Question

A source of sound of frequency 256 Hz is moving towards a wall with a velocity of 5 m/s. The number of beats per second heard by a stationary observer between the source and the wall is (Velocity of sound = 330 m/s)

• A. 8
• B. 3
• C. 5
• D. 2

Answer 1

The correct option is A 8

If the sound reaches the observer after being reflected from a stationary surface and the medium is also stationary$,$

the image of the source in the reflecting surface will become the source of the reflected sound$.$

$v=v[c–v-o/c–v_s]$

$v_o,$ $v_s$ are $+ve$ if they are directed from source to the observer and $–ve$ if they directed from observer to source$.$

$v=256[330–(-5)/330–5]=264Hz$

$\therefore$ Beat frequency $=264–256=8$

Hence,

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