Noise Pollution: an Understanding

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Noise can be defined as “unwanted sound”, but this is a subjective definition due to the fact that one person’s sound may be another person’s noise. So a better definition of noise is “wrong sound, in the wrong place, at the wrong time”.

The 20th century has been described as the “century of noise”. It has become a very important stress factor. The term ‘noise pollution’ is of recent origin and signify a vast cacophony of sounds that are produced in modern life that leads to health hazards like deafness, sleepless ness, annoyance etc.

Sources of noise: Sources of noise are plenty. Noise may arise from automobiles, factories, industries, aircrafts, trains, loud speakers etc. Noise levels are particularly much more than normal near airports, railway stations and railway lines, near traffic signals. Use of electric horns, pressure horns, loud speakers during festivals, specially festivals like Diwali in India can cause noise pollution much more than acceptable limit.

Noise has two properties. First one is loudness or intensity of noise and the second one is frequency.

(1) Loudness or intensity: It depends in the amplitude of the vibration which initiate sound/noise. The loudness is measured in unit called decibels (dB). If a sound is 50 decibels, it means that it is 50 times louder than the smallest distinguishable (audible) sound or the ‘reference’ sound pressure. The smallest audible sound have a sound pressure of 0.0002 micro bar or dyne/cm2.A dyne is one millionth of normal atmospheric pressure which is 760 mm of mercury at sea level. Normal conversation produce a noise of 60-65 dB, whispering produces 20-30 dB, boiler factories produce 120 dB, busy streets produce 60-80 dB of noise.

Human ear responds in a non uniform way to different sound pressure levels. It responds to the perceived intensity of the sound not to the loudness of sound. That means at different places and different situations some person will have different perception to same loudness of sound. For example in a house acceptable noise level is approximately

40dB. But in hospitals it is 20 to 30dB and in industrial area it is 40dB to 60dB.

(2)Frequency: The unit of frequency in use is Hertz (Hz). One Hz is equal to one wave per second. The human ear is made to hear and can hear frequencies from 20 Hz to 20,000 Hz. This range is reduced with age and certain factors. Many animals like dogs & cats can hear sounds which are not audible to humans. If frequency is less than 20 Hz they are infra audible to humans and those above 20,000 per seconds are ultra-sounds. In ultrasounds machines these above 20,000 frequency sound waves are used for diagnostic purpose. These are also used in sonar technique in submarines and ships to detect any object which may be present in front of a submarine . These ultrasonic waves are also used in breaking down of kidney stones and gall stones.

Noise is also expressed in psycho acoustic term called the phon. The phon is a psycho acoustic index of loudness. Intensity and frequency are taken in to consideration in calculating phon.

The basic instruments used in the study of sound and noise are: (1) The Audiometer to measure the hearing ability. Zero line at the top represent normal hearing. A characteristic dip in the curve is seen at 4000Hz.(2) The ‘sound level meter’ measures the intensity of sound in dB. (3) The Octane Band Frequency analyzer measures noise in octane bands. The resulting plot shows the sound spectrum and indicate characteristics of noise, whether it is high pitched, low pitched or variable pitch.

The sound levels of some common noises are given below:

Common noise

Sound level

Whisper

10 dB

Conversation

70 dB

Children shouting

79dB

Music on radio

85 dB

Children crying

80 dB

Vacuum cleaner

75 dB

Piano

86 dB

Jet take off

150 dB

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