Doppler Effect Calculator and Sound Simulator
This phenomenon is familiar to many people: a source of noise, e.g. an ambulance, passes by, its sound changes from high to low. This is the acoustic Doppler effect. Sound is made of waves, which move at a certain speed. If sound source and listener are approaching to each other, the single waves come in with a smaller distance from each other, the sound appears higher. If sound source and listener are moving away from each other, the single waves come in with a larger distance from each other, the sound appears lower.
Original sound
The formula, when source and listener are approaching to each other, is:
fs = fu * (c + vz) / (c - vs)
The formula, when source and listener are moving away from each other, is:
fs = fu * (c - vz) / (c + vs)
fs = frequency of the appearing sound
fu = frequency of the original sound
c = speed of sound
vz = speed of the listener
vs = speed of the source
The speed of sound is 767.27 mph = 343 m/s = 1234.8 km/h
It makes a difference, if source or listener are in motion. When the source moves and the listener is standing still, the tone pitch is higher as if the listener moves with the same speed and the source of sound doesn't.
The Doppler effect was predicted by Christian Doppler in 1842, surprisingly initially not for sounds but for light. When light moves toward the observer, it appears blue-shifted, when it moves away, it appears red-shifted. See the Redshift Calculator.
In 1845 the acoustic Doppler effect was demonstrated. Trumpeters stood on a train and had to hold a note while they passed musically trained listeners. The train was traveling at about 70 kilometers per hour, the note of the approaching train was about a semitone higher, that of the departing train about a semitone lower.
Light is much faster than sound, so the movement between the object and the observer must also be faster in order to be perceived. The optical Doppler effect could therefore only be detected twenty years later. At the spectroscopic analysis of the star Sirius, it was found that the light is red-shifted, meaning that the star is moving away from us.
See also Redshift and Blueshift