between waves to reduce noise.

The term active refers the use of a source of

acoustic energy in the noise reduction process.

Noise reduction:

up to 30dB at low frequencies (< 500 Hz)

Signal

processor

Loudspeaker

Noise

Microphone


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Active Noise control . . .

Applications:

For the wide variety of noise sources

Industrial fans

Air conditioning systems

Generators Transformers

Vehicle exhaust noise

Engines

For small enclosed spaces

Aircraft cockpits

Inside sports cars


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Active Vibration ControlThis is exactly the same principles and

techniques used in active noise control , can beapplied to the reduction of vibration in machinery

and structures.

Signal

processorExternal

vibrators

Noise

Accelerometer /vibration transducer


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Passive Noise control

Uses sound absorbing materials to convert

sound energy into heat.

This includes,

Insulation

Silencers

Vibration mounts

Damping treatments


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SilencersThere are two different types:

1. Absorptive silencers2. Reactive silencers

In absorptive silencers acoustic energy is converted

to heat by sound absorbing processes.

In reactive silencers sound waves are reflected

back towards the source. The acoustic energy is

dissipated in the extended flow path resulting from

internal reflections and by absorption at the source.


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Absorptive Silencers

The simplest type of silencer is a duct with walls

lined with sound absorbing material.

The attenuation produced , in dB per metre run of duct,

; P perimeter (m)

S cross sectional area (m2)

This equation is accurate for ducts with S < 0.3m2

and which are more nearly square than 2:1.

ab

Absorbing material

}S

P{R

1.4


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Absorptive Silencers

Sound attenuation is increased by splitting a

single duct into number of small parallel ducts.And all these small ducts are lined with sound

absorbing material.

Maximum attenuation is achieved with the

highest possible P/S ratio, which in effect means

that for a given cross sectional area of duct thesound is exposed to the greatest possible

surface area of sound absorbing material.


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Acoustic enclosures

Acoustic enclosures prevent the radiation of

noise from a noise source to an outside area.By doing a proper design the noise will be

reduced successfully but noise within the

enclosure may be much higher. This causes, Inconvenience to the operator

Create cooling problems

This high level of reverberant sound can be

reduced by a sound absorbing material line to

the inside walls.


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Acoustic enclosures

Insertion Loss (IL):

This is the difference in sound levels at thereception point before and after the installation of

enclosure around the machine.

IL = Lbefore Lafter

Lbefore sound level in the room before enclosure is fittedLafter sound level in the room after enclosure is fitted


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Reference book:

Acoustics and noise control

2nd edition

B J Smith, R J Peters and S Owen


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Practical schedule

3 Practical

2 - Outdoors

1 Industrial visit

Assignments:

Three (3) in-class assignments, each carry 10 marks.

3 for performance

7 for assignment

Documents

Engineering Acoustics Lecture 11