Question

A source of sound waves is travelling as shown. In which situation would the stationary observer detect the largest decrease in the observed frequency?

[Assume, medium is stationary]

• A.
• B.
• C.
• D.

Answer 1

The correct option is D

Let $f_0$ be the frequency of sound heard by the observer, when both source of sound and observer are at rest and $v$ be the velocity of sound in the stationary medium.

When a source moves towards the observer :

The frequency heard by the stationary observer $f_{app}=\left(\frac{v}{v-v_s}\right)f_0$

Since, $v-v_s<v$ , we can conclude that, $f_{app}>f_0$

Hence, when the source moves towards the stationary observer, there is no decrease in the frequency of sound.
So, options (a) and (b) are wrong.

When a source moves away from the observer :

The frequency heard by the stationary observer $f_{app}=\left(\frac{v}{v+v_s}\right)f_0$

Since, $v+v_s>v$ , we can conclude that, $f_{app}<f_0$

Hence, when the source moves away from the stationary observer, there is a decrease in the frequency of sound.

Decrement is greater when $v_s$ is greater.

Thus, option (d) is the correct answer.