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Relative motion changes the frequency that reaches the listener.
As a sound source approaches you, successive wavefronts arrive closer together. The observed frequency is higher, and the sound is heard at a higher pitch. As it recedes, wavefronts arrive farther apart and the observed pitch falls.
This is the Doppler effect. The source can be producing the same frequency throughout; the change comes from relative motion between source and observer. Loudness may change as well, but loudness and pitch describe different aspects of the sound.
A siren can sound higher before an ambulance passes and lower afterwards, even if the siren’s own emitted tone remains unchanged.
Real sirens often vary their emitted tone too. The example isolates motion to explain the effect.
Physics — Doppler Effect and Sonic Booms
Fatih Gozuacik, Denise Pattison, Catherine Tabor · OpenStax, Rice University
CC BY 4.0. Rewritten as a self-contained explanation; the example and reflection are Vakataka additions. No source images reproduced.
Source edition: 2024-12-20T16:36:28Z
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