Question

In which of the following cases a listener experiences Doppler effect?

• A. The listener and the source of sound are stationary
• B. The listener and source of sound are moving with same velocity
• C. The listener is moving towards the source of sound
• D. None of the above

Answer 1

Answer: (C)

The listener is moving towards the source of sound

The Doppler effect (or the Doppler shift) is the change in frequency or wavelength of a wave (or another periodic event) for an observer moving relative to its source.
So if both listener and source of sound are stationary or are moving with the same velocity that is relative velocity is zero, there won't be any change in frequency.
Thus, in this case, the effect will arise when a listener is moving towards the source of the sound.