Question

A point source emits sound equally in all directions in a non-absorbing medium. Two points P and Q are at distance of 2m and 3m respectively from the source. The ratio of the intensities of the waves at P and Q is :

• A. 9 : 4
• B. 2 : 9
• C. 9 : 2
• D. 4 : 9

Answer 1

Answer: (A)

9 : 4

Answer 2

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

Also 1 $\propto$ $\frac{1}{r^2}$=$\frac{I_1}{I_2}$=$\bigg(\frac{r_2}{r_1}\bigg)^2$

we have r1=2m, r2= 3m
substituting w have $\frac{I_1}{I_2}$=$\bigg(\frac{3}{2}\bigg)^2$=$\frac{9}{4}$
i.e, 9:4

Therefore, the correct option is (A): 9 : 4