Noise Pollution In Residential Engineering Essay

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History of weaving looms can be traced back to 17th century. The first power loom was invented by Edmund Cartwright in 1785. Originally Power looms were with shuttle, and they were very slow. But as the industrial demands for faster production accelerate, faster looms without shuttle came in use in early part of 20th century. As developments and innovations take place, various types of looms were developed for faster production. Today, Air-jet, Water-jet, Rapier and other computer operated looms are used to maximize production of special materials.

2.2 Indian scenario of weaving looms

Though weaving is one of the important sector for Indian textile industry, it has not been given due attention like spinning sector. Moreover structure of the industry plays a major role in making it competitive. Nature of this sector is mainly unorganized. The sector consists of fragmented, small and often, unregistered units that invest low amount in technology and practices especially in the power loom, processing, handloom and knits.

India has world's largest installed base for looms. There are approximately 5 mn (million) looms in the country. India has 1.8mn Shuttle looms which is 45% of world capacity, and 3.90 mn handlooms which is 85% of world capacity.

Power loom

The power loom sector produces more than 60% of cloth in India, and textile ministry's estimation says that more than 60% of the country's cloth exports originated from that sector. With its employment of 4.86 mn workers, the power looms sector comprised approximately 60% of total textile industry employment.

As per textile ministry of India till March 31, 2006, the power looms sector which produces various cloth products, including greige and processed fabrics consisted of 430,000 units with 1.94mn power looms. The ministry projected the number of power looms to rise to 1.95 mn in 2006-07 (Source: http://www.fibre2fashion.com).

2.3 Description of Machine

Parts of Machine and other related components:

Warp: The warp is the set of lengthwise threads attached to a loom before weaving begins, and through which the weft is woven.

Weft: The weft is the yarn that is woven back and forth through the warp to make cloth.

Shuttle: A shuttle in weaving is a device used with a loom that is thrown or passed back and forth between the threads of the warp to weave in the weft.

Shuttle

Weft

Warp

Fig. 2.1: Front view of power loom

Fig. 2.2: Rear view of power loom

Reed: Resembles a comb. It is used to push the weft yarn securely into place as it is woven, separates the threads and keeps them in their positions, keeping them untangled, and guides the shuttle as it moves across the loom.

Reed

Reed attached with power loom

(b) A separated reed

Fig. 2.3 (a) Reed attached with power loom (b) A separated reed

Motor

Fig.2.4: Motor

2.4 Sound

Sound is produced by vibrating objects and reaches the listener's ears as waves in the air or other media. When an object vibrates, it causes slight changes in air pressure. These air pressure changes travel as waves through the air, and produce sound. This can be illustrated by striking a drum surface with a stick. The drum surface vibrates back and forth. As it moves forward, it pushes the air in contact with the surface. This creates a positive (higher) pressure by compressing the air. When the surface moves in the opposite direction, it creates a negative (lower) pressure by decompressing the air. Thus, as the drum surface vibrates, it creates alternating regions of higher and lower air pressure. These pressure variations travel through the air as sound waves.

Fig. 2.5: Generation of Sound Waves

Following table lists the approximate velocity of sound in air and other media.

Approximate Speed of Sound in Common Materials

Medium Sound Velocity (ft/s) m/s

Air, dry (0C and 0.76 mm of Hg) 1,100 330

Wood (soft - along the fibre) 11,100 3400

Water (15oC) 4,700 1400

Concrete 10,200 3100

Steel 16,000 5000

Lead 3,700 1200

Glass 18,500 5500

Hydrogen (0C and 0.76 mm of Hg) 4,100 1260

The hearing mechanism of the ear senses the sound waves and converts them into information which it relays to the brain. The brain interprets the information as sound. Even very loud sounds produce pressure fluctuations which are extremely small (1 in 10,000) compared to ambient air pressure (i.e., atmospheric pressure). The hearing mechanism in the ear is sensitive enough to detect even small pressure waves. It is also very delicate; this is why loud sound may damage hearing.

