Sound: Doppler Applet
The Doppler Effect
Click the mouse button within the window to start the applet
click it again to stop it.
Left click the mouse button and drag it to show the displacememt.
Right click the mouse button and drag the mouse to show the angle.
Even if you have never heard of the Doppler effect, you have probably experienced it in your life. The Doppler effect occurs when there is relative motion between the source of sound or the observer and the medium through which the sound wave is propagating. It is named for a 19th century physicist, Christian Johann Doppler. Simply put, the frequency of the sound increases relative to a stationary observer as the source of sound approaches, and decreases as it moves away. The effect also occurs when a moving observer approaches or retreats from a stationary source. This is because as the observer or source move, the effective wavelength changes. As the observer approaches the source, or the source approaches the observer, the wavelength decreases, and since the speed of sound remains constant more wavefronts from the source hit the observer in a given period of time, so the frequency increases. The phenomenon is not unique to sound waves, but is most commonly noted in sound waves because the change in observed frequency results in a changing pitch, giving an easy way for people to hear when the Doppler effect is taking place. The change in pitch of a car horn as the car passes a stationary observer is an everyday example of the Doppler effect.
