Doppler effect is usually often we experience but sometimes we do not realize that the things that we do closely related to the Doppler effect. a unique event was first observed and described by Christian Johann Doppler in 1842 which became known as the Doppler Effect.

examples of the application of Doppler effect in our lives is when we are in a tunnel as the train moved closer to us so fast we'll hear a train sound is increasingly high. Then if the train was still ringing when the train has passed and away from us, the train was sounding voice will be more low (until eventually lost).

Illustrations that I mentioned above we can conclude that if the sound source (in this case is a passing train) and the observer or listener to move relative to one another (away from or close to) the frequency of which was captured by the observer is not the same frequency emitted by the source.
This video related to Effect Doppler:

When the sound source and observer move closer to each other, then the frequency of the captured observers (fp) will be greater than the frequency of the sound source (fs), and vice versa when the source and the observer moves away from each other, then the frequency that the observer was arrested less than the frequency of the sound source.

Hopefully this discussion useful. Congratulations learning companions !!