Noise pollution control

Gondwana University, B.Sc. II, Sem. IV, Environmental Science, Paper I, notes of Noise Pollution Control

1. Rahul K Kamble

2. Noise barriers
Mufflers or silences
 Vibration isolation
Damping
Lagging

3. Barrier = obstacle
Noise barriers are
construction of
thick and high (> 2
m) covered with an
acoustic material
on the source side.

4. Noise barriers provides appreciable
protection from noise i.e. about 15 dB
(A)
Any solid material can acts as a barrier
Sound barriers can reflect or absorb
noise
Degree of sound reduction depends
upon physical form of barrier,
specifically on its height and length.

5. Buildings, earth embankment, stored
materials and other solids acts as barrier
Effectiveness is more when placed closer
to noise source or receiving point
No leak should be there
Length should be 10 times the height
For effectiveness barrier material should
have superficial mass of 7 kg/m
2
, e.g.
brickwork, asbestos cement boards, fiber
board, wood wool slabs etc.

10. What is meant by silence ?
What are the means by which silence is
achieved?
Keep silence !!!
Silence = Silencer (the condition of no sound)
Where do you find silencers ?

11. It is a piece of pipe or duct treated or
shaped for reducing sound
At the same time it allows the flow of
gas

12. Two types of mufflers
Dissipative and reactive
Dissipative (absorptive) works on the
principle of absorption of noise. Use of
acoustic absorbing lining.

13. Reactive silencers works on the principle of
reflecting.
The performance is depends upon frequency
of sound
Dissipative silencers provides sound
attenuation over broad frequency range and
hence more popular
Reactive silencers are tuned to provide
maximum attenuation at specific frequencies
and woks in limited frequencies

16. Reduce the magnitude of vibration between
two sources attached to each other
Radiation and noise can be controlled by
impedance mismatch
Is a measure of the rate at which a unit mass
of material receives the acoustic energy.
Acoustic impedance of a material is given by
the product of density of the material and
velocity of sound in the material.

17. Material having low impedance is
inefficient sound transmitter or good
sound isolator.
Rubber between two blocks of steel can
provide a very good sound isolation.
Noisy machine should be floated on a soft
and low impedance mounting rather than
bolting it to the floor mounting.
Examples: springs, elastomers mounts,
flexible couplings to conduits, ducts and
pipes, gaskets on ducts.

19. A method to control vibration, shock
and noise.
The process involves converting
dissipating energy by heat
Damping can be applied to mechanical
systems by
Viscous, - Dry fiction, - Hysterisis
damping

21. Noise radiating surfaces are wrapped with
acoustic materials.
Applied to piping, values, vessels etc.
The porous material attenuates the sound
waves by reflecting them back into the
structure and by converting it into heat.
Fibre glass or mineral wool of 300 mm
thickness  attenuation of 60 dB (A) at
sound wave frequency of 1000 Hz.

22. e.g. of laggings: fibre, polyurethane foam, lead foil,
knee cement, PVC sheet

23. Thank you!!!