The Doppler Effect

Why the relative direction of movement of a sound source to the receiver changes its pitch

When an ambulance or a police car moves towards us with its siren on, the frequency of the siren seems to be higher than when this sound source moves away from us again. But how do they manage to always switch the pitch at that exact moment? The answer is simple: they don't. Because this impression can be explained by the time-of-flight effect in sound transmission, the so-called "Doppler effect".

When a sound source, the receiver, or both move relatively to the transmission medium (the air), the distance between the transmitter and receiver of a signal changes. A signal is "compressed" as the distance decreases and "stretched" as the distance increases, because the movement changes the wavelength - and thus the perceived frequency. If the transmitter and receiver move towards each other, the wavelength shortens and a higher frequency is perceived. If the distance increases, it appears to be lower, following the same principle in the opposite direction. The faster the sound source and receiver move relatively to each other, the stronger the perceived change in frequency.

An example: The siren of an ambulance emits two tones in succession. Let us assume sound waves with a frequency of 440 Hz for one of these tones. If the ambulance now drives at 50 km/h (approx. 14 m/s) towards a stationary receiver, the receiver perceives the frequency as higher, namely at approx. 457 Hz. When the ambulance moves away from him, he gets the opposite impression - he perceives a frequency of approx. 423 Hz.