The Doppler Effect for Sound Waves

In the Doppler Effect, the the properties of waves are influenced by motion with respect to the observer.
In the Doppler Effect, the the properties of waves are influenced by motion with respect to the observer. Dane Wirtzfeld, Getty Images

The Doppler effect is a means by which wave properties (specifically, frequencies) are influenced by the movement of a source or listener. The picture to the right demonstrates how a moving source would distort the waves coming from it, due to the Doppler effect (also known as Doppler shift).

If you've ever been waiting at a railroad crossing and listened to the train whistle, you've probably noticed that the pitch of the whistle changes as it moves relative to your position.

Similarly, the pitch of a siren change as it approaches and then passes you on the road.

Calculating the Doppler Effect

Consider a situation where the motion is oriented in a line between the listener L and the source S, with the direction from the listener to the source as the positive direction. The velocities vL and vS are the velocities of the listener and source relative to the wave medium (air in this case, which is considered at rest). The speed of the sound wave, v, is always considered positive.

Applying these motions, and skipping all the messy derivations, we get the frequency heard by the listener (fL) in terms of the frequency of the source (fS):

fL = [(v + vL)/(v + vS)] fS

If the listener is at rest, then vL = 0.
If the source is at rest, then vS = 0.
This means that if neither the source nor the listener are moving, then fL = fS, which is exactly what one would expect.

If the listener is moving toward the source, then vL > 0, though if it's moving away from the source then vL < 0.

Alternately, if the source is moving toward the listener the motion is in the negative direction, so vS < 0, but if the source is moving away from the listener then vS > 0.

Doppler Effect and Other Waves

The Doppler effect is fundamentally a property of the behavior of physical waves, so there is no reason to believe that it applies only to sound waves.

Indeed, any sort of wave would seem to exhibit the Doppler effect.

This same concept can be applied not only to light waves. This shifts the light along the electromagnetic spectrum of light (both visible light and beyond), creating a Doppler shift in light waves that is called either a redshift or blueshift, depending on whether the source and observer are moving away from each other or toward each other. In 1927, the astronomer Edwin Hubble observed the light from distant galaxies shifted in a manner that matched the predictions of the Doppler shift and was able to use that to predict the speed with which they were moving away from the Earth. It turned out that, in general, distant galaxies were moving away from the Earth more quickly than nearby galaxies. This discovery helped convince astronomers and physicists (including ​Albert Einstein) that the universe was actually expanding, instead of remaining static for all eternity, and ultimately these observations led to the development of the big bang theory.

Edited by Anne Marie Helmenstine, Ph.D.