Page 25 - Physics 10_Float

SOUND

L - L = K (log - log ) = K logI I o Table 11.1
o
I Sources of Intensity Intensity
I Sound -2 level (dB)
This difference,(L- L ), between the loudness L of an unknown (Wm )
o
o
sound and the loudness L is called the intensity level of the
o
unknown sound. Therefore, the intensity level of an Nearby jet 10 3 150
airplane
unknown sound is given by Jackhamm- 1
er/Fast train 10 130
Intensity level = K log .......... (11.3)
I 0
Siren 10 120
I
o
The value of K depends not only on the units of and but I I o Lawn 10 -2 100
mover
also on the unit of intensity level. If intensity I of any
Vacuum -5
unknown sound is 10 times greater than the intensity I o of cleaner 10 70
the faintest audible sound i.e., =10 and the intensity level I I o Mosquito -8
of such a sound is taken as unit, called bel, the value of K buzzing 10 40
becomes 1. Therefore, using K =1, Eq. (11.3) becomes Whisper 10 -9 30

Rustling of -11 10
Intensity level = log (bel) .......... (11.4) leaves 10
I
I Faintest -12 0
bel is a very large unit of intensity level of a sound. Generally, audible 10
o
sound i.e.,
a smaller unit called decibel is used. Decibel is abbreviated as Threshold
(dB). It must be remembered that 1 bel is equal to 10 dB. If the
For your information
intensity level is measured in decibels, Eq. (11.4) becomes
Logarithmic Linear
scale scale
Intensity level = 10 log (dB) .......... (11.5)
I Decibels Amplitude
I (dB) (m)
o
Using Eq. (11.5), we can construct a scale for measuring the
0 1
intensity level of sound. Such scale is known as “decibel
20 10
scale”. The intensity level of different sounds in decibel is
40 100
given in Table 11.1.
60 1,000
Example 11.1: Calculate the intensity levels of the (a) faintest 80 10,000
audible sound (b) rustling of leaves. 100 1000,000
Solution: (a) Intensity level of faintest audible sound can be 120 1,000,000
-2
calculated by substituting = I I = 10 -12 Wm in Eq. (11.5). The decibel scale is a
o
Therefore, logarithmic measure of the
amplitude of sound waves. In a
logarithmic scale, equal
Intensity level of faintest audible sound = 10 log dB
10 -12 intervals correspond to
= 0 dB multiplying by 10 instead of
10 -12 -11 -2
(b) As the intensity of the rustle of leaves is =10 W m , I adding equal amounts.

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