A moving source

A point source emits spherical waves. When the source is stationary, the wave fronts are concentric spheres, centred on the


Christian Johann Doppler. Courtesy of Austrian Post.

source. As the source starts to move, it essentially chases its own wave fronts, compressing the sound waves it has produced.

The drawing shows how successive wave fronts are pushed along to the right. The wave velocity is unchanged but the wavelength is smaller in front of the source. An observer in front of the source will receive more waves per second and measures a higher frequency than an observer behind the source.

In general, the frequency is higher when the source and the observer approach each other, and lower when they move away from each other.

7.7.3 Two Doppler effects?

An observer approaches a stationary source at one half the speed of sound in air and is subsequently approached, when

Stationary point source Source moves to the right at constant speed

stationary, by a source moving at the same speed. Is the apparent frequency the same in both cases?
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