The apparent change in frequency of a wave
When a sound source moves away from you, you tend to listen to sound increasingly serious: the Doppler Effect . The sound is a wave with a certain “wavelength”: think of a spring. If the source station starts to move away from us, the movement of the source is added to the movement of the sound wave and makes the wavelength becomes increasingly larger (meaning that the frequency is smaller). If we think of spring, when stretched from one end, the distance between two turns of the same is becoming greater .
Conversely, if the source is approaching, it is as if the spring contrajéramos and reduce the distance between turns: the wavelength of sound becomes smaller and seems to be increasingly acute (frequency increases).