When the rapid variations in pressure occur between about 20 and 20,000 times per second (i.e. at a frequency between 20 Hz and 20 kHz) sound is potentially audible even though the pressure variation can sometimes be as low as only a few millionths of a Pascal. Movements of the ear drum as small as the diameter of a hydrogen atom can be audible.

2.4.1 Sound wave

Sound is transmitted via the movement of the particles in a medium, such as air or water. Energy is transferred from one region to another via a series of compression and a tension cycle, the motion of the particles is parallel to the direction of propagation. The acoustic disturbance can be represented as a wave, with the x-axis representing time, and the y-axis the displacement of a given particle in the medium from its rest position.

Increasing the strength of the sound source extends the displacement of the particle, and so the acoustic pressure will also increase. This is heard as an increase in loudness. Exciting the sound more rapidly increases the frequency of the sound, and produces more cycles in a given period. This is heard as an increase in pitch.

Two basic quantities that can describe the nature of a sound are frequency and amplitude (of displacement or acoustic pressure). Sounds can be formed by a simple harmonic mixture of frequencies (as produced by a guitar string), an intentional mixture of frequencies and amplitude (music) or a seemingly random mixture of frequencies and amplitudes (noise).

Increased frequency sound

wave

Increased volume sound wave

Sound wave

Fig. 2.6: Graphs of simple (normal), loud and high frequency sound waves

2.4.1.1 Pitch and frequency

Frequency is the rate at which the source produces sound waves, i.e. complete cycles of high and low pressure regions. In other words, frequency is the number of times per second that a vibrating body completes one cycle of motion. The unit for frequency is the hertz (Hz = 1 cycle per second). Low pitched or bass sounds have low frequencies. High-pitched or treble sounds have high frequencies. A healthy, young person can hear sounds with frequencies from roughly 20 to 20,000 Hz. The upper frequency limit decrease with age, and so the older a person gets, the less well they can hear high nodes. Also, the male hearing range decreases more quickly than the female, and so women can generally hear higher pitch notes than men of similar age. The sound of human speech is mainly in the range of 300 to 3,000 Hz.

2.4.1.2 Decibel (dB)

The decibel scale is a logarithmic scale applicable to any parameter, used to make quantities with a wide range of values more manageable. In the measurement of sound, the amplitude of the acoustic pressure, measured in pascals (Pa). The range of acoustic pressures that the human ear can detect is very wide- from the lower limit of hearing at around 20 micro Pa (2 x 10-5 Pa) to the threshold of pain at around 20 Pa. This very wide range of values is unwieldy, so it is converted into a logarithmic scale. This changes the range of values shown above to the more manageable range of 0 dB to 140 dB. Thus 0 dB is roughly the lowest level a normal person can hear, but it is not the lowest level possible.

Acousticians use the dB scale for the following reasons:

Quantities of interest often exhibit such huge ranges of variation that a dB scale is more convenient than a linear scale.

The human ear interprets loudness on a scale much closer to a logarithmic scale than a linear scale.

2.4.1.3 How do sound levels add?

Sound pressure levels in decibels (dB) or A-weighted decibels [dB (A)] are based on a logarithmic scale. They cannot be added or subtracted in the usual arithmetical way. If one machine emits a sound level of 90 dB, and a second identical machine is placed beside the first, the combined sound level is 93 dB, not 180 dB.

If there are two sound sources in a room - for example a radio producing an average sound level of 62.0 dB, and a television producing a sound level of 73.0 dB, then the total sound level is a logarithmic sum i.e.

Combined sound level = 10 x log (10^ (62/10) + 10^ (73/10))

= 73.3 dB

For two different sounds, the combined level cannot be more than 3 dB above the higher of the two sound levels. However, if the sounds are phase related, there can be upto a 6dB increase in SPL. A simple way to add noise levels:

Table 2.1: Addition of Decibels

Numerical difference between two noise levels [dB(A)]

Amount to be added to the higher of the two noise levels [dB or dB(A)]

0

3.0

0.1 - 0.9

2.5

1.0 - 2.4

2.0

2.4 - 4.0

1.5

4.1 - 6.0

1.0

6.1 -10

0.5

10

0.0

For instance, using the example of two machines each emitting a noise level of 90 dB:

Step 1: The numerical difference between the two levels is 0 dB (90-90 = 0), using the first row.

