A van is moving with a speed of `3ms^(-1)` towards a large wall. A person standing on the line of motion of the van observes the van to be moving away from him. The horm of the van is now blown. The frequency of the sound produced by the horn is 600 Hz. what is
(i) The frequency of sound heard by the person for the sound produced directly by the horn?
(ii) The freuquency of sound reflected by the wall.
(iii) The baat frequency heard by the driver. the speed of sound in air is `330ms^(-1)?`

Explanation

(i) Since the van moves away from the person, the source is receding from the observer. Hence the frequency `v_(a)` heard is given by.
`v_(a)=v_(0)((V)/(V+V_(S)))`
or `v_(a)=600((330)/(330+3))=594.6Hz`
`~~594`Hz
(ii) Te frequency of sound reflected by the wall is the same as the frequency of sound received by the wall. now the source is approaching the wall. hence the frequency `v_(W)` of sound reflected by the wall is given by
`V_(W)=v_(0)((V)/(V-V_(S)))`
or `v_(W)=600((330)/(330-3))`
`=605.5Hz`
`=606Hz`
(iii) The beat frequency heard is the difference between the two frequencies
`therefore` Beat frequency`=606-595`
=11Hz ltbr Incidentally this beat frequency is not perceptible to the human ear because of the phenomenon of persistence of hearing. this persistance of hearing is about `(1)/(10)th` of a second. therefore the maximum beat frequency we can hear is about 10Hz.