Doppler effect sound

Change in frequency of a wave for an observer which is moving with respect  to the source is called Doppler Effect. It was first proposed by Austrian physicist Christian Doppler in 1842. In day to day life we observe this phenomenon when a vehicle sounding a siren moves towards or away from an observer. The received frequency is higher (compared to the original emitted frequency) when source is coming nearer, it is identical at the instant of passing by, and it is lower when souce is moving away.

In mathematical form we write Doppler Effect as follows:
f= (V + Vr) / (V + Vs) * fo
Where:
V is the velocity of sound waves in the medium

Vr is the velocity of the receiver relative to the medium; taken positive if the receiver is moving towards the source.

Vs is the velocity of the emitting sound source relative to the medium; taken positive if the source is moving away from the receiver.

The frequency is decreased if either receiver or sound source moving away from the other.
Analysis

In Doppler Effect actually the frequency of the sounds that the source emits does not change. Let’s take a daily life example to understand what really happens. Suppose you throw one ball every second in your friend's direction. Assume that balls travel with constant velocity. If you are stationary, your friend will receive one ball every second. However, if you are moving towards your friend, he will receive balls more frequently because the balls will be less spaced out. The inverse is true if you are moving away from your friend. So it is actually the wavelength which is affected; as a consequence, the received frequency is also affected. It may also be said that the velocity of the wave remains constant whereas wavelength changes; hence frequency also changes.