Noise control is important for many products like computer hard drives and appliances. Sound is defined as vibrations stimulating the ear, and can propagate as longitudinal or transverse waves. Sound level is measured in decibels on a logarithmic scale, where a small increase represents a large increase in sound energy. Sources of noise include industrial sources like transportation and non-industrial sources like traffic. Methods to reduce noise include improving engineering to lower noise levels, establishing regulations for manufacturers, and reducing vibration at sources.

2. INTRODUCTION:
Noise control is becoming increasingly important for a
wide variety of OEM designers. Examples of products that
take noise control considerations into account during their
design cycles include equipment such as computer hard
drives, house appliances, material handling and
transportation equipment etc,.

3. DEFINITIONS OF SOUND:
Sound can be defined as the perception of vibrations
stimulating the ear. If scientifically taken into account,
sound is a periodic disturbance in fluids density or in
the elastic strain of a solid, generated by vibrating
objects. These waves or vibrations propagate in two
basic ways.
1. longitudinal waves.
2. Transverse waves

4. BASICS:
DECIBEL – Sound level is measured in decibel. Sound
level in decibels is a logarithmic rather than a linear
measure of change in pressure with respect to a
reference pressure level. A small increase in decibels
can represents a large increase in sound energy.

5. SOUND PRESSURE LEVEL:
We have difference in decibel, 20 log10 n1/2 = 20 log10
(P1/P2). If P2 were given an absolute value and 20log10
n1/2 were known then P1 would also have an absolute
value. In audio – acoustics, P1 is frequently given the
absolute r.m.s value of 2X10-5 N/M2which is the
minimum sound pressure fluctuation discernible to
human ear. Consequently P1 must also become an r
m s pressure expressed in N/M2. 20 log10 n1/2 is still in
decibels but is now defined precisely as the sound
pressure level (SPL).

6. WEIGHTING CURVES :
The apparent loudness of sound varies with frequency as
well as with sound pressure. To adjust the frequency
response of necessary systems to be similar to that of
human ear, several weighting curves proposed.
( Available range of sound in human beings are 20-20,000 Hz )

7. TYPES OF SOURCES OF NOISE :
Sources of noise are numerous but may be
classified broadly into two classes as
1. INDUSTRIAL
2. NON INDUSTRIAL

8. INDUSTRIAL :
The industrial may included noises from various
industries operating in cities like transportation,
air crafts, rockets, defense equipments,
explosions etc. The disturbing qualities of noise
emitted by industrial premises are generally its
loudness, its distinguishing features such as
tonal or impulsive components and its
intermittency and duration.

9. NON INDUSTRIAL :
In non industrial source , major one is traffic mopeds
and scooters with have a maximum of 75 dBA and 50
dBA respectively, according to new norms from 1st
January 2003. the present norms for petrol and
diesel driven two wheelers are 50dBA and 82 dBA
respectively. Present norms for three wheelers are
82 dBA for petrol and 85 dBA for diesel will be
modified as three wheelers upto 175 cc, 75 dBA and
above 175 cc, 80dBA. Passenger cars can produce
maximum noise of 75dBA present being 82dBA.

10. INTERNAL COMBUSTION
ENGINE NOISE :
Several alternative methods can be used to
classify internal combustion engine noise.
The two most typical classification
techniques are discussed below.

12. CLASSIFICATION BY NOISE CHARACTERISTICS:
One typical engine noise classification technique
separates the aerodynamic noise, combustion noise
and mechanical noise.
1. AERODYNAMIC NOISE
2. COMBUSTION NOISE
3. MECHANICAL NOISE

13. AERODYNAMIC NOISE-aerodynamic noise includes
exhaust gas and intake air noise as well as noise
generated by cooling fans, auxillary fans or any other air
flow.
COMBUSTION NOISE- combustion noise refers to noise
generated by the vibrating surfaces of the engine
structure, engine components and engine accessories
after excitation by combustion forces.
MECHANICAL NOISE-mechanical noise refers to noise
generated by the vibrating surfaces of the engine
components and engine accessories after excitation by
reciprocating or rotating engine components.

14. CLASSIFICATION BY ENGINE NOISE SOURCES:
A second approach to the classification of piston
engine noise involves the separation of engine noise
into the following categories –
1. EXHAUST SYSTEM NOISE
2. INTAKE SYSTEM NOISE
3. COOLING SYSTEM NOISE
4. ENGINE SURFACE RADIATED NOISE

15. NOISE EVALUATION DURING ACTUAL
RUNNING :

16. ENGINE APPLICATIONS
AND NOISE :
Application of the internal combustion engine
covers a broad spectrum. The internal
combustion engine is now used to power
anything from a small hand-held weed cutter to a
large ocean liner. One typical application is as
given below.

17. Stopping it at the source
1. Improving the engineering in many noisy objects has
cut noise nearly by 30 decibels (i.e. snow mobiles)
2. Government has set up regulations to manufacturers
such as GM and Mack truck to reduce vibration in heavy
gears, axles and transmissions.
3. Reducing sound at the sources by an average of 10
decibel cuts soundness in half.