The Doppler effect is an alteration in the observed frequency of a sound due to motion of either the source or the observer. The actual change in frequency is called the Doppler shift.
where f o is the frequency observed by the stationary observer, f s is the frequency produced by the moving source, v is the speed of sound, v s is the constant speed of the source, and the top sign i...where f o is the frequency observed by the stationary observer, f s is the frequency produced by the moving source, v is the speed of sound, v s is the constant speed of the source, and the top sign is for the source approaching the observer and the bottom sign is for the source departing from the observer.