1. Doppler Effect
Doppler Effect – The apparent change in frequency of a wave due to the motion of the source and/or the observer
Stationary Source –
Moving Source –
In front of source:
Small wavelength, high freq.
Behind source:
Large wavelength, low freq.

2. Doppler Effect If you are moving toward the source, add vo
If you are moving away from the source, subtract vo
vo = velocity of observer
fo = frequency observed (heard)
fs = frequency of source
v = velocity of sound
(in air ≈ 343 m/s)
vs = velocity of source
If the sound source is moving toward you, subtract vs
If the sound source is moving away from you, add vs

3. Example 5
The siren of a police car at rest emits a frequency of Hz. What frequency will you hear if you are at rest and the police car moves at 25.0 m/s…
Toward you
Away from you

4. Example 6
A siren from a police car has a frequency of Hz and is moving with a velocity of 45.0 m/s. You are driving on the other side of the road towards the police car with a velocity of 36.0 m/s. What is the observed frequency from the police car? Assume the velocity of sound as 343 m/s.