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Here is a quick guide to understanding the doppler effect, to help you do your physics homework or assignment:
For an observer moving at a speed  towards the source of a sound moving at  , the observed frequency of the sound is
 "f_o=f_s(\frac{v+v_o}{v-v_s})")
where  is the speed of sound at 20o C. is positive when moving towards the source, and is positive when moving towards the observer.
Examples
1. Standing at a crosswalk, you hear a frequency of 560 Hz from the siren of an approaching ambulance. After the ambulance passes, the observed frequency of the siren is 480 Hz. Determine the ambulance's speed and the frequency of the source.
given,
When the ambulance is approaching,
 "560=f_s(\frac{343}{343-v_s})")
and when the ambulance is receding,
 "480=f_s(\frac{343}{343+v_s})")
solving this system gives
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