ENVIS Centre on Control of Pollution Water, Air and Noise

INTRODUCTION

Noise is, typically, defined as unwanted sound. Sound which pleases the listeners is music and that which causes pain and annoyance is noise. At times, what is music for some can be noise for others.

 

Most of the machines that have been developed for industrial purposes, for high-speed transportation, or to make life more enjoyable, by furnishing additional comfort, reducing the drudgery of everyday living, and speeding up our daily routines to provide additional leisure hours, are accompanied by noise. Noise prevention and control is important as noise affects us in hearing, ability to communicate and behavior. Undoubtedly, lesser noise can make the environment more friendly and life becomes pleasant.

SOURCES / TYPES OF NOISE POLLUTION

Noise can be broadly classified under 4 categories

1. Transport Noise
2. Occupational /Industrial Noise
3. Neighbourhood noise
4. Recreational Noise

1. Transport Noise: Transport noise mainly consists of traffic noise from road, rail, and aircraft. The number of automobiles on roads like motors, scooters, cars, motor cycles, buses, trucks and diesel engine vehicles has increased enormously, leading to noise pollution.
This can be subdivided into
• Road traffic noise
• Air craft noise
• Rail traffic noise
Noise levels in most residential areas in metropolitan cities are hovering around the border line due to increased vehicular noise pollution. In general, on urban roads there are distinct traffic peaks in the morning and evening as people travel to and from work.

2. Occupational /Industrial Noise: It is the sound having high intensity, mainly caused by industrial machines. Sources of such noise pollution are various factories’ machines, industries, and mills. Noise from mechanical saws and pneumatic drills is unbearable and a nuisance to the public. It also includes noise from domestic gadgets e.g. washing machines, vacuum cleaner etc.
Industrial workers who are exposed to noise for 8 hours per day and 6 days per week suffer from occupational noise pollution.

3. Neighbourhood noise: This implies variety of sources of noise which disturb and annoy the general public by interfering with their comfort and welfare. This type of noise includes disturbance from household gadgets and community. Common sources include musical instruments, TV, VCR, radios, transistors, telephones, music in public functions, and loudspeakers etc.

4. Recreational Noise: Harmful noise exposure is not only limited to the workplace. Some recreational activities are also dangerously loud and cause permanent damage to hearing. Additionally, many recreational activities create loud noises which interfere with the peace and quiet of the community. These activities may include sound at music concerts, firecrackers, sound at aerobic studios, personal stereo systems, children’s toys, hunting, target shooting, motorboating, waterskiing, snowmobiling, woodworking, listening music, motorcycle riding etc. Movie theatres, home entertainment centres, car stereo systems, health clubs, dance clubs, bars, and amusement centres also pose serious risk to hearing.

 

PROPERTIES AND MEASUREMENT OF NOISE

Physically, sound is a mechanical disturbance propagated as a wave motion in air and other elastic or mechanical media .such as water or steel.

Physiologically, sound is an auditory sensation evoked by this physical phenomenon. However, not all sound waves evoke an auditory sensation, e.g., ultrasound has a frequency too high to excite the sensation of hearing.

The physical properties and perception of sound or noise are expressed and measured in different concepts and units.

 

Physical Properties and Measurement
Sound waves involve a succession of compressions and rarefactions of an elastic medium such as air. These waves are characterised by the amplitude of pressurechanges, their frequency, and the velocity of propagation. The speed of sound, the frequency and the wave length are related by the equation:

Wave length = Speed of sound/frequency

 

The speed of sound in air. at a temperature of 20°^c , is approximately 344 rn/s. Sound travels much faster in solids than in air. For example, the speed of sound is 3,962 rn/s in wood and 5,029 m/s in steel.

 

Frequency is defined as the number of compressions and rarefactions per unit time (sec). Unit of frequency is hertz (Hz). Human hearing is sensitive to frequencies in the range of about 20-20,000 Hz (the audio frequency range).

 

Sound pressure is used as the fundamental measure of sound (amplitude) as it is measurable directly by instruments. The weakest sound pressure disturbance that can be detected by an "average" person at 1,OOO Hz has been found to be 20µN/m² and the largest sound pressure perceived without discomfort is of the order of 10⁷ µN/m². Because of such a wide range, the use of a linear pressure scale has been found to be impractical. It has been found convenient to employ sound pressure level, a quantity, which is proportional to the logarithm of sound pressure. By this, the sound pressure range of interest is compressed between 0 to 130, a range convenicnt to use. The sound pressure level is expressed in the unit of decibel (dB). Sound Pressure Level is defined as :

 

Lp = 10 Log10 (P/Pr)²

 

Where Lp = sound pressure level, dB

 

P = root mean square sound pressure, usually in µN/m²

 

Pr = reference sound pressure

 

Log10 = Logarithm to the base 10

 

The reference sound pressure, Pr, has an internationally agreed value of 20 µN/m².

 

Sound is measured with a sound level meter which is usually a portable, self- contained instrument incorporating a microphone, amplifier, a voltmeter and attenuators, the whole of which is calibrated to read sound pressure levels directly.

