Science & TechFact
The siren never changes its pitch. You just hear it differently.
As an ambulance approaches, it squeezes its sound waves together: they arrive more densely and the tone sounds higher. As it drives away, the waves stretch out and the pitch drops. At 50 km/h the jump is about 8 percent, a good semitone. Speed cameras use the same principle with radio waves, astronomers with starlight.
Useful: When the pitch drops, the siren has already passed you – you can relax.
Source: de.wikipedia.org