2-7 The Doppler effect
Introduction

The Doppler effect is very useful in measuring the speed of some objects.  This is done by comparing the frequency of waves coming from stationary objects and from moving objects.  Doctors use the Doppler effect to measure the blood flow in arteries and hearts, policemen use it to detect speedy cars,  and astronomers use it to find out the speed of stars.

You might have noticed the change in the nature of the sound coming from the siren of an ambulance as it moves past you.  This change is caused by the change of the frequency of the sound due to the motion of the ambulance.  When the ambulance approaches you, you hear the sound at a higher frequency than if the ambulance were stationary.  When the ambulance leaves you, you hear the sound at lower frequency than if the ambulance were stationary.  The change of the frequency caused by the relative motion between the source of sound and the detector is known as the Doppler effect or the Doppler shift, after the Austrian physicist Johann Christian Doppler who, in 1842, was the first to explain it..      

The relative motion between the microphone and the speaker causes the frequency detected by the microphone to change as the speaker rotates.  When the speaker approaches the microphone the frequency becomes higher and when the speaker move away for the microphone the frequency becomes lower.
 
Click here to hair a sound of a moving ambulance