Engineering Acoustics Lecture 8

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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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(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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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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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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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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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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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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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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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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M+P:

Double leaf partition

S


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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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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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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

Documents

Engineering Acoustics Lecture 8