Question

A source of sound S is moving with a velocity 50 m/s towards a stationary observer. He measures the frequency of the source as 1000 Hz. What will be the apparent frequency of the sound when it is moving away from the observer after crossing him? The velocity of the sound in the medium is 350 m/s.

• A. 750 Hz
• B. 857 Hz
• C. 1143 Hz
• D. 1333 Hz

Answer 1

The correct option is A 750 Hz

When the source is coming to stationary observer,$n^{\prime }=\left(\frac{v}{v-v_s}\right)n$
or $n=\left(\frac{1000\times 300}{350}\right)Hz$
When the source is moving away from the stationary observer,
$n"=\left(\frac{v}{v+v_s}\right)n=\left(\frac{350}{350+50}\right)\left(\frac{1000\times 300}{350}\right)=750Hz$