Module 5 of ME 010 702 DYNAMICS OF MACHINES

MODULE - V
ACOUSTICS AND ENVIORNMENTAL NOISE CONTROL
Binil Babu,
Dept. Of Mechanical Engineering,
SNMIMT,Maliankara

ACOUSTICS
 Sound propagation:
 Decibels
 Acceptance noise levels
 Acoustical Measurements
 Doppler Effect
 Microphone and Loud speakers
 Recording and reproduction of sound
 Acoustic impedance

 Explain sound Propagation
Sound propagates through air as a longitudinal wave.
The speed of elastic waves is related to two properties
of the medium: elasticity and density. The speed, in
meters per second, at which a sound wave travels
through a media is the speed of propagation, c.

 Decibel
 Decibel is a unit used to measure the intensity of a
sound by comparing it with a given level on a
logarithmic scale.

 Tolerance level of human ear in industrial context.
If the sound goes beyond the tolerance level, it is
classified as noise, but this depends on the
characteristics of the particular person as well as the
sound. Almost every aspect of modern life
generates noise – industrial processes, ... Human
ears respond to frequencies between 20Hz and
20,000Hz.According to OSHA (Occupational Safety
and Health Administration)there is some standard
permissible time for human under various noise level
as given below:-

Sound pressure level(dB) Permissible time (hours)
80 32
85 16
90 8
100 3
110 0.5
115 0.25
120 0.125

 Doppler Effect.
The Doppler effect (or the Doppler shift) is the
change in frequency or wavelength of a wave for
an observer who is moving relative to the wave source.
Eg: change of pitch heard when a vehicle sounding
a siren or horn approaches, passes, and recedes from
an observer. Compared to the emitted frequency, the
received frequency is higher during the approach,
identical at the instant of passing by, and lower during
the recession

 Acoustic impedance
 Acoustic impedance is a measure of the opposition
that a system presents to the acoustic flow resulting of
an acoustic pressure applied to the system.
 Sound travels through materials under the influence
of sound pressure. Because molecules or atoms of a
solid are bound elastically to one another, the excess
pressure results in a wave propagating through the
solid.
 The acoustic impedance (Z) of a material is defined
as the product of its density (p) and acoustic velocity
(V).
 Z = pV

 SOUND LEVEL METER
 A sound level meter is used for acoustic (sound that
travels through air) measurements. It is commonly a hand-
held instrument with a microphone. The diaphragm of the
microphone responds to changes in air pressure caused by
sound waves.
 They measure sound pressure and allow for the evaluation
of a variety of signal attributes such as maximum and
minimum levels, equivalent energy levels and event levels.
 Sound level meters are commonly used in noise
pollution studies for the quantification of different kinds of
noise, especially for industrial, environmental and aircraft
noise.

Module 5 of ME 010 702 DYNAMICS OF MACHINES

INDUSTRIAL NOISE CONTROL
STRATEGIES
 The noise from machines, flow induced noise and
impact of tool and work-piece etc are the main noise
sources in industries. There are many methods are
currently available for control of industrial noise. The
following are the various strategies used for noise
control.
 NOISE CONTROL AT THE SOURCE
Most economical and efficient method of noise control.
Noise control at source classified :-
(a) Through vibration control

In this method noise reduction is achieved by reducing
the vibration of machine. The vibration of machines is
mainly controlled by proper isolation ,lubrication and
replacement of existing design etc.
Lubrication – vibration and noise due to dry friction
Isolators like rubber, steel spring – foundation for heavy
machines.
 CHANGING LOCATION OR ORIENTATION OF
NOISE SOURCES
The intensity of sound decreases as square of distance
from the noise source. Therefore by placing the high
noise producing equipments at the farthest position in
industry where workers exposure to noise will be less.

 CHANGES IN DESIGN AND PROCESS
In this method by changing the tools and design of
machines etc the noise is controlled.
 USING SOUND ENCLOSURES
In this method the noise reduction is achieved by
providing suitable enclosure around the source. The
enclosures are of two types, full enclosures and partial
enclosures. Even though it is a good method the
efficiency of enclosures depends on design. It is also
very difficult to cover large machineries.
 NOISE CONTROL ALONG THE PATH
In this method the noise is controlled along the the path
which sound waves travel.

 It not only control the noise but also reduce the
reverberation. Generally highly sound absorbent
materials are used to control noise, there by the
intensity of sound reduced each time when sound
waves reflected..
 ACOUSTIC BARRIERS
 In this method acoustic barriers having heavy non
porous material or combination of two such material is
used . When sound wave strikes on these barriers
some amount of energy is absorbed.
 NOISE CONTROL AT THE RECEIVER
Ear defenders, ear muff
Ear plugs, semi inserters

Problem-1
 In the same area of a ware house there are four large
machines. M/c-1 produces a sound power of 1 W. M/C-
2,3 and 4 produces an acoustical power of 0.5 W,0.75W
and 1.25W respectively. What is the total power level
generated in the area by four machines.
Soln:
Sound power of M/c-1 = 1 W
Sound power of M/c-2 = 0.5 W
Sound power of M/c-3 = 0.75 W
Sound power of M/c-4 = 1.25W

 SOUND POWER LEVEL OF MACHINES
Sound power level Lw =10log10(W/Wref) dB, where
Wref = 10^-12 W.
Lw1 = 10 log10 (1/10-12) = 120 dB
Lw2 = 10 log10 (0.5/10-12) = 116.98 dB
Lw3 = 10 log10 (0.75/10-12) = 118.75 dB
Lw4 = 10 log10 (1.25/10-12) = 120.96 dB
SOUND INTENSITY LEVEL OF MACHINES
We know that the relation between sound intensity level
and sound power level
Lw = Li +10log10 S, where S = 4Πr2

Since area is not given ,Let us take S= 1 m2
Therefore, Lw = Li +10log10 1 ; Lw = Li
Li (1) = 120 dB
Li(2) = 116.98 dB
Li(3) = 118.75 dB
Li(4) = 120.96 dB
Now, convert Sound intensity level into sound
pressure level
The relationship between sound intensity level and
sound pressure level is given by,
Li = Lp -0.16 ; Lp = Li +0.16

Lp (1) = 120+0.16 =120.16 dB
Lp (2) = 116.98+0.16 = 117.14 dB
Lp (3) = 118.75 +0.16 = 118.91 dB
Lp (4) = 120.96 +0.16 = 121.12 dB
TOTAL SOUND PRESSURE LEVEL
Lp = 10log10 (10Lp1/10+ 10Lp2/10+ 10Lp3/10 +10Lp4/10 )
Lp = 125.59 dB
Total sound intensity level, Li = Total sound pressure
level – 0.16

Therefore Li = 125.59-0.16 = 125.43 dB
Since Li = Lw; The total sound power level of four
machines = 125.43 dB

Module 5 of ME 010 702 DYNAMICS OF MACHINES

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Module 5 of ME 010 702 DYNAMICS OF MACHINES