Question

A vehicle sounding a whistle of frequency $256Hz$ is moving on a straight road, towards a hill with a velocity of $10m{s}^{-1}$. The number of beats per second observed by a person travelling in the vehicle is velocity of sound $=330m{s}^{-1}$

• A. zero
• B. 10
• C. 14
• D. 16

Answer 1

Answer: (D)

Apparent frequency heard by the observer,
$n^{{}^{\prime }}=\left(\frac{v+v_s}{v-v_s}\right)\times n$
$=\left(\frac{330+10}{330-10}\right)\times 256$
$=\frac{340}{320}\times 256$
$=272Hz$
$\therefore$ Number of beats heard by the observer
$=272-256=16$