What phenomenon causes a siren's pitch to change as it passes?

The Doppler effect causes a siren’s pitch to change as it passes an observer. The pitch sounds higher while the vehicle approaches, then drops as the vehicle moves away.

That change happens because motion alters the frequency of the sound waves reaching the listener. As the siren approaches, successive wave crests arrive closer together, increasing the observed frequency and pitch. After the vehicle passes, the wave crests are spread farther apart, lowering the observed frequency.

The siren is not necessarily changing the note it produces. The perceived shift comes from the relative motion between source and observer. Resonance involves amplified vibration, diffraction is the spreading of waves around edges or openings, and refraction is a change in direction caused by a change of medium—none directly explains the passing-siren effect.