The Doppler Effect is a change in pitch, due to the relative motion between a source of sound and the receiver.
But, first of all....who is Doppler?!?
Christian Doppler (1805-1853) was an Austrian physicist who discovered the Doppler shift by conducting experiments with musicians on railway trains. Musicians played instruments as the train approached and receded the observers. Hippolyte Fizeau discovered independently the same phenomenon on electromagnetic waves in 1848 (in France, the effect is sometimes called "effect Doppler-Fizeau").
One of the most common examples of the Doppler effect is the change in pitch that we hear as a car horn or siren on a vehicle moves past us.
As the car speeds towards you, sirens blazing, the sound you hear is high in pitch because the sound waves in front of it are being compressed. This causes more vibrations per second to reach your ear (a higher frequency) resulting in a higher pitched sound. When the ambulance passes you, the sound becomes lower in pitch. Behind the ambulance there are fewer vibrations per second (a lower frequency), and therefore a lower pitch is heard.
The change in frequency can be calculated using the general Doppler equation:
f
'
=
f
where:
v = speed of sound (343m/s)
vo= speed of observer f ' = shifted frequency
vs = speed of source f = original frequency
This equation covers ALL cases where the source and/or the observer are moving. The upper signs are used for situations in which the source and the observer are coming closer together while the lower signs are used when they are moving away from one another. Using this formula, we can see that as the observer and the source approach each other, the frequency will increase.. As they move away from each other, the shifted frequency will be lower than the original.