Question

According to Doppler effect, which of the following changes apparently ?

• A. loudness
• B. pitch
• C. amplitude
• D. velocity
• E. acceleration

Answer 1

The correct option is B pitch

It is a common perception we feel while standing on a platform at a train station that if the train is approaching one feels as though the sound level of its horn is increasing gradually and on the other hand if the train is receding we hear a fainter sound.

This is because of the Doppler shift in its frequency, given by (for stationary observer):

(for approaching source) $f^{\prime }=f_o(1+\frac{v_s}{v})$

(for receding source) $f^{\prime }=f_o(1-\frac{v_s}{v})$

$f^{\prime }$ apparent frequency (this is what we hear), $f_o$ real frequency, $v_s$ velocity of source, $v$ velocity of wave

As the frequency changes, it is affecting the pitch of the sound.

Therefore option B is correct.