Why does the sound of an ambulance siren change as it passes?
The Doppler effect, named for Austrian scientistChristopher Doppler, who described it in 1842, is a change in the frequency of a wave due
to motion of the source relative to the observer. You can detect the Doppler effect whenever an object that makes a constant sound is moving
relative to your position. Examples include the sound of a whistle on a passing train, a siren on a police car, and even a mosquito flying by.
You can picture the Doppler effect by thinking of sound waves at a specific wavelength as compressions in the air that occur at a constant
interval. As each compression enters your ear, it causes sensors to vibrate and send a message to your brain that is interpreted as sound. Shorter wavelengths have a greater frequency—that is, compressions occur at a greater rate. A higher frequency corresponds to a higher pitch. If the source of the sound is moving toward you, each compression starts at a point that is closer to you than the previous compression. As a result, succeeding waves reach your ear more frequently than they are emitted by the source, so the sound has a higher pitch. As the source moves away from you, the waves are spread out and reach you with less frequency. The pitch of the sound is then lower. The same thing would happen if the source of the sound were stationary and you were moving quickly toward or away from it.
Reference: The Science Of Everything – Steve Miller
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Filed Under: Science
