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Chapter 5
Properties of Sound . . .
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Types of Absorbers
Absorbers may be divided into three main types:
1. Porous absorbers
2. Membrane absorbers
3. Helmholtz absorbers
The sound energy is converted into heat in all three
types of absorbers mentioned above.
But there are different frequency responses for each
type of absorber.
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Porous absorbers . . .
Porous absorbers are most effective in slowing down
air particles with a high sound velocity
Common porous absorbers include carpet, glass fiber,
glass wool, rock wool, open-cell foam, porous ceiling
tile etc.
Sound Absorption Mechanism:
The friction between air particles and pores causes
sound energy to be dissipated in the form of heat.When the pores are isolated the heat transfer process
occurs in isolated places and it will not take much
sound off by friction.
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Porous Absorbers . . .
Frequency response:
Sound absorption is large at high frequencies and small
at low frequencies.
1.0
0.8
0.6
0.4
0.2
0125 250 500 1k 2k 4k
Frequency (Hz)
Thick sampleThin sample
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Porous absorbers . . .
At all frequencies these materials have some amount of
absorption.
Sound absorption can be slightly improved by
increasing the thickness at low frequency.
A porous sound absorber is identified on drawings by a
ribbon candy symbol.
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Types of Absorbers . . .
(ii) Membrane absorbers (Panel absorbers)
The panel absorbers are non-rigid, non-porous
materials which are placed over an airspace that
vibrates in a flexural mode in response to sound
pressure exerted by adjacent air molecules
d
Backup wallPanel absorber
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Types of Absorbers . . .
(ii) Membrane absorbers (Panel absorbers)
Features:
Excellence low frequency performance
Non-fibrous
Small width and low pressure loss
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Membrane absorbers . . .
Eg: An enclosure lined with membrane absorber
material
S source of soundd width of air gap
d
enclosures
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Membrane absorbers . . .
Sound Absorption mechanism:
Panel absorbers work by transferring soundenergy firstly into vibrational energy in the panel
facing and as heat due to internal friction (damping).
P a particle of the panel (simple harmonic motion)
Natural frequency of the panel
P
x = a
x = - a
T
1f
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Membrane absorbers . . .
If fe = f resonance
The absorption is maximum at the resonance frequency ofthe panel corresponding to maximum movement.
The resonance frequency of the panel is given as,
where,
m mass per unit area of the panel kg/m2
d depth of air space (m)
fe
panel
md60f
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Membrane absorbers . . .The most common membrane absorber is the
suspended ceiling (or false ceiling)
Raised floors are also common membrane absorbers
(false floors). E.g. in Gymnasiums
Structural floor
Suspended
ceilingJoist
JoistWooden
floor
Structural floor
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Membrane absorbers . . .Frequency response:
Typical response curve is as shown below
1.0
0.8
0.6
0.4
0.2
0
63 125 250 500 1k 2k
Frequency (Hz)
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Membrane absorbers . . .
The absorption characteristic shows a peak in the
low frequency range which coincides with the
resonant frequency of the membrane.
Materials commonly used for panels such as
plywood, canvas, sheet of glass.
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Types of Absorbers . . .
Cavity absorbers (Helmhotz resonators):
Cavity absorbers are Helmhotz resonators, take the
form of an enclosed volume of air with a small
narrow neck.
Resonators
act to absorb sound in a narrow frequencyrange
include some perforated materials
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Cavity absorbers . . .
Sound absorption mechanism:
Reduce sound energy by friction at the neck and
by inter reflections within the cavity.
e.g. slotted concrete block
cavity
Multi reflection
ceiling
neck
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Cavity absorbers . . .Frequency response:
1.0
0.8
0.6
0.4
0.2
0
63 125 250 500 1k 2k
Frequency (Hz)
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Cavity absorbers . . .
The ability to tune a cavity absorber to specific
frequencies is useful for noise control.
They are used for pure tone noise attenuation for
TV, broadcasting studios and concert halls.
Usually fr
is controlled by changing the length of
neck.
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Broadband Sound Absorption
How to make effective for a wide range of
frequencies..Combination of Porous and Membrane or Cavity
absorbers can provide broadband absorption.
e.g. P + M
airgap
Backup
surface
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Broadband Sound Absorption . . .
airgap
enclosure
Porousmaterial S
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Broadband Sound Absorption . . .
Note1:
Fibrous sound absorbers are refer to as fuzz.
e.g. glass wool, rock wool
(glass fibre, mineral fibre)
Note2:
line => no air gap
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Broadband Sound Absorption . . .
A suspended ceiling can be made a broadband
absorber by replacing the hard ceiling material with
a porous material (e.g. using acoustic tiles)
A low frequency absorption of a porous absorber
can be increased by using an air gap.
Structural floorSuspended
ceilingJoist
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Broadband Sound Absorption . . .
M+P:
High frequency sound absorption of a membraneabsorber can be increased by putting porous (fibrous)
material in the air space.
Structural floor
hard ceilingJoist
Porous
material
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Broadband Sound Absorption . . .
M+P:
Double leaf partition
S
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Broadband Sound Absorption . . .
C+P:
By putting porous material in cavity high frequency
and low absorption can be slightly increased.
e.g. slotted concrete block
Fibrous
material
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Cavity absorbers . . .Frequency response:
1.0
0.8
0.6
0.4
0.2
0
63 125 250 500 1k 2k
Frequency (Hz)
C+P
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Broadband Sound Absorption . . .
P+C:
A4 thick fuzz with perforated facing (tiny holes in a
regular pattern=> behave as cavity absorbers)
B- 4 thick fuzz without perforated facing
airgap
Backup
surface
Perforated
facing
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Broadband Sound Absorption . . .
P+C . . .
Frequency response:
1.0
0.8
0.6
0.4
0.2
0
125 250 500 1k 2k 4k
Frequency (Hz)
P+C
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Assignment 3:
Time : 1.00- 3.00p.m.
Date: 11th May 2009 MondayVenue: Exam Hall No 2
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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