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7/28/2019 Engineering Acoustics Lecture 5
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Chapter 2 . . .
Noise Criteria in Sri Lanka
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Noise Criteria In Sri Lanka
Area:
Low noise area
Medium noise area
High noise area
Silent area
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Noise Criteria In Sri Lanka . . .
1) Low noise areaAn area located within any Pradeshiya sabha
area.
2) Medium noise area
An area located within any municipal council or
urban council area.
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Noise Criteria In Sri Lanka . . .
3) High noise areaAn area located within any export processing
zone.
4) Silent area
The area covered by a distance of 100m from the
boundary of a courthouse, hospital, public library,school, zoo, sacred areas and areas apart
recreation or environmental purposes.
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Noise Criteria In Sri Lanka . . .
Maximum permissible noise level at boundaries in Leq dB (A)for
industrial activities
Area Leq dB (A)Day time
6.00a.m. 6.00p.m.
Night time
6.00p.m. 6.00a.m.
Rural residential 55 45
Urban residential 60 50
Noise sensitive 50 45Mixed residential 63 55
Commercial 65 55
Industrial 70 60
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Noise Criteria In Sri Lanka . . .
Maximum permissible noise levels at boundaries in Leq dB (A)for
construction activities
Area Leq dB (A)
Day time
6.00a.m - 10.00p.m
Night time
10.00p.m 6.00a.m
Industrial/
commercial
75 60
Residential 65 50
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Noise Criteria In Sri Lanka . . .
Standards:There are standards for the rating methods.
For example, BS 4142 (1990) for industrial and
commercial activities affecting residential areas.BS 5228 for construction site noise affecting
residential areas.
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Noise Criteria In Sri Lanka . . .
Eg2:- The measurement position should be 1m from thefront of the house (front of dwelling / faade).
To find the equivalent level, the reference timeused is 5 min at night time and 60 min during the
day
1m
L 65 dB (A)
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Chapter 3
Sound Attenuation
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Sound Attenuation
Sound attenuation is due to the absorption of sound
energy by the following,
a) ground absorption
b) air absorption
And also the velocity gradients caused by
meteorological conditions.
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Sound Attenuation . . .
a) Attenuation due to ground absorption
A ground surface that is acoustically soft (eg: grass
land) as opposed to paved areas, attenuate sound
propagation.
For distances greater than 20m this effect can reduce
the sound level.
The effect is mainly significant for low propagation
heights above ground surfaces.
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Sound Attenuation . . .
a) Attenuation due to ground absorption
20m
n1 dB
h m
h
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Sound Attenuation . . .
As a rough guide the following table indicates the extra
attenuation due to propagation at various heights over
absorbing ground, assuming that source to be 0.7m
above the ground level.
Mean height of propagation (m) Ground attenuation dB (A)
6 1
4.5 2
3 3
1.5 4
0.7 5
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Sound Attenuation . . .
The sound attenuation achieved by shrubs and trees can
only be marginal.
For example, at 1000 Hz, the sound reduction per 100m
is about 0.8 dB with very leafy undergrowth.
) Attenuation due to air absorption
Sound waves attenuate during propagation in air
because the energy is absorbed by the medium. These
losses are due to the relaxation process and depend upon
the amount of water vapor present (humidity).
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Sound Attenuation . . .
If a plane wave whose intensity I0 travels a distance x, the
intensity Ix is given by,
Ix = I0 exp(-mx)
;where m is the attenuation constant per meter
When x = 1m,
log (I0/I1) = m log e = 0.434m
m = (1/0.434) log (I0/I1)
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Sound Attenuation . . .
Typical values are about 3 dB per 100m at 4000Hz dropping to 0.3 dB per 100m at 1000 Hz.
Air attenuation becomes very important for
ultrasonic frequencies and is greater than
1dB/m at 100 kHz.
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Sound Attenuation . . .
c) Attenuation due to meteorological conditions
The velocity gradients caused by air, temperature or
wind have a major influence on the propagation of sound
in the open air
i) Effect of wind
Generally the profile of wind speed is faster at
high altitude than close to the earth.
The friction between the moving air and the ground
results in decreased velocity near ground level
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Sound Attenuation . . .
The sound propagation is controlled by the vector
summation of wind and the sound velocities.
VSW = VSE + VEW
Therefore downwind from the source, sound rays are
refracted back towards the ground and the received
sound level is affected.
Upwind the sound is refracted up and away from the
ground causing acoustic shadows in which the sound
level is considerably reduced.
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Sound Attenuation . . .
acoustic shadow ( I of sound will be very low)
wind speed
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Sound Attenuation . . .
In the upwind direction shadow zones may occur
while downwind sound reaches into far distance.
ii) Effect of temperature profile
when the weather is fine during the day, the
air is heated near the earths surface by solar
radiation but gets cooler towards the upper sky.
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Sound Attenuation . . .
The sound speed at t0C is given by
C 331.5 + 0.61t
Thus the speed of sound is greater when the
temperature is higher.
According to the above expression the sound rays bend
upwards.
This causes a symmetrical acoustic shadow in which the
level is considerably reduced beyond a certain distance
from the source near the ground.
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Sound Attenuation . . .
acoustic shadow
low temp => low speed (c)
Day time
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Sound Attenuation . . .
At night or in cloudy weather the temperature
profile takes the form of an inversion.
In the case of a higher temperature in the upper air
the sound can easily return to earth at quite large
distances from the source.
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Sound Attenuation . . .
high temp => high speed (c)
Night time
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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