Question

What is the quality of the sound of a fire truck’s siren when it moves toward a stationary hearer?



What is the quality of the sound of a fire truck’s siren when it moves away from a stationary hearer?

Answer 1

You hear the siren at a higher pitch as the truck approaches and at a lower pitch as it recedes — this is the Doppler effect.

Brief explanation:
- Approaching: wavelengths in front of the moving source are compressed, so the observer receives a higher frequency (higher pitch). For a stationary observer and source speed vs, f' = f · v/(v − vs).
- Receding: wavelengths behind the source are stretched, so the observer receives a lower frequency (lower pitch). f' = f · v/(v + vs).

The loudness also rises as it comes closer and falls after it passes, and the pitch jumps from high to low at the moment the truck passes by. The siren’s timbre (harmonic content) is unchanged except that all component frequencies are shifted.