Unit 1: Doppler Effect With Sound and Ultrasound

A common example of the Doppler Effect occurs when a car, or any object emitting a sound, passes by, then there is a change in the pitch of the sound coming from the sound source.  As the sound source approaches you, the sound heard has a higher pitch (higher frequency) and as the sound source moves away from you, the sound heard has a lower pitch (lower frequency). This phenomenon is known as the Doppler Effect

IMPORTANT FACTS CONCERNING DOPPLER EFFECT
Both the SOURCE and LISTENER are STATIONARY


Consider a STATIONARY police car. The siren of the police car is ON. The siren is the source (S) of the sound. The frequency of the source (fs ) (the siren) is 800 Hz. fs = 800 Hz. There are STATIONARY LISTENERS at point A and point B. The frequency of the sound heard by the STATIONARY LISTENERS (fL) is the SAME as the frequency of the STATIONARY SOURCE (fs ) fL = fs

 

The SOURCE IS MOVING TOWARDS A STATIONARY LISTENER at point A.

When the source of sound waves is moving towards a stationary listener, the frequency detected by the listener increases (wavelength decreases) because more wavefronts are detected per second. (fL>fs) frequency is inversely proportional to wavelength

 

THE SOURCE IS MOVING AWAY FROM A STATIONARY LISTENER which is at point B.

When the source of sound waves is moving away from a stationary listener, the frequency detected by the listener decreases (wavelength increases), because less wavefronts are detected per second. (fL<fs)

 

THE LISTENER IS MOVING TOWARDS A STATIONARY SOURCE at point A:

When the listener is moving towards a stationary source, the frequency detected by the listener increases (wavelength decreases), because more wavefronts are detected per second. (fL>fs)

 

THE LISTENER IS MOVING AWAY FROM A STATIONARY SOURCE:

When the listener is moving away from a stationary source, the frequency detected by the listener decreases (wavelength increases), because less wavefronts are detected per second. (fL<fs)