Loudness of sound from an isotropic point source at a distance at a distance of 70 cm is 20 dB. What is the distance (in m) at which it is not heard

(7)

Intensity from a point source varies with distance as

Let at distance m, intensity is

Then given 20       (i)

Let for , sound level be zero. Then intensity at that point should be

and      (ii)

From Eqs. (i) and (ii), we get