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the doppler effect
The Doppler effect takes place when we listen to a sound source that is moving either away or towards us. This is usually described as an imaginary variation of the pitch (hence the frequency) of that sound. The most common situation when we hear the Doppler effect is when an ambulance is driving away from us in the street: we will have the feeling that the siren's pitch is decreasing, while in facts it remains constant.

The reason why this happens is this: the siren generate wave fronts in regular intervals, being it tuned to a single frequency; these wave fronts will travel towards us at the speed of sound, which is constant. Increasing the distance between us and the wave fronts source, will result in less wave fronts hitting our eardrums in a unit of time, which we will perceive as a lower frequency sound wave. The opposite applies when the sound source is approaching us.