Dopplar's effect

When source of sound and observer in motion either or both (or relative motion) then their will be apparent change in frequency. This phenomenon is known as Doppler's effect.
CaseI : Sound in motion observer in rest
Let us consider a source of sound which generates sound of frequency f wavelength λ and velocity V be moving toward a stationary observer O with velocity Us. Then observer feels apparent change in wave length as
λ = (V-Us)/f--------------1
Now relative motion of wave with respect to observer will be V - 0 = V
So the apparent frequency observered by the observer will be
f1 = v /λ = V/(V-Us) * f-----------2
Here V-Uo < U so clearly f1 > f SO loud sound is heared when the source moves towards stationary observer.

CASEII: When source is moving away the observer then Us will be negative and hence apparent freq.
f1 = V/(V+Us) * f--------------3

CASEIII : WHen observer in motion and source in rest
Let an observer O be in motion and source is in rest. Let the observer wave toward the source with velocity Vo, let the velocity of sound wave be V. Wave length λ and frequency f
In this case the wave length of sound remains unchanged and be given as V/P - λ
Here since the observer is moving towards source relative velocity of wave with respect to observer is V+Vo
Therefore the apparent change in frequency will be
f1 = relativevelocity/wave length = V+Vo\λ
.-.f1 = (V+Uo)/V * f------------4
Since V+Vo is greater than V the pitch of sound increases and sound will be heard louder

CASE IV:
If the observer is moving away from source relative velocity will be V-Vo apparent freq.
f1 = (V-Vo)/V * f --------------5