Two whistles Aand B produces notes of frequencies 660 Hz and 596 Hz respectively. There is a listener at the mid- point of the line joining them. Now the whistle B and the listener start moving with speed 30 m/s away from the whistle A. If speed of sound be 330 m/s, how many beats will be heard by the listener

Options

A. 2
B. 3
C. 4
D. 5

Answer

C

Explanation

There is no relative motion between the source B and the man. Hence the frequency of the note B, heard by the man will be 596 Hz.
But the man is moving away from A.
`:.` The apparent frequency of A as heard by the man is
`n_(A) = ((v - v_(0))/(v)) n = ((330 - 30)/(330)) xx 660 = 300 xx 2 = 600 Hz`
`:.` No. of beats/sec `= 600 - 596 = 4`