A siren emitting a sound of frequency 1000 Hz moves away from you towards a cliff at a speed of 10 m/s. (a) What is the frequency of the sound you hear coming directly from the siren ? (b) What is the frequency of the sound you hear reflected off the cliff (c) What beat frequency would you hear ? Take the speed of sound in air as, 330 m/s.

Explanation

The situation is as shown in figure.

(a) Frequency ofsound reaching directly to us (by S)
`f_1 (v/(v +v_s)) f = (330/(330 +10)) (1000) = 970.6 Hz`
(b) Frequecny of sound which is reflected of from the cliff is same as from S.
`f_2 = (v/(v -v_s)) f = ((330)/(330 " " 10))(1000)`
` = 1031.3 Hz`
(c ) Beat frequncy `=f_2 -f-1 =60.7 Hz`
(Too high to be heared as beats)