NOISE POLLUTION AND ITS CONTROL

1
NOISE POLLUTION AND ITS CONTROL
Goal: The goal of this module is to introduce the reader the basic concepts of noise pollution and
its control.
Learning Objectives: To achieve the goal, the following learning objectives (LO) with selected
thrust areas are identified.
LO-1: What is noise ? (Definition, theoritical concept)
LO-2: How it is measured ? (Concept, mathematical formulae)
LO-3: Frequency analysis (Concept)
LO-4: Where does noise emanate from? (Concept, application)
LO-5: Why bother about noise ? (Concept)
LO-6: Control of noise pollution (Concept, technic, application)
LO-7: How to document the reports? (Concept)
LO-8: What does the Regulatory guidelines prescribe? (Concept, application)
The emphasis of this module is to make the reader understand the basic concepts of noise pollution
and its control and co-relate with the happenings at outside world.
By the time you finish reading this module, you can
Identify the sources of noise pollution
Grasp the various adverse impacts of noise pollution
Quantify the noise levels
Develop methodologies to control noise pollution
Document the noise levels in a systemmatic approach
Get familiar with the Statutory limits for both the ambient noise levels and the noise levels at a
workspace environment
Besides
Be able to execute assignments in your locality pertaining to noise pollution identification,
quantification and control.
We often hear from people who have returned from shopping in the market that, there is a lot of
noise pollution in the city. They often blame the two, three - and four wheelers besides commercial
units for increasing ambient noise pollution. On a special day like festivals, marriage functions,
birthday parties etc., we can hear loud speakers drilling the common man with severe noise
pollution. Let us get familiar with some basic concepts pertaining to noise pollution and its control.2
What is noise?
In simple terms, noise is unwanted sound. Sound is a form of energy which is emitted by a
vibrating body and on reaching the ear causes the sensation of hearing through nerves.
Sounds produced by all vibrating bodies are not audible. The frequency limits of audibility
are from 20 HZ to 20,000 HZ.
A noise problem generally consists of three inter-related elements- the source, the
receiver and the transmission path. This transmission path is usually the atmosphere
through which the sound is propagated, but can include the structural materials of any
building containing the receiver (See Fig. 1)
Source Path Receiver
Barrier
Source Path Receiver
Fig. 1 Inter-relationship between the elements of noise
Noise may be continuous or intermittent. Noise may be of high frequency or of low
frequency which is undesired for a normal hearing. For example, the typical cry of a child
produces sound, which is mostly unfavorable to normal hearing. Since it is unwanted
sound, we call it noise.
The discrimination and differentiation between sound and noise also depends upon the
habit and interest of the person/species receiving it, the ambient conditions and impact of
the sound generated during that particular duration of time. There could be instances that,
excellently rendered musical concert for example, may be felt as noise and exceptional
music as well during the course of the concert!
Sounds of frequencies less than 20 HZ are called infrasonics and greater than 20,0000 HZ
are called ultrasonics. Since noise is also a sound, the terms noise and sound are
synonymously used and are followed in this module.
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LO - 13
How it is computed?
The intensity of sound is measured in sound pressure levels (SPL) and common unit of
measurement is decibel, dB. The community (ambient) noise levels are measured in the A
- weighted SPL, abbreviated dB(A). This scale resembles the audible response of human
ear. Sounds of frequencies from 800 to 3000 HZ are covered by the A - weighted scale. If
the sound pressure level, L1 in dB is measured at r1 meters, then the sound pressure level,
L2 in dB at r2 meters is given by,
L2 = L1 - 20 log10 (r2/r1) ...... (1)
If the sound levels are measured in terms of pressure, then, sound pressure level, LP is
given by,
LP = 20 Log10 (P/Po) dB(A) ...... (2)
The Lp is measured against a standard reference pressure, Po = 2 x 10
-5
N/m
2
which is
equivalent to zero decibels. The sound pressure is the pressure exerted at a point due to a
sound producing source (see. Fig. 2)
o ) ) ) ) ) *)SPL
Fig. 2 Definition of sound pressure
Day-night equivalent noise levels (Ldn): The day night equivalent noise levels of a
community can be expressed as -
Ldn , dB(A) = 10 x log10 [15/24 (10
Ld/10
) + 9/24 (10
(Ln + 10)/10)
] ............... (3)
where, Ld = day-equivalent noise levels (from 6AM - 9 PM), dB (A)
Ln = night equivalent noise levels (from 9 PM - 6 AM), dB (A)
The day hours in respect to assessment of noise levels, is fixed from 6 AM - 9 PM (i.e., 15
hrs) and night hours from 9 PM - 6 AM (i.e., 9 hrs). A sound level of 10 dB is added to Ln
due to the low ambient sound levels during night for assessing the Ldn values.
