First slide

Doppler

Question

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

Moderate

A

750 Hz
B

857 Hz
C

1143 Hz
D

1333 Hz

Solution

When the source is coming to the stationary observer,
$n^{'} = (\frac{v}{v - v_{s}}) n or 1000 = (\frac{350}{350 - 50}) n$
or $n = (\frac{1000 \times 300}{350}) Hz$
When the source is moving away from the stationary obsrver.
$n^{''} = (\frac{v}{v + v_{s}}) n$
= $(\frac{350}{350 + 50}) (\frac{1000 \times 300}{350})$
= 750 Hz

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