Sound Perception and Measurement

The magnitude of a sound, as perceived by human ear is called its loudness. Ear is not equally sensitive at all frequencies and amplitudes of sound pressure. For this reason, even though, the sound pressure levels of two different noises may be same, the first may be judged to be louder than the second if the sound energy of the first is concentrated in a frequency region where the ear is more sensitive.

Fig. I shows a set of equal loudness curves for an average young person with good hearing, listening to pure tones.

 

To obtain levels which bear a closer relationship to loutiness judgements than sound pressure levels, so called frequency weighting networks, are incorporated in sound level meters. Such levels are termed as Sound Levels. Generally, three types of filters having frequency response curves A, B & C, as shown in figure 2, are employed to match the ear response at low, medium and high loudness, respectively. However, extensive experience has shown that A-filter usually provides the highest correlation between physical measurements and, subjective evaluations of loudness of noise. Levels on the A-scale are commonly expressed in dB (A).

Most Sound Level Meters integrate instantaneous sound pressures over some time constant viz. Fast-, Slow- and Impulse-response times. Fast-response corresponds to a time constant of 0.125 s and is intended to approximate the time constant of human hearing system. Thus, all measurements of sound pressure levels and their variation over time should be made using the Fast response time, except where it is not possible. Sound Pressure Meters include a Slow-response time with a time constant or I s, which may be used, for convenience, in non-recording Meters. Sound Pressure Meters also include Impulse- response time with a time constant of 0.035 s, for measurement of impulse noise, such as noise from firecrackers, gun-shot etc.

Equivalent Continuous Sound Pressure Level, Leq is the level of that steady sound which over the same interval of time, contains the same total energy (or dose) as the fluctuating sound.

Equivalent sound level Leq can be obtained from variable sound pressure level, L, over a time period T, by using following equation:

Equivalent continuous sound level has gained widespread acceptance as a scale for the measurement of long-term noise exposure.

 

EFFECTS OF NOISE POLLUTION

Problems caused by noise pollution include stress related illnesses, speech interference, hearing loss, sleep disruption, and lost productivity. The hazards are summarized below:

1. Noise pollution affects both human and animal health. It leads to:
i. contraction of blood vessels,
ii. making skin pale,
iii. Excessive adrenalin in the blood stream which is responsible for high blood pressure,
iv. Blaring sounds are known to cause mental distress,
v. Heart attacks, neurological problems, birth defects, and abortion.

2. Muscle contraction leading to nervous breakdown, tension, etc.

3. The adverse reactions are coupled with a change in hormone content of blood, which in-turn increases heartbeat, constriction of blood vessels, digestive spasms, and dilation of the pupil of the eye.

4. Adversely affects health, work efficiency, and behaviour. Noise pollution may cause damage to the heart, brain, kidneys, liver, and may produce emotional disturbance.

5. The most immediate and acute effect of noise is impairment of hearing that diminishes some part of the auditory system. Eardrum is damaged when exposed to very loud and sudden noises. The hair cells in the inner ear are chronically damaged. Prolonged exposure to noise of certain frequency pattern leads to chronic damage to the inner ear and leads to hearing loss.

6. Impulsive noise may cause psychological and pathological disorders.

7. Ultrasonic sound can affect the digestive, respiratory, cardiovascular system, and semi-circular canals of the internal ear.

8. The brain is adversely affected by loud and sudden noise by jets and airplanes. People are subjected to psychiatric illness.

9. Recent reports suggest that blood is thickened by excessive noise. Eosinophilia, hyperglycaemia, hypokalaemia, and hypoglycaemia are caused by alteration in the blood due to noise.

10. The optical system of human beings is also affected by noise pollution. Severe noise pollution causes:
i. Pupillary dilation
ii. Impairment of night vision and
iii. Decrease in rate of colour perception

Intensity of common sources of sound

1. Sounds of Normal Conversations:

Sound Intensity: 40-60 dB
Health Hazard: Sound less than 80 dB is safe for the ear.

2. Sounds emanating from Tape recorders or an Orchestra:

Sound Intensity: 70 dB
Health Hazard: It is safe for ear.

3. Sounds of Heavy Traffic:

Sound Intensity: 90 dB
Health Hazard: Constant exposure to sound greater than 80 dB causes temporary hearing loss and if they are not treated immediately, causes permanent impairment.

4. Sounds of Pneumatic drills and other machines:

Sound Intensity: 100 dB
Health Hazard: Constant exposure causes temporary hearing loss and if they are not treated immediately, causes permanent impairment.

5. Sounds of Aircraft engine:

Sound Intensity: 100-200 dB
Health Hazard: Higher noise level of 160 dB cause total deafness, rupturing eardrums, damaging inner ear. It also causes high blood pressure, ulcer in stomach, palpitation, nervous problems, irritation, anger, and affects pregnant women’s embryo.