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LO - 24
Addition of sound levels: The effective sound levels form two or more sources cannot
be simply added algebraically. For example, the effective sound level from two air
conditioners 60 dB(A) each, say is not 60 + 60 = 120 dB (A) but 60 + 3 = 63 dB(A). (See
table 1). Similarly, the effective sound level of 57 dB, 63 dB, 63 dB, 66 dB and 69 dB is 72
dB. The computation is illustrated below.
57
63
66
63 72
69
66
72
69
Table 1 Addition of sound levels, L1 and L2 (L1 > L2)
L1 - L2, dB Add to L1
0 or 1 3 dB
2 or 3 2 dB
4 - 8 1 dB
9 or more 0 dB
Source : Ref. (9)5
Frequency analysis
(3,9 )
The frequency analysis allow to separate the main components of the signals by dividing
the frequency range of interest into smaller frequency bands using a set of filters. We may
distinguish between noises that consist of regularly repeated or periodic sounds and those
that consist of aperiodic sounds. The simplest periodic sound is a pure tone i.e., a
pressure disturbance that fluctuates sinusoidally at a particular frequency. The lower the
frequency, the longer is the wave length (wavelength = velocity of sound/frequency).
The noise produced by most sources of community noise, such as automobiles or aircraft
engines, are examples of aperiodic sounds. Such sounds cannot be subdivided into sets
of harmonically related pure tones but can be described in terms of components extending
over finite frequency bands. Such frequency analysis are often done in bands of octaves
or 1/3 octaves.
An octave band is a frequency band with upper and lower cutoff frequencies having a ratio
of 2. The cut off frequencies of 707 HZ and 1414 HZ define an octave band, whose band
centre frequency is 1000 HZ and would be referred to as the 1000 HZ octave band.
Frequency analysers can be divided into two groups viz. constant band width analyser and
constant percentage bandwidth analyser. In the constant bandwidth analyser the filter
bandwidth is kept constant throughout the frequency range while in the constant
percentage bandwidth analyser, the bandwidth is proportional to the centre frequencies.
The constant percentage bandwidth analyser is widely used. The nine preferred centre
frequencies for noise level measurement are 31.5, 63, 125, 250, 500, 1000, 2000, 4000
and 8000 HZ.
(3)
As already mentioned, dB(A) values give emphasis to sounds in the range of about 800 to
3000 HZ. Since the sound generating frequencies are not fully covered under dB(A), for
detailed evaluation and engineering design, the multiple-number descriptions provided by
frequency analysis are often required.
Noise measurement instruments
(3)
Noise measurement is an important diagnostic tool in noise control technology. The
objective of noise measurement is to make accurate measurement which give us a
purposeful act of comparing noises under different conditions for assessment of adverse
impacts of noise and adopting suitable control techniques for noise reduction. The various
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LO - 36
equipment used for noise level measurement are summarised at Table 2. The principle
and the components of noise measuring instruments is summarised below.
A sound level meter consists basically of a microphone and an electronic circuit including
an attenuator, amplifier, weighting networks or filters and a display unit. The microphone
converts the sound signal to an equivalent electrical signal. The signal is passed through a
weighting network which provides a conversion and gives the sound pressure level in dB.
The instructions laid down by the noise level meter manufacturers shall be followed while
using the instruments.
The time constants used for the sound level meter standards are
(3)
S (Slow) = 1 second
F (Fast) = 125 milli seconds
Relatively steady sounds are easily measured using the "fast" response and unsteady
sounds using "slow" response. When measuring long-term noise exposure, the noise level
is not always steady and may vary considerably, in an irregular way over the measurement
period. This uncertainty can be solved by measuring the continuous equivalent level,
which is defined as, the constant sound pressure level which would have produced the
same total energy as the actual level over the given time. It is denoted as Leq. The display
of Leq facility is also available in certain models of sound level meters. This is the desired
parameter for assessment of ambient noise levels.
Table 2 Equipment used in the measurement of noise levels
S.No. Equipment Specification/Area of usage
1. Sound level meter Type-0 : Laboratory reference standard
Type-1: Lab use and field use in specified controlled
environment
Type-2: General field use (Commonly used)
Type-3: Noise survey
2. Impulse meters For measurement of impulse noise levels e.g. hammer
blows, punch press strokes etc.
3. Frequency analysers For detailed design and engineering purpose using a set of
filters.
4. Graphic recorders Attached to sound level meter. Plots the SPL as a function
of time on a moving paper chart.
5. Noise dosimeters Used to find out the noise levels in a working environment.
Attached to the worker
6. Calibrators For checking the accuracy of sound level meters.
Source: Ref. (3)7
Noise Sampling
Bureau of Indian Standards (BIS) has published several code books for sampling and
analysis of noise pollution and guidelines for control of noise pollution from domestic and
industrial sources. The reader is advised to refer to the BIS code books (table 3) for a
better understanding of methods of noise sampling. For sampling of noise levels from
industrial sources, noise levels in the different octave bands are measured by a sound
level meter in conjunction with octave - band filters at the workers ear level or at about a
distance of one meter from the source of noise.