Step 2: The number corresponding to this difference of 0, taken from the right hand column, is 3.

Step 3: Add 3 to the highest level, in this case 90. Therefore, the resulting noise level is 93 dB.

When the difference between two noise levels is 10 dB (A) or more, the amount to be added to the higher noise level is zero. In such cases, no adjustment factor is needed because adding in the contribution of the lower in the total noise level makes no perceptible difference in what people can hear or measure. For example if workplace noise level is 95 dB(A) and another machine that produces 80 dB(A) noise, the workplace noise level will still be 95 dB(A).

2.4.1.4 Basic rules of working with decibel (dB) units

As decibel [dB, and also dB (A)] is a logarithmic scale, for mathematical calculations using

dB units, we must use logarithmic mathematics. However, in our day-to-day work we do not need such calculations.

The use of dB unit makes it easy to deal with the workplace noise level data provided we use a set of simple rules as summarized in Table 2.2.

Table 2.2: Relation between change in Sound Level & the corresponding change in Sound Energy

Change in dB Change in sound energy

3 dB increase Sound energy doubled

3 dB decrease Sound energy halved

10 dB increase Sound energy increased by factor of 10

10 dB decrease Sound energy decreased by factor of 10

20 dB increase Sound energy increased by factor of 100

20 dB decrease Sound energy decreased by factor of 100

2.4.1.5 Sound intensity

This may be defined as the rate of sound energy transmitted in a specified direction per unit area normal to the direction. With good hearing the range is from about 0.000000000001 Watt per square meter to about 1 Watt per square meter (12 orders of magnitude greater). The sound intensity level is found from intensity I (W/m2) by:

Sound Intensity Level = 10 x log (1/1.0E-12) dB

(Note: 1.0E-12 W/m2 normally corresponds to a sound pressure of about 2.0E-5 Pascals which is used as the datum acoustic pressure in air.)

Sound intensity meters are becoming increasingly popular for determining the quantity and location of sound energy emission.

2.4.1.6 Sound decay with distance

The way sound changes with distance from the source is dependent on the size and shape of the source and also the surrounding environment and prevailing air currents. It is relatively simple to calculate provided the source is small and outdoors, but indoor calculations (in a reverberant field) are rather more complex.

If the noise source is outdoors and its dimensions are small compared with the distance to the monitoring position (ideally a point source), then as the sound energy is radiated it will spread over an area which is proportional to the square of the distance.

This is an 'inverse square law' where the sound level will decline by 6dB for each doubling of distance. Line noise sources such as a long line of moving traffic will radiate noise in cylindrical pattern, so that the area covered by the sound energy spread is directly proportional to the distance and the sound will decline by 3dB per doubling of distance.

Close to a source (the near field) the change in SPL will not follow the above laws because the spread of energy is less, and smaller changes of sound level with distance should be expected.

In addition, it is always necessary to take into account attenuation due to the absorption of sound by the air, which may be substantial at higher frequencies. For ultrasound, air absorption may well be the dominant factor in the reduction.

2.4.2 Sound pressure

Sound pressure is the amount of air pressure fluctuation a noise source creates. We "hear" or perceive sound pressure as loudness.

Sound pressure also depends on the environment in which the source is located and the listener's distance from the source. The sound produced by the drum is louder two meters from the drum if it is in a small bathroom, than if it is struck in the middle of a football field. Generally, the farther one moves from the drum, the quieter it sounds. Also if there are hard surfaces that can reflect the sound (e.g. walls in a room), the sound will feel louder if it is heard from the same distance, in a wide-open field.

