Doppler Effect

The response of sound waves to moving bodies is illustrated in the example of the sounding of the locomotive whistle of a moving train. When the train blows its whistle while it is at rest in the station, stationary listeners who are either ahead of the engine or behind it will hear the same pitch made by the whistle, but as the train advances, those who are ahead will hear the sound of the whistle at a higher pitch. Listeners behind the train, as it pulls further away from them, hear the pitch of the whistle begin to fall.

The faster the train moves the greater will be the effect of the rising and falling of the pitch. Also, if the train remains at rest but the listeners either move toward the sounding train whistle or away from it, the effect will be the same. Those who move toward the train will hear a higher pitch, while those who travel away from the train will hear a lower pitch.

When the train is at rest it is the center of the sound waves it generates in circles around itself. As it moves forward, it ceases to be the center of the sound waves it produces. The sound waves move in the same direction of the train's motion. The train is chasing or crowding its waves up front, compressing them, so that the listener in front of the direction of its movement hears more waves per second, thus producing the effect of a higher frequency. The listener standing behind the train hears a lower pitch because the waves have spread out behind the forward motion of the train. Thus, there are fewer waves per second. The listener is now hearing a lower frequency than is actually being produced by the whistle.

In 1845, the Doppler effect received further confirmation in an elaborate experiment devised by a Dutch meteorologist, Christopher Heinrich Buys-Ballot. He placed a band of trumpet players on an open railroad flatcar and had it ride by listeners with perfect pitch who recorded their impressions of the notes produced by the whistle. Their written recordings of the pitches clearly demonstrated the Doppler wave effect.