Doppler effect is the phenomenon in which there is an apparent change in the frequency or increase or decrease when there is a relative motion between the listener and the observer.
When the listener and the observer are approaching each other then the frequency of the wave will get increased.
When the listener and the observer are moving away from each other then the frequency of the wave will get decreased.
Therefore, under the Doppler effect, an observer behind a moving wave source observes decreased wave frequency.
