Doppler Effect

Everyone has noticed a passing train's whistle sound distinctly different -- higher-pitched as it approaches, lower-pitched as it recedes -- even though the train's whistle itself is producing one unchanged frequency the entire time. The exact same shift in perceived pitch happens if instead the LISTENER moves toward or away from a stationary sound source. This phenomenon, first identified in 1842 by the Austrian physicist Christian Doppler (1803-1853), is called the Doppler effect: whenever a sound source and a listener are in RELATIVE motion with respect to each other, the frequency the listener actually hears differs from the frequency the source is actually emitting. …