Answer to Question #19558 in Mechanics | Relativity for miks
Explain how a driver of a moving car hears a siren but
can NOT hear the pitch of the siren changing.
The phenomenon has to do withthe Doppler effect or shift. The driver, i.e. receiver of the acoustic signal, is moving at a certain non-zero speed, so the perceived acoustic wave frequency (which determines the pitch) is different from that emitted by the source (i.e. the siren).