Doppler effect – a moving sound source
PhysicsOscillations & WavesAges 16–17
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Sign in to playA sound source (a vehicle with a siren) drives at constant speed past a listener standing a distance d from the road; the wavefronts are circles emitted from the source's earlier positions, so they bunch up ahead of it and spread out behind. The frequency the listener receives, f', is worked out from the velocity components along the sound's path and shown as a number, as an f'(t) graph and as sound: high while the source approaches, gliding lower as it moves away. The listener can also move against or with the source.
Lesson: Sound waves: the Doppler effect for a moving source or observer
What it shows
The Doppler effect is the change in the frequency heard when a sound source and a listener move relative to each other. A moving source emits each wave crest from a new position, so crests bunch up ahead of it and spread out behind. When the two get closer, crests arrive more often and the listener hears a higher frequency; when they move apart, a lower one. In still air, f' = f·(v + vo')/(v − vs'), where vs' and vo' are the velocity components along the line joining them. As the source passes, these components change smoothly, so the pitch glides down like a passing ambulance siren.
How to use
Press the speaker button to hear the sound, then watch one run: f' starts high, glides down past the source frequency and ends low. Compare the circle spacing ahead of and behind the source. Change Source speed and Source frequency, drag the listener to change Distance from road, or set Listener to Moving opposite to the source. Use Pause, Run and Reset to replay a pass.
Parameters you can change
- Source speed 0–170 m/s
- Source frequency 200–1000 Hz
- Listener At rest, Moving opposite to the source, Moving with the source
- Listener speed 0–100 m/s
- Distance from the listener to the road 10–60 m
Questions to explore
- Why does f' fall gradually, instead of jumping from high to low, as the source passes the listener?
- Why does f' equal f slightly after the source is level with the listener, not at that moment?
- At a source speed of 170 m/s, how do the wavelengths ahead and behind compare with the wavelength at rest?
