Question

A motor car blowing a horn of frequency 124 vibration/sec moves with a velocity 72 km/hr towards a tall wall. The frequency of the reflected sound heard by the driver will be (velocity of sound in air is 330 m/s)

• A. 109 vibration/sec
• B. 132 vibration/sec
• C. 140 vibration/sec
• D. 248 vibration/sec

Answer 1

Answer: (C)

140 vibration/sec

Answer 2

In the given condition source and listener are at the same position i.e. (car) for given condition

$n' = \frac{v + v_{car}}{v - v_{car}}.n = \frac{330 + 20}{330 - 20}.n$ = 140 vibration/sec