Noise & it's Types
INTRODUCTIONS:-
Our ears are very
sensitive to the small pressure fluctuations caused by sound waves. This is
appropriate because sound is an integral component of the environment, and
plays a very important role in our normal development and survival by providing
a valuable channel of sensory-environmental contact. Often, however, sound may interfere with our activities;
disturb sleep, lead to auditory or other physiologic harm, or cause irritation
and annoyance. When this occurs, sound is generally unwanted or
undesirable, and is often referred as noise.
Sound becomes noise only when it is physiologically or psychologically defined
as unwanted. This subjective definition of noise involves intricate (complex)
physiologic, emotional, and psychological responses to sound. (Burns, 1973;
Environmental Protection Agency (EPA), 1973; kryter, 1970).
Noise is derived from the Latin word “nausea” implying ‘unwanted
sound’ or ‘sound that is loud, unpleasant or unexpected’. The noise
originates from human activities, especially the urbanization and the
development of transport and industry. Though, the urban population is much
more affected by such pollution, however, small town/villages along side roads
or industries are also victim of this problem. Noise is becoming an
increasingly omnipresent, yet unnoticed form of pollution even in developed
countries.
Noise may be defined as
sound which is objectionable, annoying or disturbing to some individuals and
may have damaging effect regardless of intensity that in many cases is not the
main issue.
Next air pollution and water
pollution noise is the third major pollution. The damaging effect of noise is
not only hearing loss or acoustic trauma but also other disagreeable or even
serious sign, symptoms and condition such as
Ø Rustle
in ear
Ø Increased
in blood pressure
Ø Permanent
functional changes
Ø Changes
in work effectiveness
Ø Extend
of fatigue after working hours
Ø -Decreased
quality of sleep
A noise hazard is an unwanted
sound that may damage person’s hearing.
Excessive can be generating within a business by planting machinery and loots,
compressed air steam process as well as externally from traffic or industrial
premises.
Types Of Noise :
Noise can be classified based on-
1) Frequency composition
2) Source of noise
3) Time duration.
Sources
of noise
Environment noise is a worldwide problem.
However, the way the problem is dealt with differs greatly from country to
country and is very much dependent on culture, economy, and politics. But the
problem persists even in areas where extensive resources have been used for regulating,
assessing and damping noise sources or for creation of noise barriers.
There are various sources from which
noise is exposed. Some sources are described below,
Resident
and community noise
In normal day to day activities,
various home appliances in our residence produce noise. Some of the major
sources are exhaust fans, lawn movers, mixis , grinders , cooling and heating
system , TV and music , motors used for pumping etc.
Community noise is one of the
major sources for affecting a large number of people living within the community.
It occurs due to various activities of community during religious festivals, fairs,
marriage, or public functions. The use of amplifiers provides one of the
frequent sources of disturbances. Loudspeakers systems are in almost continual
use at many open air events. Loudspeakers and amplified music are violently
used in unrestricted way at restaurant , marriage functions and clubs , religious and festive gathering and even in
sale of lottery tickets . There is also the perennial of the use of portable
radios and cassette players on beaches or in places where either people have
gone in search of peace and quiet. Most of the problems which occur in public ,
originated from these main source amplified sources i.e. loudspeakers , public
gatherings , festivals , indecency behaviour of people , rash driving with various kinds of horns music
with unnecessary use of vehicle.
Industrial
noise
Industrial machinery and processes
are composed of various noise sources such as motors , stators , gears , fans ,
vibrating panels , turbulent fluid flow impact processes electrical machine ,
internal combusting engines etc . The mechanism of noise generation depend on
the particularly noisy operations and equipment including crushing , riveting ,
blasting (quarries and mines ) , shake out (laundries ), punch presses , drop
forges , drilling, lathes pneumatic equipment ( e.g.:- jack hammers, chipping hammers,
etc),tumbling barrels, plasma jets, cutting
torches, sandblasting, electric furnaces, boiler making, machine tools for
forming, dividing and metal cutting such as punching pressing and shearing lathes,
milling machines and grinders, as well as textile machines, beverage filling
machines and print machines, pumps and
compressor, drive unit, hand-guided machines, self-propelled working machines,
in-plant conveying system and transport vehicles. On the top of this, there are
the information technology devices which are being encountered more and more in
all areas.
Noise is therefore a common
occupational hazard in a large number of work places such as the iron and steel industry, foundries, saw
mills, textile mills, airports and aircraft maintenance shops, crushing mills,
among many others.
