Loudness of sound from an isotropic point source at a distance of 10m is 20dB. If the distance at which it is not heard is 10 K in meters find K.
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(2)
Intensity from a point source varies with distance as 
where ϕ is the energy flux.
Loudness in dB = 10 log 
where I r is the reference intensity
L = 10 log 
.....(i)
For r = 10 m,
L = 20 dB gives,
20 = 10 log 
......(ii)
Subtracting I and II and putting L = 0 we get
20 = 10 log 
20 = 20 log 
1 = log 
r = 10 2 m
Hence power of 10 is 2.
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