11.4. Doppler Effect http://www.ck12.org
11.4 Doppler Effect
Objective
The student will:
- Understand the Doppler effect.
Vocabulary
- Doppler effect:
Introduction
Have you ever heard a car honking its horn or ambulance blaring its siren as it zooms past you at high speed? It
has a distinctive sound, like “eerrrrrrrroooooooom”. It is not just that the sound gets louder, but it changes pitch, as
well. In other words, it changes in both amplitude and frequency. The sound is illustrated in the video below:
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/62087The car horn distinctly sounds like it has a higher pitch as it is getting closer, and a lower pitch as it goes away. Why
does this happen?
Moving Source, Stationary Observer
Sound is a vibration that travels through the air. Once the sound leaves the car, it travels at its normal wave speed
through the air, regardless of how the car is moving. So if the car is moving quickly, it is catching up to the sound
waves that it is sending forwards. This means that the wavelength is effectively shorter by exactly how far the car
moves over one period. When the car passes the observer and is moving away, then the car is moving away from
the sound waves it sends out, so the wavelength is longer. SeeFigure11.25.
When the wavelength is shorter, as seen to the right in theFigure11.25, the waves arrive together more frequently.
The higher frequency means a higher note or pitch, When the waves are farther apart, as in the left of theFigure
11.25, the waves arrive less frequently. Frequency decreases, meaning a lower note or pitch. This matches what we
know from the wave equation: