Question

A trumpet player is playing a musical note. A boy is running at a constant velocity toward the trumpet player, then stops quickly when he is 2.0 meters away. As the boy stops running, what change in pitch, if any, occurs to the musical note he perceives?

The pitch becomes higher
The pitch remains the same
The pitch becomes lower

Answer 1

According to Doppler's law when a listener move towards a source of sound at rest, the frequency observed by the listener is greater than the actual frequency of sound coming from the source. This observed frequency by the listener is given as

f' = (V + v) f/V

where f = actual frequency of the sound from source

V = speed of sound , v = speed of listener towards the source.

Since (V+v)/v > 1

hence f'> f

but when the boy stops , he starts hearing the actual frequency of sound from the source which is smaller to what he was hearing before stopping. hence

The pitch becomes lower