The Doppler Effect results from the
relative motion between the sound source and the
detector. The basic idea is that when the motion
is to decrease the distance between source and detector,
the frequency or pitch of sound goes up. When they
separate, the frequency or pitch lowers. Everybody
has heard the Doppler shift, so all you've got to
do is remember what you know about the real world.
As the car approaches, the radio or horn or engine
sounds higher pitch, and as it goes away from you,
it sounds lower. Like the sounds at a race track.
And if the velocity of separation between source
and detector is zero, there is no shift in pitch.
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The demo shown at the left is a rotating horn.
There is a reed in the funnel and as the device
spins air is scooped into the funnel and the reed
vibrates --- much like a harmonica. But as the horn
rotates, for part of the cycle it is coming closer
to you and you hear a higher pitch. For another
part of the cycle, it is going away from you and
you hear a lower pitch. |
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Have you ever heard an electronic organ at a
hockey or baseball game that has a distinct vibrato?
You know, that slight up and down in pitch of
every note? Before complete reliance on electronics,
organs used to have a rotating speaker inside.
That vibrato you heard was caused by the speaker
alternately coming toward you and going away from
you. Really. It's true. Look it up.
See also: Beats
- Visualization and Beats
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