Engineering Acoustics Lecture 6

7/28/2019 Engineering Acoustics Lecture 6

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Chapter 4

Sound Generation Mechanism


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Sound Generation Mechanism

1. Airborne sound

2. Structure-borne sound

Airborne sound

The sound generated directly into the air

is called the airborne sound.

Eg: voice, loudspeaker, traffic noise, aircraft noise,

musical instrument etc.

Airborne sound is transmitted mainly

through the air.


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Structure-borne sound

The sound produced by sources which act

directly on the structure is called structure-borne sound.

eg: foot steps, slamming of doors, windows, vibrating

machinery

Structure-borne sound is transmitted mainly through

the structure. This type of noise is really a combination

of both airborne and impact noise because the impacts

will produce airborne noise.


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Propagation of Airborne sound

a) Point source

Consider a point source of power Wradiating uniformly into free space. To find the sound

intensity at a distance r from the source construct a

sphere of radius r.

Intensity at r,

I =


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For example consider the sound intensity at a

distance r from a point source.I = W/4r2

L = Lw 20 logr 10 log 4

L = Lw 20 log r - 11


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Example 1:

Obtain an expression for sound intensity at a distance

r from a source placed on hard reflecting ground.

Average sound intensity at a distance r from the

source,I = W/2 r2

L = Lw 20 log r -8


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Example2:

Obtain an expression for sound intensity at a distancer from a sound source placed on soft absorbing

ground.

I = (W/2) / 2r2

L = Lw 20 log r - 11


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Example 3:

Two sounds of power 4W & 8W are produced atground level at a distance of 6m and 7m respectively

from a point of observation. If the ground is

unobstructed and non-absorbing what will be thesound level observed?


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Answer

L = Lw 20 log r - 11


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b) Line source

Many sound sources in a row can beconsidered as an infinite line source.

e.g. vehicles on a busy motor way (assume vehicles

are identical)

The sound will radiate cylindrically.

Intensity at r,I = W / 2rl

where W acoustic power

per unit length


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For a given W,

I 1/r

In this case I is inversely proportional to distance.

L = Lw 10 log r 8


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Example

Calculate the reduction of sound level for doubling ofthe distance


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Answer

For every doubling of distance the sound level isreduced by 3 dB.


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c) Real noise sources

They differ from simple point sources in two

ways.

i) the source has a finite size

ii) it may radiate different amounts of acoustic

energy in different directions

e.g. Machines with vibrating surfaces

However provided the distance from the source

is great enough, real noise sources do behave like

simple point sources.


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Then the sound level in any direction will decrease

at the rate of 6 dB per doubling of distance away

from source.

The region close to a real source (such as a

machine) is called the near sound field or simply the

near field.

In the near field sound radiating from the various

portions of the source combine in a complex way.

It is difficult to predict local vibrations (how it varies

with distance) in sound intensity near to the source.


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The region further away from the real source is called

the far sound field or simply the far field.

In the far field sound intensity distribution obeys an

Inverse Square Law of distance.

The extent of the near field region depends on

the dimensions of the machine and on the wavelength

of sound being radiated. There is no sharp division

between the two regions.

.


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L Vs log(r)

Ideally a minimum distance of 1 or 2 wavelengths or

1 or 2 machine lengths from the source is taken as

the far field (whichever is the greater).


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Directionality of sound sources

Directionality of radiation may arise because a sound

source can be inherently directional.For example, a machine may radiate more noise from

the front than the back (e.g. loud speaker).

The directionality of a sound source may be described interms of a directivity factor, Q defined as ,

Q = (sound..) / (avg sound intensity)


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The average intensity is the intensity which would be

produced at that distance if the total energy of thesource were to be equally distributed in all directions.

If for example measurements are made at six points

evenly spread over sphere surrounding the source givingintensities; I1, I2, I3, I4, I5, I6

Iav = (I1+I2++I6) / 6

Then directivity factor in a given

direction (say direction 3)

Q3 = I3 / Iav


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It is more convenient to take measurements as sound

levels.

The directivity index D in any direction is defined as,

D = 10 log (I/Iav)

D = 10 log Q

where Q is the directivity factor in the direction of

interest.

D= 10 log (I/I0 x I0/Iav)= 10 log(I/I0) 10 log (Iav/I0)

= L - Lav


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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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Engineering Acoustics Lecture 6