6. Sounds of Rockets during Take off:

Sound Intensity: 200 dB
Health Hazard: It is dangerously causing total deafness by rupturing the eardrums and damaging the inner ear. It also causes high blood pressure, ulcer in stomach, palpitation, nervous problems, irritation, anger, and affects pregnant women’s embryo.

CONTROLLING MEASURES OF NOISE POLLUTION

Some measures which can be adopted in this direction are as follows:

1. Prescribing noise limits for vehicular traffic
2. Ban on honking (usage of horns) in certain areas
3. Creation of silence zones near schools and hospitals
4. Redesigning buildings to make them noise proof
5. Reduction of traffic density in residential areas
6. Giving preference to mass public transport system.
7. Minimum use of loudspeakers and amplifiers especially near silence zones.
8. Banning pressure horns in automobiles.
9. Framing a separate Noise Pollution Act.

CONTROL MEASURES

I. Suppression / isolation of noise at Source
II. Transmission path intervention (Path Control)
III. Receptor control

I. Suppression / isolation of noise at Source

This includes source modification such as acoustic treatment to machine surface, design changes, limiting operational timings, etc. The noise pollution can be controlled at the source of generation itself by employing techniques, such as,

• Reducing the noise levels from domestic sectors:

The domestic noise generated from radio, tape recorders, television sets, mixers, washing machines, cooking operations etc. 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 help to reduce the noise levels.

• Control over vibrations:

The vibrations of materials may be controlled using proper foundations, rubber padding etc.

• Low voice speaking:

Speaking in 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 habitat 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 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. Similar 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.

• Oiling:

Proper oiling reduces noise from the machine.

• Tree plantation:

Planting green trees and shrubs along roads, hospitals, educational institutions etc. help in noise reduction to a considerable extent.

• Acoustic zoning:

Increased distance between source and receiver by zoning of noisy industrial areas, bus terminals and railway stations, aerodromes etc. away from the residential areas would go a long way in minimising noise pollution. There should be silence zones near the residential areas, educational institutions, and above all, near hospitals.

II. Transmission path intervention (Path Control)

This includes containing the source of noise inside a sound insulating enclosure, constructing a noise barrier or provision of sound absorbing materials along the path. In many cases, noise control at source is very difficult or quite uneconomic. In such cases, noise control is achieved in the path:

• Putting major noise sources at one place & separating it from quitter area.
• Using acoustic barriers, sound absorbing linings, & sound insulating partitions.
• Complete or partial enclosure of many equipments
• Providing silencers on the intake as well as exhaust side of flow machines like blowers, fans etc.
• Insertion of damping material between machine bases and foundations and use of anti-vibration mountings.
• Sound travels through the cracks that get left between the door and the wall. For reducing noise, this space (jamb frame gap) should be packed with sound absorbing material.
• Sound insulation can be done by constructing windows with double or triple panes of glass and filling the gaps with sound absorbing materials.
• Acoustical tiles, hair felt, perforated plywood etc. can be fixed on walls, ceilings, floors etc. to reduce noise (especially for sound proof recording rooms etc.)

III. Receptor control

This includes protection of the receiver by altering the work schedule or provision of personal protection devices such as ear plugs for operating noisy machinery. The measure may include dissipation and deflection methods. For people working in noisy installations, ear-protection aids like ear-plugs, ear-muffs, noise helmets, headphones etc. must be provided to reduce occupational exposure.

RECENT DEVELOPMENTS

This Section emphasizes the recent developments in the field of "Noise Pollution Control".

1. Noise cancellation technology

2. Acoustic camera

3. Desk noise meter

1. Noise cancellation technology
A technology has been developed for actively reducing noise pollution. It is based on a physical phenomenon called as destructive interference. According to this phenomenon, when waves are travelling into the space and they are hit with the similar waves of the same volume and the same frequency, but shifted by 180°; then waves will interfere with each other and cancel each other.

Using this technology, the noise reduction can be achieved upto 10 dB. For this, specially designed chips are available. The same levels of reduction are achieved when using an industrial vacuum cleaner, kitchen hoods, and ventilating systems. The technology can either be embedded on the source of the noise or can create a “bubble” in open spaces. The technology captures ambient noise, creating a quiet zone.

For more information: http://www.silentium.com/

2. Acoustic camera
An acoustic camera is an imaging device used to locate sound sources and to characterize them. It is an optimal tool for visualization of sound level measurements. By taking real time measurements and displaying 3D acoustic fields, the instrument can be used for sound source localization on targets that are stationary, outdoors, moving, or in wind tunnels.

It consists of a group of microphones, also called as microphone array, that are simultaneously acquired to form a representation of the location of the sound sources.

3. Desk noise meter
The noise meter sits on a desk and takes power from a computer's USB port. It clearly shows the surrounding noise levels, allowing one to make decisions based on facts rather than just perception. The instrument shows the noise level in dB on its digital display and also shows it as a bar, giving an instant indication of whether the levels are low or high, and which way they are changing.

The desk noise monitor is ideal for:
• Monitoring office noise levels, including open offices,
• Call center noise assessment and control,
• Hospitals ward noise levels.