Table 3 Selected BIS code books on noise pollution
BIS Code Description
IS-4954-1968 Noise abatement in town planning recommendations
IS-3098-1980 Noise emitted by moving road vehicles, measurement
IS-10399-1982 Noise emitted by stationary road vehicles, methods of measurement of
IS-6098-1971 Air borne noise emitted by rotating electrical machinery, method of
measurement of
IS-4758-1968 Noise emitted by machines, methods of measurements of
IS-3483-1965 Noise reduction in industrial buildings, code of practice for
IS-1950-1962 Sound insulation of non-industrial buildings, code of practice for
IS-9167-1979 Ear protectors
Source Ref. (3 )8
Sources of noise
Where does it generate from? The sources of noise may vary according to daily
activities. They sources may be domestic (movement of utensils, cutting and peeling of
fruits/vegetables etc.) natural (shores, birds/animal shouts, wind movement, sea tide
movement, water falls etc.), commercial (vendor shouts, automobiles, aeroplanes,
marriages, laboratory, machinery etc.) industrial (generator sets, boilers, plant operations,
trolley movement, transport vehicles, pumps, motors etc.). The noise levels of some of the
sources are summarised at table 4.
Typical surveys pertaining to causes of noise pollution, reveal the various sources of noise
pollution and frequency variation of their occurrences. The results of a survey conducted in
Central London, way back in 1961-62 reveals the presence of noise pollution even in the
early ‘60s (Table 5). Road traffic is identified as the major source of noise pollution while at
home or outdoors or at work.
Table 4 Typical noise levels of some point sources
Source Noise level dB(A) Source Noise level, dB(A)
Air compressors 95-104 Quiet garden 30
110 KVA diesel generator 95 Ticking clock 30
Lathe Machine 87 Computer rooms 55-60
Milling machine 112 Type institute 60
Oxy-acetylene cutting 96 Printing press 80
Pulveriser 92 Sports car 80-95
Riveting 95 Trains 96
Power operated portable saw 108 Trucks 90-100
Steam turbine (12,500 kW) 91 Car horns 90-105
Pneumatic Chiseling 118 Jet takeoff 120
Source: Ref. 3,4
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LO - 49
Table 5 Noise that disturb people- 1961-62 Central London Survey
S.No. Description of noise No. of people disturbed per 100 questioned
When at home When outdoors When at work
1 Road traffic 36 20 7
2 Aircraft 9 4 1
3 Trains 5 1 0
4 Industry / Construction work 7 3 10
5 Domestic appliances 4 0 4
6 Neighbors impact 6 0 0
7 Children 9 3 0
8 Adult voices 10 2 2
9 Radio/TV 7 1 1
10 Bells/alarms 3 1 1
Source: Ref. ( 9)
The variations in the emission of noise levels in a particular environment can be assessed
from the statistical distribution of noise levels in that environment (See Fig 3). To draw a
statistical distribution curve, terms like L10, L50 and L90 play an important role.
The Sound levels exceeding 10%, 50% and 90% of the total time intervals during a
particular period are designated as L10, L50 and L90 respectively.
From figure, it can be seen that, 90% of the sound levels are about 64 dB(A). Local
disturbances increased the sound levels (L10) to 76 dB(A), i.e., during 10% of the total
time. L90 represents the background noise levels.
Fig. 3 Statistical distribution of noise levels10
The equivalent noise levels, Leq can also be calculated as
(5)
Leq = L50 + (L10 - L90)
2
/ 60
Noise Climate (NC): It is the range over which the sound levels are fluctuating in an
interval of time
(5)
NC = L10 - L90
Hence, Leq in the above example is -
Leq = 69 + (76 - 64)
2
/60 = 71.4 dB.
and noise climate, NC = 76 - 64 = 12 dB/sampling time.
Think a bit and do it
· Observe the activities in your house which produces annoyance to you. Try to record the
frequency of their occurrence in a day/week/month etc.
· Identify the noise generating sources in your neighborhood.
· Compute reduction of effective noise levels at your house from your neighborhood using
equation (1).
· Tabulate and analyse. what did you find?
Aircraft Noise
The noise of aircraft is described in terms of Perceived Noise Levels (PNL), a scale of
noisiness, expressed in pNdB. There is no simple relationship between the dB(A) value
and pNdB value for all noises. However, a useful statement is that, the pNdB value for a
noise is approximately 13 units greater than the dB(A) value for the noise.