Examples
of machinery noise sources
a) Industrial
gas jets:- Industrial jet noise probably rank
third as a major cause of hearing
damage after that of impact and material handling noise. Air jets are use
expensively for cleaning, for drying and ejecting part, for power tools, for
blowing of compressed air, for steam valves, pneumatic discharge vents, gas and
oil burners, etc. Typical sound pressure level at 1m from a blow-off nozzle can
reach 105dB (A). The flow velocity through the nozzle can become sonic i.e.,
reaches the speed of sound. This result s in a high generation of broad-band
noise with the highest values at a frequency band between 2 to 4HHz.
b) Ventilator
and exhaust fans:-
It’s rare not to find one or more
ventilators or exhaust fans in each department of an industrial or
manufacturing complex. Industrial fans are usually low speed, low static
pressure and have a large volume flow rate. There basic noise sources are:
Ø Broad band aerodynamic noise generated by the
turbulent flow.
Ø Discrete tones at the blade passing frequency
Fp(Hz).
Ø Mechanical noise due to mounting, bearing,
etc.
C) Compressor
Compressors are usually very
noisy machines with high pressure. The basic noise sources are caused by
trapping a definite volume of field and carrying it around the case to the
outlet with higher pressure. The pressure pulse from compressors are quite
severe, and equivalent sound pressure level can exceed 105dB (A). The noise
generated from compressors is periodic with discrete tones and harmonics
present in the noise spectrum.
d) Electric Motors
Noise from electrical equipment such as
motors and generators is generally a discrete low frequency. Super imposed on a
broadband cooling system noise. There are three basic sources involved in the
noise generated by electric motors:
Ø Broad
band aerodynamic noise generated from the end flow at the inlet/outlet of the cooling fan. The
cooling fan is usually dominant noise source.
Ø Discrete
frequency component caused by the blade passing frequencies of the fan.
Ø Mechanical
noise caused by bearing, casing vibration, motor balancing shaft misalignment
and motor mounting.
e) Woodworking
machines
Wood working machinery uses
operations, such as cutting, milling, shaping etc. Three basic noise sources
are involved:
Ø Structure
vibration and noise radiation of the work piece or cutting tool (Such as a
circular saw blade) and machine frame, especially at the mechanical resonance
frequencies.
Ø Aerodynamic
noise caused by turbulence, generated by tool rotation and the work place in
the air flow field.
Ø Fan
dust and chip removal air carrying systems.
Equivalent sound pressure levels in the
furniture manufacturing industry can reach 106dB (A).
f) Pneumatic
tools
There are three basic types of
sources that dominate the noise generated:
Ø Noise
produced by contact between the machine and the working surface. The vibration
transmitted from the tool trends to vibrate the working surface and work bench,
generating high vibration.
Ø Exhaust
air noise caused by the turbulent flow generated in the air exhaust.
Ø Sound
radiation from tool vibration caused by air flow inside the tool.
MUSIC:
The "music and entertainment sectors" are defined as "all
work places where live music is played, or recorded music is played in a
restaurant, bar, public house, discotheque,
or night club ,or alongside live music or a live dramatic or dance
performance". This definition has a wide application including orchestras,
bands,concert halls,theatres,rock concerts,marching bands,pubs,clubs etc. It
encompasses not just musicians,but also,for example,sound engineers and
security and bar staff.
Music is perceived as pleasurable an is sometimes loud to produce its
effect. The sound of a pneumatic drill is not regarded as pleasurable,but both
music and pneumatic drills cause pressure fluctuations (noise) and if this
noise is too high,or carries on for too long,people's hearing may be damaged.
TRAFFIC:
Road
Traffic Noise:
Road traffic is the most widespread source of noise in all countries and
the most prevalent cause of annoyance and interference. It's directly
proportional to the volume of vehicles. Increasing of population is increasing
of vehicles and hence increasing of noise pollution. The major sources of noise
in automobiles are exhaust, intake, engine and fan, and tires at high speed.
The noise output of all conponents increases with speed. As an example, keeping
all the conditions same, an approximately 10dB(A) increase with speed of a car
from 30mph to 60mph. The road traffic noise not only depends on volume of
vehicles and also depends on several factors,some of them are below:
Ø Road
conditions
Ø Traffic
clearance
Ø Condition
of vehicles
Ø Speed
of the vehicles
Ø Common
sense
The people living near roadside (highway ) are mainly exposed .
