Doppler effect is given by the change observed in the frequency of sound wave, when the observer and the source are in a relative motion. The changes in the wave frequency can be described as follows. If the source is moving towards the observer, every crest is generated from a closer position to the observer. So the wave will reach to the observer very fast. Therefore, the arrival time is comparatively less, hence the frequency increased. If the object is moving away from the observer, wave is generated from the farther position compared to the observer. So, there is a time lag in the arrival of the wave to the observer. Hence there is a reduction in the frequency of the propagating wave.
= Apparent frequency.