http://www.ck12.org Chapter 12. Waves and Energy Transfer
sound you hear will have shorter wavelengths than the sound that was emitted by the trumpet player. The frequency
of the sound you hear would be double the frequency that was being emitted.
If the truck was moving away from you, the opposite change of frequency would occur. That is, the received
wavelength would be longer than the one emitted and the received frequency would be lower than the frequency
emitted. You may have noticed frequency changes in automobile sounds when you stand near a highway. Or that an
emergency vehicle’s siren is different when approaching you than when leaving.
You can also hear the Doppler shift in the classroom simply by striking a tuning fork and then moving the tuning
sharply toward you or away from you. Even though the tuning fork generates a constant tone, you will hear wavering
tones as you move the tuning fork around.
In terms of the listener, the train sketched on the left above is not moving and therefore the wavelengths and frequency
heard in all directions will be the same as the frequency being emitted. For the train on the right, however, since the
train is moving toward the right, the wavelengths measured to the right will be shorter than those emitted and the
wavelengths measured to the left will be longer than those emitted.
Christian Doppler (1803 –1853) did experiments in 1842 with trumpeters playing a single note as they sat on a
railroad flatcar and were pulled back and forth past a stationary observer. The Doppler effect also occurs when the