Doppler Effect

    A sudden change in pitch of a passing siren on a police car or the horn of a train is called the Doppler Effect.  Explained by Christian Doppler in 1842, the Doppler Effect is the shift in wavelength and frequency of waves which results from the source of the sound moving towards or away from the receiving medium (ie. a person listening).  A person experiencing the Doppler Effect will hear a higher pitch when the source is approaches and a lower pitch when it departs. 

A stationary sound source emits waves that look like those in figure 1.  A listener that near this sound source will experience the same sound regardless of whether or not they are in front or behind the source.

Fig 5

When the sound source is moving the sound waves move in a pattern shown by figure 2.  This is what the waves look like when a person is experiencing the Doppler Effect.  The waves in front of the moving sound source are bunched up because even though the wave fronts are the same frequency, they are now slightly displaced to the right.  When the waves in front of the source are bunched up then a listener behind  the sound source will hear a lower frequency and an listener in front of the sound source will hear a higher frequency.

Fig 6

A sound source moving at much faster speeds such as Mach 1, the speed of sound,  will cause all of the waves in the front to bunch up at a single point.  An listener who witnesses this will hear nothing until the sound source arrives.  If the sound source is moving faster than the speed of sound, a listener will hear a sonic boom.  Because the sound source is moving faster than the sound waves it creates then the source can pass a stationary listener before they even hear the sound created.  Because the sound waves are so compressed, after the sound source passes a listener  hears all of the combined wave fronts at the same time.  A sonic boom.

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