12.3. Doppler Shift http://www.ck12.org
12.3 Doppler Shift
- Define the Doppler shift.
- Explain the Doppler shift with sound.
- Make calculations based on the Doppler shift.
At normal speeds, when an object generates sound, the sound travels away from the object and people hear the same
sound that was generated by the object. In some unusual cases, like this airplane, the object can travel as fast or
faster than the sounds it makes. The sound wave moves along with this airplane; as the airplane continues to generate
more sound, this sound is added to the old sound. When this extra large sound wave front arrives, the sound is much
louder than the sound that was originally generated. When this happens with airplanes, the sound is called a “sonic
boom.”
Doppler Shift
If an emitted sound is moving at a significant fraction of the speed of sound relative to the receiver, it is possible for
the sound emitted by a source and the sound received by a receiver to be different. This is called aDoppler shift.
Suppose the trumpet player in the sketch below is playing a constant note with a wavelength equal to 1.00 m/s. If you
are sitting down the road a ways and the truck is stationary, then the wavelength of the sound that reaches you will
be 1.00 m long and you will hear a note corresponding to that wavelength. Suppose now, that the speed of sound is
333 m/s and this truck is moving toward you at 166. m/s. When the trumpet player begins playing the tone, the first
crest will move toward you at 333 m/s. In the first 0.003 seconds, that first crest will move exactly 1.00 m (which
happens to be one wavelength for this sound). After the trumpet player generates the first crest, however, the truck
also moves and in the first 0.003 seconds, the truck moves 0.50 m. After the first 0.003 seconds, the trumpet player
creates a second crest and sends it on its way. When this second crest is generated, the first crest is only 0.50 m
away. If the truck did not move, the crests would reach your ear with a wavelength of 1.00 m, but because the truck
moves toward you at very high speed, the crests reach your ear at a distance of only 0.50 m apart. Therefore, the