Doppler Effect for Sound
by Ron Kurtus (revised 6 November 2012)
The Doppler Effect for sound is the change in pitch or frequency that you hear when the source of sound is moving with respect to you. When the source of sound is moving toward you, the frequency is higher and the sound has a higher pitch. When the source is moving away from you, the sound has a lower frequency.
Note: You can also experience the Doppler Effect when you are moving toward or away from the source of the sound. However, you usually only experience the effect when the source is moving. Thus, we will emphasize that application.
The Doppler Effect is a result of the source of the sound moving to a different position for each vibration, thus shortening or lengthening the wavelength, while the speed remains the same.
The same effect occurs when sound is reflected off a moving object. This phenomenon is used in Doppler sonar to determine the speed of an object.
Questions you may have include:
- How does sound change when the source is moving?
- What causes the changes in frequency?
- What happens when sound reflects off a moving object?
This lesson will answer those questions. Useful tool: Units Conversion
Sound from a moving source
The sound you hear or detect from a moving source has a different frequency than the sound that comes from a stationary source. This shift in frequency is called the Doppler Effect.
For example, when you hear the sound of an object moving toward you—such as a siren from and emergency vehicle—the frequency of that sound is higher than if the vehicle was standing still. Likewise, as the vehicle moves away from you, the siren would have a lower frequency.
Note that the speed of the source must be less than the speed of sound. An aircraft flying at the speed of sound or greater creates a sonic boom, which is a different effect.
(See Traveling Faster than Sound for more information.)
Why frequency changes
When the source of sound is moving, the frequency of the sound that you hear is different, depending on your position with respect to the moving source. An important fact concerning this phenomenon is that the speed of sound in air is constant no matter what the speed of the source is.
When the source is moving in the same direction as the sound is moving, the source catches up to the previous wave crest, thus shortening the wavelength between crests. This increases the frequency. When the source is moving away from the direction that the sound is moving, the crests become spread apart.
Wavelength and frequency of sound from a moving source change according to your position with respect to the source
(See Doppler Effect for Waveforms for more information.)
Reflected off moving object
If a stationary source sends out sound waves and they are reflected back off a moving object, the sound waves heard or detected will be at different frequency, according to the direction the object is moving. This is as if the moving object was sending out the sound waves.
This phenomenon is used in Doppler sonar. While standard sonar uses sound waves to measure the distance to an object, Doppler sonar also measures the speed of the object.
The pitch of the sound you hear from a moving source will be either higher or lower than the emitted frequency, depending on the direction the source is moving. This is called the Doppler Effect. Knowing the initial frequency, the velocity of the source and the speed of sound, equations are available that allow you to calculate the new frequency. The angle between the source and the line-of-sight adds another factor to the equations.
Resources and references
Dan's Depot - MP3 of the Doppler effect for a train rolling by
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Doppler Effect for Sound