Suppose a stationary observer is hearing a sound wave of frequency f₀ emitted by a stationary source.
When there is a relative motion between the source and the observer, the observed frequency of the sound wave is different from the actual frequency. The apparent shift from the actual frequency is called the Doppler's shift and this effect is called the Doppler's effect.
If the observer is moving with a velocity Vo, and the source of sound is moving with a velocity Vs and the velocity of sound in the stationary medium is V, then
apparent frequency heard = f = f₀ (V + Vo) / ( V - Vs)
If source and observer are approaching each other, then this is the formula. If they are separating from each other, the + and - signs will reverse.