The Doppler effect in sound is the change in the pitch you hear when a sound source and a listener move relative to each other. It explains why an ambulance siren sounds higher in pitch as it approaches and lower in pitch after it passes. This matters in music, transportation, weather sensing, and astronomy because waves carry information about motion.
The effect is not caused by the siren changing its actual frequency, but by the way wavefronts reach the listener.
Key Facts
- Higher observed frequency means higher perceived pitch.
- For a moving source and stationary listener: f' = f v / (v - vs) when the source approaches.
- For a moving source and stationary listener: f' = f v / (v + vs) when the source moves away.
- v is the speed of sound, about 343 m/s in air at 20°C.
- When a source moves toward you, wavefronts are compressed, so wavelength decreases and frequency increases.
- When a source moves away from you, wavefronts are stretched, so wavelength increases and frequency decreases.
Vocabulary
- Doppler effect
- The Doppler effect is the change in observed frequency of a wave due to relative motion between the source and the observer.
- Frequency
- Frequency is the number of wave cycles that pass a point each second, measured in hertz.
- Pitch
- Pitch is how high or low a sound seems to a listener and is mainly determined by frequency.
- Wavelength
- Wavelength is the distance from one wave crest or compression to the next.
- Wavefront
- A wavefront is a line or surface showing points of a wave that are in the same phase, such as the circular compressions spreading from a siren.
Common Mistakes to Avoid
- Saying the siren itself changes frequency is wrong because the source emits the same frequency while the listener receives wavefronts at a different rate.
- Using the approaching formula for a source moving away is wrong because the sign changes whether wavefronts are compressed or stretched.
- Confusing loudness with pitch is wrong because loudness depends mostly on amplitude, while pitch depends mostly on frequency.
- Ignoring units for speed is wrong because the Doppler formula only works when the sound speed and source speed use the same units, such as m/s.
Practice Questions
- 1 An ambulance siren emits 700 Hz while moving toward a stationary listener at 25 m/s. Using v = 343 m/s, what frequency does the listener hear?
- 2 The same ambulance passes and moves away from the listener at 25 m/s while the siren still emits 700 Hz. Using v = 343 m/s, what frequency does the listener hear?
- 3 Explain why the pitch change is sudden as the ambulance passes the listener, even though the siren itself keeps producing the same tone.