Question

A person carrying a whistle emitting continuously a note of $272Hz$ is running towards a reflecting surface with a speed of $18km/hour.$ speed of sound in air is $45m/s.$ The number of beats heard by him is$:$

• A. 4
• B. 6
• C. 8
• D. 3

Answer 1

The correct option is C 8

According the concept of sound image$:$

$n^{\prime }=\frac{v+v_{person}}{v-v_{person}}\times 272$

$=\frac{345+5}{345-5}\times 272=280Hz$

$\Delta n=Numberofbeats=280-272=8$
Hence,

option $\left(C\right)$ is correct answer.