Engineering Acoustics Lecture 5

7/28/2019 Engineering Acoustics Lecture 5

1/30

Chapter 2 . . .

Noise Criteria in Sri Lanka


2/30

Noise Criteria In Sri Lanka

Area:

Low noise area

Medium noise area

High noise area

Silent area


3/30

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.


4/30


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.


5/30


6/30


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


7/30


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


8/30


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.


9/30


10/30


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)


11/30

Chapter 3

Sound Attenuation


12/30

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.


13/30

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.


14/30


a) Attenuation due to ground absorption

20m

n1 dB

h m

h


15/30


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


16/30


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).


17/30


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)


18/30


19/30


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.


20/30


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


21/30


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.


22/30


acoustic shadow ( I of sound will be very low)

wind speed


23/30


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.


24/30


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.


25/30


acoustic shadow

low temp => low speed (c)

Day time


26/30


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.


27/30


high temp => high speed (c)

Night time


28/30


29/30


30/30

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 5