Post on 03-Apr-2018
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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