Astudent hears a police siren. What would change the frequency that the student hears?
The Doppler effect
The Doppler effect is caused by a difference between the velocity of the source (e.g. siren) and the velocity of the detector (e.g. ear).
The reason is that the period (time between cycles) of the waves is changed from that of the source, because each successive cycle has a slightly different distance to travel, so takes a slightly shorter (or longer) time to get to the detector.
This means that the cycles appear to be closer together (or further apart) to the detector than they would if both were at a constant distance apart (e.g. both stationary or both traveling at the same velocity)
The pitch or frequency (number of cycles per second) of the siren will be higher when the source and detector are getting closer to each other and lower when they are moving apart.