A source of sound of frequency 256 Hz is moving towards a wall with a velocity of 5 m/s. How many beats per second will be heard by an observer O standing in such a positoin that the source S is between O and wall? `(c=330(m)/(s)`)

Options

A. 8
B. 6
C. 10
D. 4

Answer

A

Explanation

(i) The observer is at rest and the ambulance is moving away from him. The apparent frequency of the siren heard by him is
`n_(1) = ((v)/(v + v_(s))) n = (330 xx 256)/((330 + 5))`
`= 256 xx (66)/(67) div 252 Hz`
(ii) He also hears the sound reflected from the wall. The apparent frequency of the reflected sound is
`n_(2) = ((v)/(v - v_(s)))n`
`= ((330)/(330 - 5)) xx 256 div 260 Hz`
`:.` No. of beats heard/sec `= 260 - 252 = 8`