Environmental Science (2+1)

Measurement of Noise

• Measurement of Noise: Sound consists of repeated alternate compressions and expansions of air. The pitch of sound is determined by frequency and intensity of sound. The intensity of noise is measured by ‘decibels’ dB. (deci = 10 and the name of scientist Alfred Graham Bell).


dB = 10 x log 10 I/I0

• I = Sound Intensity
• I0 = Softest audible sound intensity


• Human ear is known to be sensitive to an extremely wide intensity from 0 to 190 dB. Here, ‘0’ dB is threshold of hearing while 180 dB is threshold of pain. Some people will feel discomfort with 85 db and most people may not feel discomfort even with sound of 115 dB. Pain is usually felt at 140 dB. The sound intensity is measured by instrument Larnharometer. The effect of sound as man depends upon its frequency. Frequency = Number of vibrations per second.It is denoted by Hertz. One Hz = 1 vibration / second.People can hear sound from 16 to 20000 hz, but this range is reduced with age. Why the sound which at one time gives soothing touch to the ear becomes noise at another occasion.


Ambient and quality standards for noise

Place	Limits dB
	Day time	Night time
Industrial area	75	70
Commercial area	65	55
Residential area	55	45
Silent zone	50	40