SEE THE WAVEFRONTS. HEAR THE SHIFT.
The Doppler effect.
Change their motion live. Keep the positions, wavefronts and frequency history.
Follow the sound
Sound speed: 343 m s⁻¹ · still air
Each ring represents 100 sound cycles. The tone uses the actual frequency. Sound was already being emitted before the clock started.
Hear the difference
Sound is off. Press the button to enable it; audio plays only while the simulation runs.
Switch between the received and original tone to compare their pitch. Loudness is kept independent of distance so you can focus on frequency.
Received frequency against time
The graph keeps the simulated history. Live control changes affect the motion from that time onwards; earlier data is preserved. The dot marks the inspected time.
Solid · received frequencyDotted · source frequency at each time
The Doppler formula
- Frequency received by the observer (listener), in Hz.
- Frequency emitted by the source, in Hz. For sound arriving now, use its value at emission.
- Speed of sound in the medium: 343 m s⁻¹ in this still-air model.
- Observer velocity relative to the medium at reception: positive towards the source, negative away.
- Source velocity relative to the medium at emission: positive towards the observer, negative away.
Source now, sound later
A source velocity change affects the sound emitted from that moment. The listener hears it after the sound travels between them. Existing fronts keep expanding from their original emission positions. A listener velocity change affects the rate of receiving the incoming fronts immediately.
Change velocity live
Move either velocity slider while the simulation runs. Increase or reduce the speed, or change its sign to reverse direction. The source and listener keep their current positions, and the clock, sound and graph continue. Each chosen velocity stays constant until you change it again.
Model and sound display
The source starts at −100 m and the listener at +100 m. Both move along a straight line through still air. Their velocities are capped at ±150 m s⁻¹, below the sound speed; shocks and sonic booms are not modelled. The source and listener keep their assigned velocities between changes. Each circular front is centred on the position where it was emitted, and expands at 343 m s⁻¹ afterwards.
To keep individual fronts visible, one ring represents 100 oscillations rather than one. The audible sine tone uses the true source or received frequency. The simulation begins with an already-established train of sound; there is no artificial wait for a first signal. At the exact instant when the ideal point source and listener coincide, the received frequency has different one-sided limits, so no single value is shown. The sound fades during that instant. Real finite-sized sources and listeners would smooth the passing transition.
The time slider lets you inspect the run. Changing a control at an earlier inspected time replaces the later motion with a new continuation from that point. Restart run clears the history and returns to the starting positions using the current controls; Reset experiment also restores the defaults.
Explore the Doppler effect with sound
Change source and listener velocities while they move, without restarting the run. Watch circular wavefronts, hear the received pitch, compare it with the source tone and inspect the frequency history. Rightward velocities are positive and leftward velocities are negative.
Supports Doppler-effect study in IB Physics and A Level Physics, with sound-wave and graph-reading practice for AP Physics. Source changes reach the listener after the sound-travel delay; the model preserves previous motion and emitted wavefronts. Audio starts only after you enable it.