Sound and Oscillation

Sound intensity level with examples

“Sound energy passing per second through a unit area held perpendicular to the direction of propagation of sound waves is called intensity of sound.”The sound waves transfer energy from the sounding body to the listener. The intensity of sound depends on the amplitude of sound waves. Intensity is a physical quantity and can be measured accurately. The unit of intensity of sound is watt per square meter (Wm-2).

Sound intensity level

The human ear responds to intensities ranging from 10-12 Wm-2 to more than 1Wm-2 (which is loud enough to be painful). Because the range is so wide, intensities are scaled by factors of ten. The barely audible and the faintest intensity of sound is 10-12 Wm-2 is taken as reference intensity, called zero bel (a unit named after Alexander Graham Bell).

The loudness of a sound depends not only on the intensity of sound but also on the physical conditions of the ear. The human ear is more sensitive to some frequencies rather than others.

The loudness (L) of a sound is directly proportional to the  logarithm of intensity i.e.,

L ∝ log I

L =K log I  ……(1)

Where k is a constant of proportionality. Let Lbe the loudness of the faintest audible sound of  intensity Iand L be the loudness of an unknown sound of intensity I, then by equation (1), we can write

L0 =K log I0……..(2)

Subtracting equation (2) from (1), we get:

L – L0=K(log I -log I0)

=K log I/I0

This difference,(L-L0), between the loudness L of an unknown sound and the loudness L0 is called the intensity level of the unknown sound. Therefore, the intensity level of an unknown sound is given by

Intensity level = k log I/I0    ……..(3)

The value of k depends not only on the units of I and I0 but also on the unit of intensity level. If intensity I of any unknown sound is 10 times greater than the intensity Iof the faintest audible sound i.e., I=10 I0, and the intensity level of such a sound is taken as a unit, called bel, the value of k becomes 1. Therefore, using k=1 in equation (3) becomes:

Intensity level = log I/I0 (bel)   …..(4)

Bel is a very large unit of the intensity level of a sound. Generally, a smaller unit called decibel is used. Decibel is abbreviated as (dB). It must be remembered that 1 bel is equal to 10 dB. If the intensity level is measured in decibels, then equation (4) becomes:

Intensity level = log I/I(dB) ……….(5)

See Also: Sound Vs Noise

Using equation (5), we can construct a scale for measuring the intensity level of sound. Such scale is known as the “decibel scale”.The intensity level of different sounds in decibel is given in the table:

Sources of sound Intensity (Wm-2) Intensity level (dB)
Nearby jet airplane   103 150
 Fast train   101  130
Siren 100 120
Lawn mover 10-2 100
Vacuum cleaner 10-5 70
Mosquito buzzing 10-8 40
whisper 10-9 30
Rustling of leaves 10-11 10
Faintest audibel sound 10-12 0

The decibel scale is a logarithmic measure of the amplitude of sound waves. On a logarithmic scale, equal intervals correspond to multiplying by 10 instead of adding equal amounts.

Logarithmic scale Decibels (dB) Linear scale Amplitude (m)
0     1
20       10
  40        100
60         1,000
   80          10,000
100           1000,000
120           1,000,000

Watch also video about intensity of sound:

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