Sound pressure is usually expressed in units called pascals (Pa). A healthy, young person can hear sound pressures as low as 0.00002 Pa. A normal conversation produces a sound pressure of 0.02 Pa. A gasoline powered lawn mower produces about 1 Pa. The sound is painfully loud at levels around 20 Pa. Thus the common sounds we hear have sound pressure over a wide range (0.00002 Pa - 20 Pa).

It is difficult to work with such a broad range of sound pressures. To overcome this difficulty we use decibel (dB, or tenth (deci) of a Bel). The decibel or dB scale is more convenient because it compresses the scale of numbers into a manageable range. The decibel is named after Alexander Graham Bell, the Canadian pioneer of the telephone who took great personal interest in the problems of deaf people.

2.4.2.1 Sound pressure level

Sound pressure converted to the decibel scale is called sound pressure level (Lp). The zero of the decibel scale (0 dB) is the sound pressure of 0.00002 Pa. This means that 0.00002 Pa is the reference sound pressure to which all other sound pressures are compared on the dB scale. This is the reason the decibels of sound are often indicated as dB re 0.00002 Pa.

Table 2.3: Comparison of Sound Pressure Level and Sound Pressure

2.4.2.2 Sound power

The sound power is the sound energy transferred per second from the noise source to the air. A noise source, such as a compressor or drum, has a given, constant sound power that does not change if the source is placed in a different environment.

Power is expressed in units called watts (W). An average whisper generates a sound power of 0.0000001 watts (0.1 microwatt (µW), a truck horn 0.1 W, and a turbo jet engine 100,000 W. Like sound pressure, sound power (in W) is usually expressed as sound power levels in dB.

Table 2.4: Comparison of Sound Power Level and Sound Power

2.4.2.3 Sound power level

Sound power level, Lw, is often quoted on machinery to indicate the total sound energy radiated per second. The reference power is taken as 1pW.

For example, a lawn mower with sound power level 88 dB (A) will produce a sound level of about 60 dB (A) m at a distance of 10 meters. If the sound power level was 78 dB (A) then the lawn mower sound level would be only 50 dB (A) at the same distance.

2.4.2.4 Relation between sound pressure and sound power

Because the sound power of a noise source is constant and specific, it can be used to calculate the expected sound pressure.

The calculation requires detailed information about the noise source's environment. Usually a noise source with a lower sound power generates less sound pressure. The manufacturer can often provide the sound power of equipment. A number of international standards are available for labelling machines and equipment with their noise emission levels. From the sound power of a compressor, one can calculate the expected sound pressure and sound pressure level at a certain location and distance. This information can be helpful in determining possible noise exposures and how they compare to the noise guidelines.

2.4.2.5 Loudness

Loudness is the human impression of the strength of a sound. The loudness of a noise does not necessarily correlate with its sound level. Loudness level of any sound, in phons, is the decibel level of an equally loud 1 kHz tone, heard binaurally by an otologically normal listener. Historically, it was with a little reluctance that a simple frequency weighting "sound level meter" was accepted as giving a satisfactory approximation to loudness. The ear senses noise on a different basis than simple energy summation, and this can lead to discrepancy between the loudness of certain repetitive sounds and their sound level.

A 10dB sound level increase is considered to be about twice as loud in many cases. The sone is a unit of comparative loudness with 0.5 sone=30 phons, 1 sone=40 phons, 2 sones=50 phons, 4 sones = 60 phons etc. The sone is inappropriate at very low and high sound levels where subjective perception does not follow the 10dB rule.

Loudness level calculations take account of "masking" - the process by which the audibility of one sound is reduced due to the presence of another at a close frequency. The redundancy principles of masking are applied in digital audio broadcasting (DAB), leading to a considerable saving in bandwidth with no perceptible loss in quality.

2.5 Hearing

Of the five senses, hearing is one of the most important. Audible sounds enable communication, and they can tell us what and where things are. They certainly have a significant effect on how we feel. The human ear is an organ of complex design and function. The ear forms the receiver and transmission line to the brain, which then processes this information and converts the received signal into something that we can understand. The sound is then perceived as loud or soft, as a high or a low note, or on a more general level, perhaps as noise, or as music.

