Two whistles A & B produces notes of frequencies 660 Hz and 596 Hz respectively. There is a listener at the middle point of the line joining them. Both the whistles 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. 4
C. 6
D. 8

Answer

B

Explanation

Since B and listener are moving with same velocity the frequency of note from B remains unchanged, freq. of note from A will be
`f_(A) = (330 - 30)/(330 - 0) xx 660 `
= 600 Hz.
Beat freq. = 600 - 596 = `4s^(-1)`.
correct choice is (b).