Progressive wave : Intensity and sound level
If we hear the sound produced by violin, flute or harmonium, we get a pleasing sensation in the ear, whereas the sound produced by a gun, horn of a motor car etc. produce unpleasant sensation in the ear.
The loudness of a sound depends on intensity of sound wave and sensitivity of the ear.
The intensity is defined as the amount of energy crossing per unit area per unit time perpendicular to the direction of propagation of the wave.
Intensity is measured in W m?2.
The intensity of sound depends on (i) Amplitude of the source (I α a2), (ii) Surface area of the source (I α A), (iii) Density of the medium (I α ρ), (iv) Frequency of the source (I α n2) and (v) Distance of the observer
from the source (I α 1/r2).
The lowest intensity of sound that can be perceived by the human ear is called threshold of hearing. It is denoted by Io.
For sound of frequency 1 KHz, Io =10?12 W m?2. The level of sound intensity is measured in decibel. According to Weber-Fechner law,
decibel level (β) = log10[I/I0]
where Io is taken as 10?12 W m?2 which corresponds to the lowest sound intensity that can be heard. Its level is 0 dB. I is the maximum intensity that an ear can tolerate which is 1W m?2 equal to 120 dB.
β = 10 log10(1/10-12)
β = 10 log10(1012)
β = 120 dB
List gives the decibel value and power density (intensity) for various sources.
Intensity of sound sources
Source of sound Sound Intensity
intensity(dB) (W m?2)
Threshold of pain 120 1
Busy traffic 70 10?5
Conversation 65 3.2 ? 10?6
Quiet car 50 10?7
Quiet Radio 40 10?8
Whisper 20 10?10
Rustle of leaves 10 10?11
Threshold of hearing 0 10?12
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