Sound is the sensation produced when longitudinal vibrations of the molecules in the external

environment, i.e. alternate phases of condensation and rarefaction of the molecules, strike the tympanic membrane. The waves travel through air at a speed of approximately 344 m/s (770 miles/h) at 20 °C at sea level. The speed of sound increases with temperature and with altitude. Other media in which humans occasionally find themselves also conduct sound waves but at different speeds. For example, the speed of sound is 1450 m/s at 20 °C in fresh water and is even greater in salt water. It is said that the whistle of the blue whale is as loud as 188 decibels and is audible for 500 miles.

Generally speaking, the loudness of a sound is correlated with the amplitude of a sound wave and its pitch with the frequency (number of waves per unit time). The greater the amplitude, the louder the sound; and the greater the frequency, the higher the pitch. However, pitch is determined by other poorly understood factors in addition to frequency, and frequency affects loudness, since the auditory threshold is lower at some frequencies than others. Sound waves that have repeating patterns, even though the individual waves are complex, are perceived as musical sounds; aperiodic nonrepeating vibrations cause a sensation of noise. Most musical sounds are made up of a wave with a primary frequency that determines the pitch of the sound plus a number of harmonic vibrations (over-tones) that give the sound its characteristic timbre (quality). Variations in timbre permit us to identify the sounds of the various musical instruments even though they are playing notes of the same pitch.

The amplitude of a sound wave can be expressed in terms of the maximum pressure change at the eardrum; but a relative scale is more convenient. The decibel scale is such a scale. The intensity of a sound in bels is the logarithm of the ratio of the intensity of that sound and a standard sound. A decibel (dB) is 0.1 bel. Therefore,

Number of dB = 10 log

Intensity of sound

Intensity of standard sound

Sound intensity is proportionate to the square of sound pressure. Therefore,

Number of dB = 20 log

Pressure of sound

Pressure of standard sound

The standard sound reference level adopted by the Acoustical Society of America corresponds to 0 dB at a pressure level of 0.000204 x dyne/cm2, a value that is just at the auditory threshold for the average human. As decibel scale is a log scale, a value of 0 dB does not mean the absence of sound but a sound level of an intensity equal to that of the standard. Furthermore, the 0- to 140-dB range from threshold pressure to a pressure that is potentially damaging to the organ of Corti actually represents a 107- (10 million)-fold variation in sound pressure. In another way, atmospheric pressure at sea level is 15 lb/in.2 or 1bar, and the range from the threshold of hearing to potential damage to the cochlea is 0. 0002-2000 μ bar.

The sound frequencies audible to humans range from about 20 to a maximum of 20,000 cycles per second (cps, Hz). In other animals, notably bats and dogs, much higher frequencies are audible. The threshold of the human ear varies with the pitch of the sound, the greatest sensitivity being in the 1000- to 4000-Hz range. The pitch of the average male voice in conversation is about 120 Hz and that of the average female voice about 250 Hz. The number of pitches that can be distinguished by an average individual is about 2000, but trained musicians can improve on this figure considerably. Pitch discrimination is best in the 1000- to 3000-Hz range and is poor at high and low pitches.

Fig. 2.7: Human audibility curve. The middle curve is that obtained by audiometry under the usual conditions. The lower curve is that obtained under ideal conditions. At about 140 decibels (top curve), sounds are felt as well as heard.

2.5.1 Hearing Mechanism

The sound waves are collected by the external ear upto some extent. They pass through the external auditory meatus to the tympanic membrane which is caused to vibrate. The vibration are transmitted across the middle ear by the malleus, incus and to the stapes bones. The latter fits into the fenestra ovalis. The perilymph of the internal ear receives the vibrations through the membrane covering, the fenestra ovalis. From the perilymph the vibrations are transferred to the scala vestibuli of cochlea and then to scala media through Reissner's membrane. Thereafter, the movements of endolymph and tectorial membrane stimulate the sensory hairs of the organ of Corti. The impulses thus received by the hair cells are carried to the brain (temporal lobe of each cerebral hemisphere) through the auditory nerve where the sensation of hearing is felt (recognised).

