Question

A source of sound, of (true) frequency 1122 Hz is moving with a velocity of 10 m/s towards a wall. An observer, at rest, is positioned between the source and the wall. The velocity of sound, in air, is 340 m/s. The frequency of beats, heard by this (stationary) observer, would be

(1) Zero

(2) One

(3) Two

(4) Three

Answer 1

(1) Zero

The two sounds, reaching the stationary observer, are (i) the direct sound and (ii) the reflected (from the wall) sound. For both these sounds the ‘source’ (real or virtual (the reflected sound)) is moving towards the (stationary) observer with the same speed. Hence, the apparent frequencies of both the sounds, are equal to each other. Hence no beats would be heard (by the stationary observer).