A further refinement resulting from the study of aircraft noise is the Effective Noise Level, a
scale of noisiness of a time-varying event, expressed in EPNdB. It is used to describe the
noise of a single aircraft activity. In order to describe the noise exposure associated with
an airport, the EPNdB values are supplemented with such information as the number of
flights of each aircraft type, the flight paths that the aircraft use and the time of day at
which the operations occur. The resulting picture is often presented in such terms as
Noise Exposure Forecast (NEF) contours, which are intended to represent the long-term
average noise exposure in communities around airports. More detailed information on the
subject is available with Ref. 9.11
Impacts of noise
Why bother about noise? Often neglected, noise induces a severe impact on humans
and on living organisms. Some of the adverse effects are summarised below.
· Annoyance: It creates annoyance to the receptors due to sound level fluctuations.
The aperiodic sound due to its irregular occurrences causes displeasure to hearing
and causes annoyance.
· Physiological effects: The physiological features like breathing amplitude, blood
pressure, heart-beat rate, pulse rate, blood cholesterol are effected.
· Loss of hearing: Long exposure to high sound levels cause loss of hearing. This is
mostly unnoticed, but has an adverse impact on hearing function.
· Human performance: The working performance of workers/human will be affected as
they'll be losing their concentration.
· Nervous system: It causes pain, ringing in the ears, feeling of tiredness, thereby
effecting the functioning of human system.
· Sleeplessness: It affects the sleeping there by inducing the people to become
restless and loose concentration and presence of mind during their activities
· Damage to material : The buildings and materials may get damaged by exposure to
infrasonic / ultrasonic waves and even get collapsed.
Have you observed?
· Any change in your personal reactions like annoyance, tiredness, loss of concentration while
executing your daily activities?
· Any noise polluting source in your surroundings? If so, for how much time, you are exposed to
the noise levels?
· Any noise level measurements being taken at your surroundings? What did they say?
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LO - 512
Control of Noise Pollution
Noise generation is associated with most of our daily activities. A healthy human ear
responds to a very wide range of SPL from - the threshold of hearing at zero dB,
uncomfortable at 100-120 dB and painful at 130-140 dB
(3)
Due to the various adverse .
impacts of noise on humans and environment (See LO-5), noise should be controlled. The
technique or the combination of techniques to be employed for noise control depend upon
the extent of the noise reduction required, nature of the equipment used and the economy
aspects of the available techniques.
The various steps involved in the noise management strategy is illustrated at Fig 4.
Reduction in the noise exposure time or isolation of species from the sources form part of
the noise control techniques besides providing personal ear protection, engineered control
for noise reduction at source and/or diversion in the trajectory of sound waves.
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LO - 613
Fig. 4 Noise Management Strategy (Source : Ref : )
The techniques employed for noise control can be broadly classified as
(4,5,9,10)
· Control at source
· Control in the transmission path
· Using protective equipment.
NOISE SURVEY
NO
RISK
NOISE HAZARD
ZONE
REDUCE
Leq
REDUCE
WORKING LIFE
REDUCE
NOISE LEVEL
REDUCE
EXPOSURE TIME
NOISE
REDUCTION
AT SOURCE
INTERRUPT
TRANSMISSION
PATH
ISOLATE
PERSON
PERSONAL
EAR
PROTECTION
NOISE
ATTENUATING
REFUGES
JOB
ROTATION
MONITORING
AUDIOMETRY14
Noise Control at Source
The noise pollution can be controlled at the source of generation itself by employing
techniques like-
· Reducing the noise levels from domestic sectors: The domestic noise coming
from radio, tape recorders, television sets, mixers, washing machines, cooking
operations can be minimised by their selective and judicious operation. By usage of
carpets or any absorbing material, the noise generated from felling of items in house
can be minimised.
· Maintenance of automobiles: Regular servicing and tuning of vehicles will reduce the
noise levels. Fixing of silencers to automobiles, two wheelers etc., will reduce the noise
levels.
· Control over vibrations: The vibrations of materials may be controlled using proper
foundations, rubber padding etc. to reduce the noise levels caused by vibrations.
· Low voice speaking: Speaking at low voices enough for communication reduces the
excess noise levels.
· Prohibition on usage of loud speakers: By not permitting the usage of loudspeakers
in the habitant zones except for important meetings / functions. Now-a-days, the urban
Administration of the metro cities in India, is becoming stringent on usage of
loudspeakers.
· Selection of machinery: Optimum selection of machinery tools or equipment reduces
excess noise levels. For example selection of chairs, or selection of certain
machinery/equipment which generate less noise (Sound) due to its superior
technology etc. is also an important factor in noise minimisation strategy.