Fig. 2.8: Schematic representation of the conduction of sound vibrations in the ear

The external and middle ears serve to transmit sound waves to the internal ear. In the internal ear, transformation of the vibrations into nerve impulses for relay to the brain takes place. During loud sound, some sound waves are transferred from scala vestibule to scala tympani through helicotrema. From scala tympani the sound waves are transmitted to the tympanic or middle ear cavity through the membrane covering the fenestra rotunda. From the tympanic cavity the sound waves are transferred to the pharynx through the Eustachian Tube.

2.6 Noise

Noise is an important form of pollution caused by unwanted sound. At low levels noise can be a nuisance, but exposure to sustained high levels, for example in a noisy workplace, can cause hearing loss. Impulsive noise, such as the sound of a pneumatic tool, or tonal noise, such as the whine of a machine, can be particularly irritating. But what some people consider as noise, others can tolerate, or may even like, and so the study of noise has to recognize these different subjective responses.

2.6.1 Difference between sound and noise

Sound is what we hear. Noise is unwanted sound. The difference between sound and noise depends upon the listener and the circumstances. Rock music can be pleasurable sound to one person and an annoying noise to another. In either case, it can be hazardous to a person's hearing if the sound is loud and if he or she is exposed long and often enough.

2.6.2 Noise an important workplace hazard

Noise is one of the most common occupational health hazards. In heavy industrial and manufacturing environments, as well as in farms, cafeterias, permanent hearing loss is the main health concern. Annoyance, stress and interference with speech communication are the main concern in noisy offices, schools and computer rooms.

To prevent adverse outcomes of noise exposure, noise levels should be reduced to acceptable levels. The best method of noise reduction is to use engineering modifications to the noise source itself, or to the workplace environment. Where technology cannot adequately control the problem, personal hearing protection (such as ear muffs or plugs) can be used. Personal protection, however, should be considered as an interim measure while other means of reducing workplace noise are being explored and implemented. As a first step in dealing with noise, workplaces need to identify areas or operations where excessive exposure to noise occurs.

2.6.3 Kinds of noise

Noise can be continuous, variable, intermittent or impulsive depending on how it changes over time. Continuous noise is noise which remains constant and stable over a given time period. The noise of boilers in a power house is relatively constant and can therefore be classified as continuous.

Most manufacturing noise is variable or intermittent. Different operations or different noise sources cause the sound changes over time. Noise is intermittent if there is a mix of relatively quiet periods and noisy. Impulse or impact noise is a very short burst of loud noise which lasts for less than one second. Gun fire or the noise produced by punch presses is examples of such noise.

2.7.2 Auditory Effects of Noise Pollution

2.7.2.1 Auditory fatigue

It appears in the 90 dB region and greatest at 4000 Hz. It may be associated with side effects such as whistling and buzzing in the ears.

2.7.2.2 Deafness or hearing loss

The most serious pathological effect is deafness or hearing loss. The victim is generally unaware of it in early stages. The hearing loss may be temporary or permanent. Temporary hearing loss results from a specific exposure to noise; the disability disappears after a period of time up to 24 hours following the noise exposure. Most temporary hearing loss occurs in frequency range between 4,000 to 6,000 Hz. Repeated or continuous exposure to noise around 100 decibels may result in a permanent hearing loss; in this, the inner ear damage may vary from minor changes in the hair cell endings to complete destruction of the organs of Corti. When this occurs as a result of occupation in industries, it is called 'occupational hearing loss'. Exposure to noise above 160 dB may rupture the tympanic membrane and cause permanent loss of hearing.

2.8 Measurement of Noise

Noise is measured by means of a sound level meter. The sound level meter is positioned in a desired location, with no obstruction from the sound source, and then reading is taken. Whenever possible, the sound level meter should be mounted on a tripod and the operator should be at least 0.5 m away from the nearest edge of the level meter. The outdoor measurements are made 1.2 to 1.5 m above the ground and at least 3.5 m away from the reflecting surfaces such as buildings. Indoor sound measurements should be made at least 1.2 to 1.5 m above the floor, at least 1.0 m from walls and 1.5 m from windows. Indoor measurements are normally made with windows closed. Measurements that deviate from these recommended distances should be specified accordingly.

