Question

Tina is waiting at the bus stop. She sees and hears an ambulance as it approaches and passes by her. Which two changes in the loudness of the siren does Tina hear?
a.The sound becomes less loud as the ambulance approaches her.
b.The sound becomes louder as the ambulance approaches her.
c.The loudness of the sound remains the same as long as she can see the ambulance.
d.The sound stops when it reaches her.
e.The sound becomes less loud as the ambulance moves away from her.

Answer 1

Option B i.e The sound becomes louder as the ambulance approaches her,

Option E i.e The sound becomes less loud as the ambulance moves away from her, are the two changes in the the loudness of the Siren which Tina would hear.

The above changes are due to the Doppler Effect. According to Doppler Effect, effective wavelength is shortened when an approaching source moves closer during the period of sound wave and thus gives a higher pitch.  Likewise the pitch of a receding sound source will be lowered.

Answer 2

Tina observes B and E occur.

This increase and decrease in sound amplitude is due to the Doppler effect.