· Maintenance of machines: Proper lubrication and maintenance of machines,
vehicles etc. will reduce noise levels. For example, it is a common experience that,
many parts of a vehicle will become loose while on a rugged path of journey. If these
loose parts are not properly fitted, they will generate noise and cause annoyance to
the driver/passenger. Similarly is the case of machines. Proper handling and regular
maintenance is essential not only for noise control but also to improve the life of
machine.15
Control in the transmission path
(9,10)
Please recall the Fig 4 wherein the inter-relationship between elements of noise was
represented. The change in the transmission path will increase the length of travel for the
wave and get absorbed/refracted/radiated in the surrounding environment. The available
techniques are briefly discussed below.
Installation of barriers: Installation of barriers between noise source and receiver can
attenuate the noise levels. For a barrier to be effective, its lateral width should extend
beyond the line-of-sight at least as much as the height (See Fig. 5). It may be noted that,
the frequencies, represented on the X-axis of the graph in Fig. 5, are the centre
frequencies of the octave band. The barrier may be either close to the source or receiver,
subject to the condition that, R <<D or in other words, to increase the traverse length for
the sound wave. It should also be noted that, the presence of the barrier itself can reflect
sound back towards the source. At very large distances, the barrier becomes less effective
because of the possibility of refractive atmospheric effects. Another method, based on the
length of traverse path of the sound wave is given at Fig. 6.
Barrier
Source Receiver
Barrier close o
to source R
D
Barrier
Source Receiver
Barrier close o
to receiver R
D16
Fig. 6 Attenuation of noise levels using barriers
Design of building: The design of the building incorporating the use of suitable noise
absorbing material for wall/door/window/ceiling will reduce the noise levels. The
approximate reduction of outside noise levels using typical exterior wall construction is
given at Table 6. The reduction in noise levels for various frequencies and the A-weighted
scale are shown. Variations in spectrum shape may change this A-weighted value by as
much as +/- 3 dB.
· Installation of panels or enclosures: A sound source may be enclosed within a
paneled structure such as room as a means of reducing the noise levels at the
receiver. The actual difference between the sound pressure levels inside and outside
an enclosure depends not only on the transmission loss of the enclosure panels but
also on the acoustic absorption within the enclosure and the details of the panel
penetrations which may include windows or doors.
The product of frequency of interest and surface weight of the absorbing material is
the key parameter in noise reduction through transmission loss. With conventional
construction practices, the high-frequency transmission loss of a panel becomes
limited to around 40 dB, owing to the transmission of sound through flanking paths
other than the panel itself. Examples of such flanking are structural connections or
ducts joining the two spaces on either side of the panel of interest. Procedures for
detailed design examples are given at Ref.: 9.17
Table 6 Approximate reduction of outside noise provided by typical exterior wall
construction
Octave Band
Center
Frequency (Hz)
A B C D E F G H
63 0 9 13 19 14 24 32 21
125 0 10 14 20 20 25 34 25
250 0 11 15 22 26 27 36 30
500 0 12 16 24 28 30 38 37
1,000 0 13 17 26 29 33 42 42
2,000 0 14 18 28 30 38 48 44
4,000 0 15 19 30 31 43 53 45
8,000 0 16 20 30 33 48 58 46
approx. dB(A) 0 12 16 24 27 30 38 33
A: No wall; outside conditions.
B: Any typical wall construction, with open windows covering about 5% of exterior wall area.
C: Any typical wall construction, with small open air vents of about 1% of exterior wall area, all windows
closed.
D: Any typical wall construction, with closed but operable windows covering about 10-20% of exterior wall
area.
E: Sealed glass wall construction, 1/4-in glass thickness over approximately 50% of exterior wall area.
F: Approximately 20 lb./ft
2
solid wall construction with no windows and no cracks or openings.
G: Approximately 50 lb/ft
2
solid wall construction with no windows and no cracks or openings.
H: Any typical wall construction, with closed double windows (panes at least 3/32” thick, air space at least 4
in.) and solid-core gasketed exterior doors.
Source : Ref. 9
· Green belt development: Green belt development can attenuate the sound levels.
The degree of attenuation varies with species of greenbelt. The typical attenuation of
noise levels by shrubs and trees is presented at Fig. 7 (a) and (b).The statutory
regulations direct the industry to develop greenbelt four times the built-up area for
attenuation of various atmospheric pollutants, including noise.
Using protection equipment
Before employing the use of protective equipment, please recall the Fig. 4, wherein the
various steps involved in the noise management strategy are illustrated. Protective
equipment usage is the ultimate step in noise control technology, i.e. after noise reduction
at source and/or after the diversion or engineered control of transmission path of noise.18
Fig. 7 (a) & (b) Noise level attenuation by shrubs and trees
The first step in the technique of using protective equipment is to gauge the intensity of the
problem, identification of the sufferer and his exposure to the noise levels. For the
Regulatory standards pertaining to time of exposure vs. maximum noise levels permitted
in a workspace environment, please refer to LO-8.
The usage of protective equipment and the worker's exposure to the high noise levels can
be minimised by -
· Job rotation: By rotating the job between the workers working at a particular noise
source or isolating a person, the adverse impacts can be reduced.
