Question

A point source emits sound equally in all directions in a non absorbing medium. Two points P and Q are at distances of 9m and 25m respectively from the source. The ratio of the amplitudes of the waves at P and Q is

• A. 5 : 3
• B. 3 : 5
• C. 25 : 9
• D. 625 : 81

Answer 1

The correct option is C 25 : 9

$I=\frac{p}{4\pi r^2}$

$andI\propto A^2$

$So,A^2\propto \frac{1}{r^2}$

$orA\propto \frac{1}{r}$

$So,\frac{A_1}{A_2}=\frac{{}^1{/}_{r_1}}{{}^1{/}_{r_2}}$$=\frac{r_2}{r_1}$$=\frac{25}{9}$