Why does a Fire truck siren change pitch when it passes us?
The pitch of the siren of a Fire truck appears to change as the truck passes us due to the Doppler effect. Of course, to an observer on the truck, the pitch does not change at all. Since the speed of sound in air is essentially fixed, the perceived pitch of a tone is related to the wavelength of the sound. The shorter the wavelength, the higher the pitch, and vice-versa. The wavelength can be conveniently considered to be the distance between successive “crests” of the waves of pressure that constitute the sound wave. Consider a tone emitted in the forward direction in which the Fire truck is moving and consider the launching of two successive wave crests. The first crest is launched and travels in the (assumed still) air at the speed of sound (relative to the air). If the truck were not moving, the second crest would be launched after the first had travelled one wavelength. However, because the truck is moving in the same direction as the sound, the distance between the first crest an
