Engineering Acoustics Lecture 7

7/28/2019 Engineering Acoustics Lecture 7

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Chapter 4 . . . .

Sound Generation Mechanism



Anechoic Room



Anechoic Room



Directionality of sound sources . . .

Note: Semi-Anechoic Room

A room with anechoic walls and ceilings but with

a hard reflecting floor.

Sources which would be omni directional in free space

become directional to a certain extent if the radiation is

restricted to less than a complete spherical space.

eg: When the sound source is positioned with respect

to surfaces such as walls, floors and ceilings.




The directivity of an omni directional source when

positioned at various points in a room:

Position of Source Part of sphere into

which source can radiateQ D (dB)

Center of room Whole sphere 1 0

Center of wall, floor, ceiling 1/2 2 +3

Junction of two planes

eg. wall & ceiling, wall & floor1/4 4 +6

Corner (junction of three

planes)1/8 8 +9



Example

Find the directivity index of a noise source

situated on open but hard reflecting ground.




It is only free to radiate into a hemispherical space and

half the power which would have traveled downwards

is reflected upwards.

Average intensity over a half sphere

; W – acoustic power of the

source

Average intensity over complete sphere

=>

2av2W I'

r

2av4W I

r

2II' Qav

av

3 210log Q10log D



Example

Show that the average sound level over the

complete sphere, Lav = L’av -3, where L’av is the

average of measured sound levels when the

sound source is placed on hard reflecting

ground



Answer

3LL'

2log10}IIx

II'{log10

2log10}I

I'{log10D

2

W I'

4

W

I

avav

av

0

0

av

av

av

2av

2av

r

r



Example

The sound pressure level is measured as

92dB (A) at 4 m from a sound source of sound

power level 103 dB (A).

Find directivity factor (Q) in that direction.

Assume the source is placed on hard reflectingsurface.



Answer

87.9

10QQlog10

(A)dB9.04

84log20103-92

8r log20LL D

L-L D

8r log20LL

0.9

w

av

wav





Properties of Sound

1) Absorption, Reflection & Transmission

2) Total absorption

3) Sound Absorbers

a –

Porous absorbersb – Membrane absorbers

c – Cavity absorbers

4) Broadband sound absorption

5) Sound transmission

6) Mass law

7) Wave propagation in solids

8) Sound Insulation



Absorption, Reflection & Transmission

Sound absorption is defined, as the incident sound

that strikes a material that is not reflected back.

Reflection of sound takes place when there is a

change of medium. The laws of reflection for sound

are similar to those for light.

1. The angle of incidence is equal to the angle of

reflection

2. The incident wave, the reflected wave and the normal

all lie in the same plane.



Absorption, Reflection & Transmission . . .

Ei – amount of acoustic energy incident

Er – amount of acoustic energy reflected

Ea – amount of acoustic energy absorbed

Et – amount of acoustic energy transmitted

Ei

Er

Et

Ea

Medium1

Medium2




Energy cannot be created or destroyed, by the law of

conservation of energy.

Ei = Er + Ea + Et

Sound Absorption coefficient:

The absorption coefficient of a material is ideally

the fraction of the randomly incident sound power

which is absorbed, or otherwise not reflected.

i

ta

i

r i

E

EE

E

EE α




Sound absorption coefficient is a key factor in

selecting sound absorptive material.

For open window, Er = 0, Ea = 0 Ei = Et

When the surface is a perfect reflector,

Ei = Er

depends on the frequency of incident sound.

It also depends on how the sound incident on the

surface.

1 α α max

0 α α min

α''




For normal incidence, α is called normal incidence absorption coefficient and is denoted

by α0.

In the case of oblique incidence the coefficient is

expressed as αθ




For sounds incident at all angles the coefficient is

called random incidence absorption coefficient, α.



Measurement of absorption coefficient

It is essential to know the absorption coefficient values

of the materials used to build a design.

Methods of measurement:

1) Reverberation chamber method

2) Impedance tube method

Reverberation chamber method allows all angles of

incidence. So the measured coefficient will be “Random

incidence absorption coefficient”.

Impedance tube method only measures the

absorption coefficient at normal incidence.




Reverberation chamber method:

• The chamber is made by hard surfaces which reflectsound.

• Non-directional or 'diffuse‘ sound field is produced

•Also appropriate for measurement of sound absorptionand transmission loss characteristics of materials.

The value obtained by measurement in a reverberation

chamber is generally designated α and is used in

practice as the absorption coefficient.




Impedance tube method:

Measured absorption coefficient is,

• A useful indication of the sort of absorbent properties of

a material• Mainly used in theoretical and research work

• Used in quality control for the production of acoustic

absorbent materials



Total absorption

The effective area of absorption of a particular surface

= αi Si

; αi – absorption coefficient

Si – area of surface

Total absorption or total effective area of absorption ,

A = αi Si

is the sum of the contributions of all

surfaces.



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.



Types of Absorbers

Porous absorbers (Dissipative absorbers):

Porous absorbers are the most commonly used soundabsorbing materials. These materials allow air to flow

into a cellular structure where sound energy is

converted to heat.

Porous materials are light weight , spongy and have

interconnected pores.

An effective porous absorber will pass air undermoderate pressure.

A simple test to identify a porous absorber is to blow

through it.



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.



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)

α

Thick sampleThin sample



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.



Types of Absorbers

Membrane absorbers (Panel absorbers):



Membrane….

2) Panel Absorbers: Typically, panel absorbers are non-rigid, non-porous materials which are placed over an airspace thatvibrates in a flexural mode in response to sound pressureexerted by adjacent air molecules. Common panel(membrane) absorbers include thin wood paneling overframing, lightweight impervious ceilings and floors, glazingand other large surfaces capable of resonating in response tosound. Panel absorbers are usually most efficient at absorbinglow frequencies. This fact has been learned repeatedly on

orchestra platforms where thin wood paneling traps most of the bass sound, robbing the room of “warmth.”



Cavity….

3) Resonators: Resonators typically act to absorb sound in a narrow frequencyrange. Resonators include some perforated materials and materials thathave openings (holes and slots). The classic example of a resonator is theHelmholtz resonator, which has the shape of a bottle. The resonantfrequency is governed by the size of the opening, the length of the neckand the volume of air trapped in the chamber. Typically, perforated

materials only absorb the mid-frequency range unless special care is takenin designing the facing to be as acoustically transparent as possible. Slotsusually have a similar acoustic response. Long narrow slots can be used toabsorb low frequencies. For this reason, long narrow air distribution slotsin rooms for acoustic music production should be viewed with suspicionsince the slots may absorb valuable low-frequency energy.



Impedance tube method



Reference book:

Acoustics and noise control

2nd edition

B J Smith, R J Peters and S Owen



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 7