Doppler Effect

You hear the high pitch of the siren of the approaching ambulance, and notice that its pitch drops suddenly as the ambulance passes you. That is called the Doppler effect.

Doppler Effect

When a vehicle with a siren passes you, a noticeable drop in the pitch of the sound of the siren will be observed as the vehicle passes. This is an example of the Doppler effect. An approaching source moves closer during period of the sound wave so the effective wavelength is shortened, giving a higher pitch since the velocity of the wave is unchanged. Similarly the pitch of a receding sound source will be lowered.

Doppler Wavelength Change

The speed of sound is determined by the medium in which it is traveling, and therefore is the same for a moving source. But the frequency and wavelength are changed. The wavelengths for a moving source are given by the relationships below. It is sometimes convenient to express the change in wavelength as a fraction of the source wavelength for a stationary source:

Doppler Effect Frequency Calculation