· Exposure reduction: Regulations prescribe that, noise level of 90 dB (A) for more
than 8 hr continuous exposure is prohibited. Persons who are working under such
conditions will be exposed to occupational health hazards. The schedule of the
workers should be planned in such a way that, they should not be over exposed to the
high noise levels.
· Hearing protection: Equipment like earmuffs, ear plugs etc. are the commonly used
devices for hearing protection. Attenuation provided by ear-muffs vary widely in
respect to their size, shape, seal material etc. Literature survey shows that, an average
noise attenuation up to 32 dB can be achieved using earmuffs
(7)
Details of some of .
the suppliers of the protective equipment are given in Ref. 3.19
Documentation of noise measurements
Please recall the Fig. 4, where noise survey is the first step of noise management
strategies. By now, the reader might be conversant with the terminology, impacts,
significance and control technology of noise pollution. Hence, it is felt to place this section
just before the end of the module to avoid any confusion for the reader.
Noise surveys will be conducted in an area (or zone) to find out the ambient noise levels or
noise levels at the work environment. The field data will be analysed and documented for
decision making. The parameters to calculate however, vary with the objective. But in
most cases, Leq, Ldn, NC are the likely deciding parameters (See Table 7 and 8).
For a systematic presentation, the noise survey reports for a typical industrial noise level
survey should contain the following information
(3)
· Reference to individual standard(s)
· Description of the machine and its conditions of installation and operation
· Description of the test environment with respect to its ability to reflect, dissipate or
absorb noise and location of the machine.
· Number of workers exposed and duration of exposure
· Description of the measuring apparatus used and method of calibration
· Time constant and weighting network used
· Position of measuring points.
· Results of SPL instruments either A -scale or octave band analysis
· Background noise levels and sound pressure values corrected for background noise, if
any.
The same principles can be applied for documentation of community (ambient) noise
levels and is left as an exercise to the reader.
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LO - 720
Regulatory guidelines
Statutory Regulatory guidelines were prescribed both for the ambient noise levels (Table
7) and for workspace environment noise levels (table 8). Factories Act, 1948 prescribes
the protection of workers against high noise levels (noise level > 90 dB (A)). The State
Pollution Control Board and Inspector of Factories have powers to administer the control
of noise pollution.
Table 7 Indian Standards for ambient noise levels
1
Area Noise Limits, Leq, dB (A)
Day Time
2
Night Time
3
Silence zone
4
50 45
Residential area 55 45
Commercial area 65 55
Industrial area 75 65
1. Ministry of Environment and Forest (MOEF) Guidelines vide Environment (Protection) Act, 1986 third
amendment rules, dated 26/12/89 (Ref. 6)
2. Day time from (600 hrs to 2100 hrs, IST)
3. Night time from (2100 hrs to 600 hrs IST)
4. Silence zone: Up to 100m around hospitals, educational institutions and courts. The zones are to be
declared by competent authority. Use of vehicle horns, loud speakers and bursting of crackers shall
be banned in these zones.
Source: Ref. (3,5)
Table 8 Damage risk criteria for hearing loss Occupational Safety & Health Administration
(OSHA) regulations
Maximum allowable duration per day
hours
Sound pressure level,
dB (A)
8 90
6 92
4 95
3 97
2 100
1.5 102
1 105
0.75 107
0.5 110
0.25 115
No exposure in excess of 115 dB(A) is permitted.
Source: Ref. (3,5)
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LO - 821
Application in Environmental Impact Assessment (EIA) studies
The EIA study will be carried out to evaluate and assess the impacts of any proposed (or
existing) activity on the environment. Noise is one of the environmental attributes, on
which the likely impacts due to the proposed (or existing) activity need to studied.
The likely steps to be carried out while conducting noise level studies for an EIA project
are summarised below. The EIA wiil be carried out for either proposed or existing
activities. The sequential steps involved will be same for both the activities.
· The likely activities that generate noise from the proposed activity are to be identified
· The typical sound (noise) levels of the noise generating sources are to be assessed
either from literature or from a similar source
· The likely exposure time of a worker at a noise generating source is to be assessed
from the plant / utility records
· The workspace environment noise levels are to be checked with OSHA standards
(Table 1.8). If the noise exposure levels are higher, suitable noise control measures
like personal protective equipment, installation of barriers, enclosures etc., need to be
suggested
· The EIA will usually be carried out in an impact circle of radius 3 Km to 25 Km or even
more depending on the objective and the likely activities of the proposed project. The
representative baseline (or back ground) status of the ambient noise levels need to be
collected by monitoring at various stations in the study zone
· The ambient noise levels are to be analysed for the prescribed parameters like, Leq,
Ldn etc., and compared with the ambient noise level standards (Table 1.7) for the
study region. If these values are higher than the prescribed limits, the likely causes for
the high values need to be assessed
· The likely impact of the noise levels from the proposed activity on the local
environment keeping in view the baseline status of noise levels need to be predicted
· If the predicted impact is adverse, suitable measures for attenuating the noise levels
like, green belt development, in-plant control measures etc., need to be suggested.
