Doppler effect for sound waves — FAQ

The pitch of a siren changes due to the Doppler effect. As the siren approaches, sound wavefronts are compressed, leading to a higher observed frequency (pitch). As it moves away, the wavefronts are stretched, causing a lower observed frequency (pitch).

No, the Doppler effect applies to all types of waves, including electromagnetic waves like light. However, the specific formulas and implications (e.g., red/blue shift for light) differ from those for sound waves due to their fundamental differences.

If the observer is moving towards a stationary source, they will encounter more wavefronts per second, leading to a higher observed frequency. If they move away, they encounter fewer wavefronts, resulting in a lower observed frequency. While the formula given in this lesson is for a moving source, the Doppler effect still occurs due to the relative motion. A different formula, $f_o = f_s (v \pm v_o) / v$, is used for a moving observer.

If the source's speed $v_s$ is greater than the speed of sound $v$, the wavefronts cannot get away from the source and build up into a conical shock wave. This shock wave is perceived by a stationary observer as a sonic boom when it passes over them.