Rail Traffic Noise:
Noise from the trains include : 1) Diesel exhaust, 2) Engine & Air
intake, 3) Cooling fans, 4) Wheel-rail interaction, 5) Electric generator and electric
traction motor, 6) Miscellaneous noise generated in freight and passenger cars
etc. Other intermittent noises includes car impact sounds when trains are
braking and accelerating and the sounds of sirens or horns that produces 10 to
20dB(A) higher than from other sources. The noise generated in communities
having long haul or rapid transit vehicles (like metros) in their environment
will depend primarily on track observer distance, frequency and train length
and condition of track. The rapid transit trains arw short, so that their noise
duration time is much less than that of good's trains. Rapid transit vehicles
produce lower level of noise than compare to lengthy trains.
Aircraft Noise:
Noise from commercial aircraft is only a problem around airports as this
is where aircraft converge at low altitude and high engine power. Increasing
air traffic with different types of aircraft's and city expansion will
exacerbate the noise problems. Noise pollution of an aircraft will increase
with the speed. The aircraft noise reduction; trafic and flight path
restrictions can alleviate them. The invention of supersonic aircraft's has
added more noise for the plight of persons who live near Aerodromes. Taking off
and landing of an aircraft produces unbearable noise. It has been observed that
supersonic jet planes are one of the biggest irritants in today's noisy world.
The noise of these planes may sometimes break window panes , crack on walls and
shakes the buildings also. By these effects of noise one can very easily
understand that what would be the effects of such noise on human body.
OTHERS:
Agricultural Machines:
Tractors, thrashers, harvesters, tube wells, powered tillers etc. have
all made agriculture highly mechanical but , at the same time highly noisy.
Noise level 90dB to 98dB due to running of farm machines have been recorded in
the state of Punjab.
Defence Equipment:
A lot of noise pollution is added to the atmosphere by artillery,tank,
launching of rockets, explosions, exercising of military airplanes and shooting
practices. Screams of jet engines and sonic booms have a deafening impact on
the ears and in extreme cases have been known to shatter the window panes and
old dilapidated buildings.
BASED ON FREQUENCY COMPOSITION:
White Noise:
White noise is a sound that contains every frequency within the range of
human hearing( generally from 20HZ to 20KHZ) in equal amounts. Most people
perceive this sound as having more high frequency content than low, but this is
not the case. This perception occurs because each successive octave has twice
as many frequencies as the one preceding it. For example, from 100Hz to 200Hz,
there are one hundred discrete frequencies. In the next octave(from 200Hz to
400Hz), there are two hundred frequencies.
White noise can be generated on a sound synthesizer. Sound designers can
use this sound , with some processing and filtering, to create a multitude of
effects such as wind, surf, space whooshes, and rumbles.
Pink noise is a varient of white noise. Pink noise is white noise that
has been filtered to reduce the volume at each octave. This is done to
compensate for the increase in the number of frequencies per octave. Each
octave is reduced by 6dB, resulting in a noise sound wave that has equal energy
at every octave.
Broadband Noise:
Broadband noise has a frequency spectrum where there are no discrete or
dominant tones. Sound pressure fluctuations of broadband noise are non-periodic
in nature with relatively random phase and amplitude. Although devoid of
discrete frequencies , the acoustical energy of broadband noise may still be
largely concentrated in one or more areas of the spectrum. Example of broadband
noise are shop air blow-offs, gas fired burners, jet engines and grinding
tools.
Speech Noise:
This is an outcome of white noise. It is obtained by filtering white
noise above 1KHz at the rate of about 12dB per octave. This has more energy in
the low frequency spectrum than white noise and resembles like the spectrum
speech.
Complex Noise:
This is a type of broadband noise having a low frequency fundamental
plus multiple of the fundamental. Usually the base frequency varies between
60-120Hz and acoustic energy is present upto 4KHz. Energy decreases as
frequency increases. The noise is buzzing like quality and mask most
effectively in the lower frequency . There are two types of complex noise-
a) Square wave noise: - This is achieved
by generating a square wave and its harmony.
b) Saw tooth noise: - This is achieved
by generating a saw tooth noise and taking multiple of the reparative wave.
Bands stop Noise:
It is generated by using band stop filter which block certain range of
frequency and allows frequency either side of this range to be passed. For
example , one may wish to block 'Beta wave' ranging from 16 to 32 Hz from
reoccurring but, record will be the other frequency i.e., 0-15Hz and 33-100Hz.