· The objective of the EIA study is to make ensure that, the local environment, say
noise, will not get affected by the noise levels emanated from the proposed activity. If
the ambient noise levels are high, then control measures be suggested to the project
proponent to ensure that, ambient noise levels will not increase due to the proposed
activity22
Summary
Whether knowingly or unknowingly, everyone of us contribute to noise pollution, because
most of our day-to-day activities generate some noise. Often neglected, noise pollution
adversely affects the human being leading to irritation, loss of concentration, loss of
hearing.
identify the sources of noise pollution. Once identified, the reason(s) for increased noise
levels to be assessed. Now, efforts shall be made to reduce the undesired noise levels
from (unwanted) noise generating sources. This leads to marginal reduction of noise
levels. It is still un-bearable scientific methods of noise control shall be employed.
The statutory Regulations have prescribed the noise level exposure limits. The public may
complain to the statutory Board for violation of noise level limits by any noise generator.
Suitable action will be taken to attenuate the noise levels and controlling pollution. It is
advisable that suitable noise control measures be taken and reduce the interference of
Statutory Board. It is high time that everyone should do this bit in curbing the noise
pollution, which is otherwise becoming as effective as SLOW POISONING.23
Glossary
Noise
Noise is unwanted sound.
Sound
Sound is a form of energy emitted by a vibrating body and on reaching the ear it causes
the sensation of hearing through nerves.
Infrasonics
The sound of frequency less than 20HZ.
Ultrasonics
The sound of frequency more than 20,000 HZ.
Decibel, dB
It is measurement unit of sound, represented by dB.
Ldn
The day-night equivalent value of sound level. The day is counted from 6AM to 9PM
(15hrs) and night from 9PM- 6AM (9hrs).
Frequency analysis
It allows to separate the main components of the signals by dividing the frequency bands
using a set of filters.
A-weighted scale
It resembles the audible response of human ear. Represented as dB(A).
Equivalent sound level, Leq
It is the constant sound pressure level which would have produced the same total energy
as the actual sound level over the given time. It is denoted as Leq.
L10, L50 and L90
The Sound levels exceeding 10%, 50% and 90% of the total time intervals during a
particular period are designated as L10, L50 and L90 respectively.
Perceived Noise Levels (PNL)
The noise of aircraft is described in terms of Perceived Noise Levels (PNL), a scale of
noisiness, expressed in pNdB.24
Review Questions
1. What is the difference between sound and noise?
2. What is the frequency range of infrasonic and ultrasonics?
3. What is the purpose of frequency analysis?
4. What are the frequencies used for frequency analysis?
5. List out typical sources of noise pollution.
6. What are the impacts of noise?
7. What are the methods to control noise pollution?
8. What are the noise exposure limits in a workspace environment?
9. What are the ambient noise limits?
10. How to document the noise levels ?
11. Write short notes on - (a) aircraft noise (b) application of noise pollution and its control in EIA studies (c)
noise reduction at source (d) noise reduction by engineered control of its transmission path
12. Find out the reduction in noise levels if the source is at (i) 2m (ii) 4m, (iii) 6m (iv) 10m (v) 100m from your
place (Hint: use equation (1))
13. Find out the noise levels in decibels, if the sound pressure level measured in N/m
2
was
2 x 10
-4
(ii) 6 x 10
-3
(iii) 8 x 10
-2
(iv) 10 x 10
-3
(v) 3 x 10
-1
.
14. Find out the day-night equivalent noise levels if Ld = 70 dB(A) and Ln = 52 dB(A). If Ldn value were to be
in safe limits, which is the best suited habitant zone. Give reasons.
15. Find out the barrier dimensions required for a noise reduction of 15 dB at (a) 500 Hz (b) 1000 Hz (c) 2000
Hz (d) 4000 Hz (e) 8000 Hz
16. Find out the effective noise level from five sources of 50dB, 55 dB, 62 dB, 64 dB and 65 dB noise
generation.
You may try this
· Collect the map of your colony and divide into suitable number of zones. Try to -
1. Identify the noise sources from each zone
2. Find out the noise levels of each zone
3. Compute Ldn values of the colony
4. Compare with the regulatory standards and find out whether noise levels in your colony are
permissible under the respective selected zone or not.
5. Identify the likely causes of noise from noise sources.
6. Develop methodologies to solve the problem
7. Attempt to solve
How good are you?
· Apply the same principles and methodologies and extend the problem area to your municipal ward,
panchayat, village, school, club, college etc.
