The pitch of a sound detected by our ears is determined by the period of oscillation of our ear-drums as they respond to the minute pressure modulations. If there is relative motion between source and detector (ear) then the rate at which crests of pressure waves reach the ear is effected and hence we sense a modified frequency.
If the detector (ear) moves towards the source with a velocity
then the eardrum intercepts
maxima in a time t and hence the frequency of oscillation of the
ear drum, which is the perceived frequency of the sound, is
If the detector moves away from the source then
There is a similar effect when the source moves such that in summary we can write
where the upper signs which increase 
correspond to
motion which decreases the source detector distance. You
have noticed this effect when an emergency response vehicle
drives by you with sirens on. The frequency appears higher when
it comes towards you as compared to it moving away from you.
We have an example of the Doppler effect:
If I swing a source round in a circle
it moves against you at one point in its trajectory
and away from you at another point. we have
which should produce a shift of
Putting in numbers we get
We hear a characteristic chirping sound which is the result of
Doppler shifted frequency.