Question

A sound source is moving with speed $5m/s$ towards a wall. If the velocity of sound is $330m/s$, then the stationary observer would hear beats equal to (frequency of source $=240Hz$):

• A. $0$
• B. $9$
• C. $8$
• D. $4$

Answer 1

The correct option is C $8$

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.

$v=v\left(\frac{c–v_o}{c–v_s}\right)$

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

$v=256[330–(-5)/330–5]=264Hz$

∴ Beat frequency $=264–256=8$