· Keep updating yourself with the latest developments in technology.
For your information
The results of a recently conducted study shows that the ambient noise levels in the mining city of
Dhanbad are exceeding the statutory regulations
( )
.You may also report about your own locality .
Aren't you?25
Solved examples
Ex 1: If the distance from a noise source is doubled, find out the noise levels.
Sol: Given, r2 = 2r1
We have, L2 = L1 - 20log10 (r2/r1)
Substituting, we get,
L2 = L1 - 20 log10 (2r1/r1)
= L1 - 20 log10(2)
i.e., L2 = L1 - 20 x 0.301
= L1 - 6.02
i.e., the noise level will decrease by 6 dB for doubling of distance from the source.
Ex 2: The noise levels at a particular location are 65dB, 70dB and 78dB measured during
an hour of the day. Find out the average noise levels at the location.
Sol: Given, L1 = 65dB, L2 = 70dB, L3 = 75dB
The noise levels are to be logarithmacally averaged.
Average of L1, L2 & L3 = L (say). Convert the noise levels from decibels to bels.
i.e. L1 = 65dB or 6.5 bels,
L2 = 70dB or 7.0 bels
L3 = 78dB or 7.8 bels
L = 10 x log10 ([10
6.5
+ 10
7.0
+ 10
7.8
] / 3 )
= 10 x log10 [25419337.37]
= 10 x 7.405 = 74.05dB.
\ Average noise level is 74.05 dB.
Ex 3: The sound pressure level is measured at 5 x 10
-4
N/m
2
Find out the noise level in .
dB.
Sol: Given, P1 = 5 x 10
-4
N/m
2
We know P0 = 2 x 10
-5
N/m
2
(reference pressure)
Noise level in decibels, L = 10 log10 [P1/P0]
2
dB.
\ L = 10 log10 (5 x 10
-4
) / (2 x 10
-5
(
= 10 log10 [625] = 10 x 2.795
= 27.95dB.26
Ex 4: It is required to find out the day-night equivalent noise levels at a location. The
three-hourly day average values in dB are 48, 54, 56, 52, 61 and three-hourly night
average values in dB are 36,42,48. Compute Ldn.
Sol : (i) Find out day equivalent noise levels.
Lde = 10 x log10 ([10
4.8
+ 10
5.4
+ 10
5.6
+ 10
5.2
+ 10
6.1
]/5)
= 56.29 dB.
(ii) Find out night - equivalent noise levels.
Lne = 10 x log10 ([10
3.6
+ 10
4.2
+ 10
4.8
]/3)
= 44.41dB.
(iii) Find out day-night equivalent noise level, Ldn.
Ldn = 10 x log10 [15/24 (10
Lde/10
) + 9/24 (10
((Lne +10)/10)
[(
= 10 x log10 [15/24 x 10
5.629
+ 9/24 x 10
5.441
[
= 10 x log10 {265999 + 103522}
= 55.68dB.
Ex.5. What barrier dimensions are necessary in order that the barrier provide 20 dB
attenuation at 500 HZ.
Sol: From fig 1.5, we see that, H
2
/R must be atleast 10ft in order to achieve the desired
attenuation. This can be accomplished by selecting different values for H and R for
example,
H = 5.5 ft, R = 3ft; H = 10 ft, R= 10 ft; H = 17.5 ft, R = 30 ft etc.27
REFERENCES
1. Assessment and Control of Noise Pollution in Mining Industry, Proceedings of National
Seminar held by Institution of Engineers at Madras, 1989.
2. Environmental Noise Pollution and its Control, Proceedings of Specialist Course,
Institution of Engineers, Roorkee, India 1992.
3. Industrial Safety and Pollution Control Handbook, Associate (Data) Publishers Pvt.,
Secunderabad, 1991 ed.
4. Muralikrishna, K V S G, Air Pollution and Control, Kaushal & Co., Kakinada, AP, 1995
ed.
5. Rao, P.R., Noise Pollution and Control, Encyclopedia of Environmental Pollution and
Control, Vol.-2, Environmedia Publications, India, 1995 ed.
6. Jeva, R. et al, Noise Impact Assessment - an approach, Energy Environment Monitor,
Vol.: 12, September 96.
7. Mathur, J.S.B., Noise Control : Legislation, Planning and Design, Industrial Effluent
Treatment, Vol.:2, Applied Science Publishers Ltd., London, 1981 ed.
8. Mathur J.S.B., Noise Control : Methods of Reduction, Industrial Effluent Treatment,
Vol. : 12, Applied Science Publishers Ltd., London, 1981 ed.
9. Franken A. Peter, Community Noise Pollution, Industrial Pollution, Van Noistrand
Reinhold Company, New York, 1974 ed.
10. Noise : A Leaflet Published by Steel Authority of India Limited.