Weighting network

Rectifier

Display meter

Output jack

Amplifier

Preamplifier

Microphone

Fig.2.9: Block diagram of the basic parts of a sound level meter

Fig. 2.10: Sound Level Meter

Normally, we measure root mean square (r.m.s,) sound pressure levels of complex waves with sound level meter instead of measuring the peak pressures of the wave. The basic parts of most sound level meters include a microphone, amplifiers, weighting networks, and a display reading in decibels (dB). As the sound pressure coming from a source reaches the microphone, a pressure transducer (such as a diaphragm) mimics the pressure pattern of the source and converts the signal into a small current of electricity. The electric signal thus produced is then boosted in magnitude at the preamplifier. The electric signal is then modified by the weighting network, followed by a further boosting in magnitude through the amplifier. The rectifier, then, converts the amplified electrical signal from alternating current (A.C.) to direct current (D.C.) to cause the needle of the display meter to register the sound pressure level directly in decibels. Now-a-days, digital display meters are preferred over the analog display meter. A calibrator is used to produce a reference sound. It is attached to the microphone to calibrate the sound level meter.

2.8.1 Weighting networks

Sound pressure that the sound level meter receives is not the same as what the human ear would perceive. Due to this reason, an electronic circuitry called a weighting network is built into the sound level meter so as to alter the measured signal in a similar fashion as the human hearing mechanism. From the measured equal loudness level contours, three internationally accepted weighting network curves were developed for different conditions. These are the A, B and C weighting networks. The A, B and C weighting curves are obtained by inverting selected phon contours. The A-weighting network approximates human response for low sound levels, the B-weighting network approximates the human response for moderate sound levels, and the C- weighting network approximates the human response for high sound levels.

Fig.2.11: Relative frequency response of the A, B and C weighting networks of a sound level meter.

By examining the curves in Fig.2.10, one can observe that the A- weighting network deletes low-frequency sound, while the C-weighting network produces little change in the measured signal (i.e., it provide a relatively flat response across the frequency range). The network circuitry electronically subtracts the actual sound pressure level of a particular frequency that reaches the microphone according to in which network the sound level meter is set. For example, using the A-weighted network, at a sound frequency of 200 Hz, the actual pressure level reaching the sound level meter is subtracted by 11; while using B-weighted network, at a sound frequency of 200 Hz, the actual pressure level reaching the sound level meter is subtracted by 2. Depending on whether the weighting network used is A, B or C, readouts are in dB (A), dB (B), or dB (C) respectively. Since the readings are not actually the true sound pressure levels, the readouts from the A, B and C weighted networks are called sound levels (not sound pressure levels). Due to this same reason, the sound level meters are called so (and not sound pressure level meters).

The A-weighted network is more commonly used and has been adopted in many laws and ordinances of almost all countries to compensate for human hearing characteristics regardless of the sound intensity level. There is another weighting network, called D-weighting network, which has been recommended as a correlation with human response from noise around airports.

2.9 Standard Noise Levels of different source

Table 2.5: The Decibel Scale

Sound Source

Decibel, dB

Launching of space rocket

170

Jet plane at take off

150

Threshold of pain

140

Pneumatic riveter

130

Running motor cycle

118

Jet fly over at 150 m

115

Rock band

111

Jet fly over at about 300

103

Farm tractor

98

Motor cycle at 25 ft

90

Heavy city traffic

85

Alarm clock

80

Average city traffic

70

Normal conversation

60

Living room

45

Library

35

Broadcasting studio or A quiet room at night

20

Rustling of leaves

10

Threshold of hearing

0

Table 2.6: Psychological and Physical Effects at Different Decibel Levels

135 dB

Painful

110 dB

Discomfort

88 dB

Hearing impairment on prolonged exposure

80 dB

Annoying

65 dB

Intrusive

Table 2.7: Acceptable noise levels for different outdoor and indoor situations

Location

Acceptable level range, noise dBA

Outdoor

Rural

25-35

Residential (urban)

35-45

Business (urban)

45-50

City

45-55

Industrial area

50-60

Indoor

Radio & TV studies Hospitals, classrooms.