Threshold Equalisation noise:
It is a broadband noise designed to produce almost equal masked
threshold in dBSPL over a wide range of frequency of normal hearing listener
and listener with hearing impairment with dead region.
Brown/Red Noise:
Red noise or brown noise is a kind of signal noise produced by Brownion
motion. The graphic representation of the sound signal mimics a Brownion
pattern. Its spectral density is proportional to 1/F square, meaning it has
more energy at lower frequency even more so than pink noise. It decreases power
by 6dB per octave and is air like soft quality compare to white and pink noise.
It can be generated by integrating white noise.
Blue Noise:
Blue noise power density increases 6dB per octave with increasing
frequency over a frequency range. In computer graphics the term blue noise is
sometimes used more loosely as any other noise with minimal low frequency
component and low concentrated spikes in energy.
Purple Noise:
Power density of this noise increases 6dB per octave with increasing frequency
over a frequency range. It is also known as violet noise.
BASED ON DURATION
Based of Duration the noise mainly based on two types :
i) Steady Noise
ii) Non-steady Noise
Steady Noise:
Continuous Noise:
Continuous noise is what it says on
the tin. It is noise that is produced continuously by machinery that keeps
running without interruption. This could come from factory equipment, heating
or ventilation system.Non steady Noise:
- Fluctuating Noise:
Fluctuating noise refers to random noise. It is a noise whose level
varies continuously and to an appreciable extent during the period of
observation .
- Impulsive Noise:
In a broad sense,
impulses can be generated in two ways and in two extreme types of environments:
1) the result of explosions (burst balloon, firecracker, gunfire), and 2) the
result of the collision of two objects (Hammer and nail, punch press etc).
Furthermore each type of impulse can occur either in anechoic (free field
conditions) or in reverberant surroundings.
An explosive discharge
in a free field yields the most simple p-t configuration and is referred as an
“A” type of impulse/ Friedlander wave. And the time course of type A impulse is
usually described by the duration of the condensation phase/ Positive phase of
the wave.
When an A type of
impulse is generated in not completely free field conditions, i.e., if there
are reflecting surfaces nearby, the p-t history reflect their presence by
showing the reflected components. These impulses are often referred to as “impact noise” or as a “B” type of wave. The duration of the
impact noise is defined as the duration from the beginning of the wave to a
point on envelop of the impact that is either 10 or 20 dB down from the peak
(Coles et al., 1968). These are often product of the collision of two excited
objects (Hammer and nail, punch press etc) in closed surroundings.
Impact noise can also
be described as one type of structure-borne noise, occurs when an object
strikes a partition and sets into mechanical vibration. In most cases it is
generated in floor-ceiling systems of multistory buildings by footsteps,
dropped objects, moved furniture etc. An impact sound from adjoining spaces has
an intrusive character and is often more annoying than a continuous noise. The impact insulation class (IIC) is a
single number indicating how a floor-ceiling system reduces impact noise. The
higher the IIC rating, the larger the floor-ceiling insulation for impact
sounds.
Note: Intermittent
noise is more annoying than continuous noise (if the duration is maintained).
Parameters
for evaluating Impulse/ Impact noise:
·
Peak Level of impulse
(Lpp):
Single round of
exposure damages the middle ear. In case of multiple rounds of exposure the
damage occurs both in the cochlea as well as middle ear. There is a high
positive correlation seen between the degree of the hearing loss and Lpp.
·
Pulse duration (T):
In general as time
increases, damage to auditory system also increases. With shorter time the
middle ear is more prone to damage. Longer the pulse duration, the damage
spreads to cochlea.
·
Number of impulses:
As number of rounds increases,
over all hearing loss increases. The middle ear is exclusively damaged in the
first round of exposure. When number of rounds is more than three, damage
spreads to cochlea.
·
Inter impulse interval:
When inter impulse
interval is equal to 0.1sec the damage would be extendable to cochlea and the
loss is less severe. But if the inter impulse interval is between 10 to 60 sec
the damage is more severe.
·
Spectral content in the
impulse:
When Lpp is maintained
constant but the duration increased then low frequency energy is increased. But
if the duration is maintained and Lpp is increased then the high frequency
energy is increased. Energy with low frequency is less harmful than higher
frequency energy.
- Intermittent Noise:
Intermittent noise is a noise level that increases and decreases rapidly . This might be a freight train passing by, factory equipment that operates in cycles or aircraft overhead.
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