Anybody who has been passed by an ambulance at high speed has experienced a physical effect called the Doppler shift: As the ambulance moves toward the listener, its motion compresses the siren's sound waves and raises the sound pitch. As the ambulance moves away from the listener, the sound waves get dilated and the pitch is lowered. A listener wearing a blindfold could use this Doppler shift pattern to track the motion of the ambulance.
* This article was originally published here