25-30

Apartments, hotels and conference rooms

35-40

Private offices and court rooms

40-45

Public offices, stores and banks

45-50

Table 2.8: Speech Interference Level and Voice Levels Required for Communication

Speech Interference

Level (SIL), dB

Voice level and distance within which communication is possible

45

Normal Voice at 3 m (Relaxed Conversation)

55

Normal Voice at 1 m (Continuous conversation in work areas)

65

Raised Voice at 0.6 m (Intermittent communication)

75

Very loud voice at 0.3 m (Minimum communication)

85

Shouting at 0.3 m (Unavoidable communication)

Table 2.9: Permissible Exposure Limits to Various Noise Levels

Noise levels, dBA

Permissible duration of exposure (hours / day)

90

8

95

4

100

2

105

1

110

1/2

115

1/4

Table 2.10: Noise Threshold Limit Values as per American National Standard Specifications for sound level meters

Sound levels,

dBA

Permissible duration of

exposure (hours per day)

80

16

85

8

90

4

95

2

100

1

105

1/2

110

1/4

115

1/8

Table 2.11: Threshold Limit Values of Impulses or Impacts per day

Sound level, dB (Decibels peak sound pressure level)

Permissible number of Impacts per day

140

102

130

103

120

104

Table 2.12: Ambient Noise Standards

S. No.

Area*

Day Time**

Night Time***

1

Industrial Area

75

70

2

Commercial Area

65

55

3

Residential Area

55

45

4

Silence Zone

50

40

* Mixed Areas should be declared by the competent authority and the corresponding limit be applied.

** Day Time - 6.00 AM to 9.00 PM (15 hours)

*** Night Time - 9.00 PM to 6.00 AM (9 hours)

2.10 Control of Noise Pollution

Due to the various adverse impacts of noise on humans and environment, 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 techniques employed for control of noise pollution from power looms can be broadly classified as:

Control at source

Control in the transmission path

Other methods

Movement of the shuttle is the major contributor to noise generation. Shuttle holds weft in its groove and moves to and fro between thread of warp and strikes at both ends. If rubber or other shock absorbing pads are placed at those ends, it may reduces noise production to some extent. Noise is also generated by reed, which moves up and down (according to pattern). Unlike shuttle, noise from reeds cannot be control directly.

The vibrations of machines may be controlled using proper foundations, rubber padding etc. to reduce the noise levels caused by vibrations. Proper lubrication and maintenance of power loom will reduce noise levels. Proper handling and regular maintenance is essential not only for noise control but also to improve the life of machine.

The change in the transmission path will increase the length of travel for the wave and get absorbed/refracted/radiated in the surrounding environment. Installation of barriers between noise source and receiver can attenuate the noise levels. The design of the building incorporating the use of suitable noise absorbing material for wall/door/window/ceiling will reduce the noise levels.

2.11 Noise Pollution Control in India

The Noise Pollution (Regulation and Control) Rules, which came into being in February 2000, stipulate that,

The state government, as the implementing authority, should initiate the process of controlling noise pollution by classifying areas as industrial, commercial, residential or silence zones.

It should ensure that noise does not exceed the prescribed limits.

It should also consider noise pollution and its effect on people while carrying out development projects.

It should ensure that loudspeakers and public address systems are not used without written permission from the authority and at night between 10:00 pm and 6:00 am. This law clearly shows that loudspeakers cannot be used at night between 10:00 pm and 6:00 am.

The designated authority is either the district magistrate, the police commissioner or and any officer given the mandate to implement these rules.

Noise pollution is treated as part of air pollution and the Central Pollution Control Board (CPCB) is the monitoring agency.

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