Question

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

• A. zero
• B. 10
• C. 14
• D. 16

Answer 1

Answer: (D)

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