A motor car blowing a horn of frequency $124\,vib/sec$ moves with a velocity $72\, km/hr$ towards a tall wall. The frequency of the reflected sound heard by the driver will be .... $vib/sec$ (velocity of sound in air is $330\, m/s$)
Diffcult
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(c) The frequency of reflected sound heard by the driver
$n' = n\,\left( {\frac{{v - ( - {v_0})}}{{v - {v_s}}}} \right) = n\,\left( {\frac{{v + {v_0}}}{{v - {v_s}}}} \right)$
$ = 124\,\left[ {\frac{{330 + (72 \times 5/18)}}{{330 - (72 \times 5/18)}}} \right] = 140\, vibration/sec.$
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