Physics
Doppler effect
Let an ambulance drive past an observer, watch the wavefronts and compare the emitted and received tone.
Launch simulationHow it works
The source emits wavefronts that travel through the air at the speed of sound c = 343 m/s, each from the place where it was emitted. A moving source chases its own wavefronts, so they are closer together ahead of it and the observer hears a higher frequency; behind it they are further apart and the tone is lower. A moving observer meets the wavefronts more or less often. Only the velocity component along the direction from which the sound arrives matters.
f′ = f₀ · (c − n·vₒ) / (c − n·vₛ)source heading straight at you: f′ = f₀ · c / (c − vₛ)
Try it yourself
- Choose “Ambulance pass-by” (160 m/s, 600 Hz). Long before it passes, the received tone approaches 600 · 343 / 183 ≈ 1125 Hz; long after, about 409 Hz.
- Choose “Slower pass-by” (30 m/s, about 108 km/h). Over the timeline the tone falls only from 638 to 571 Hz.
- Click “Close to the road” (35 m). The change in pitch happens faster because the direction to the source turns over a shorter stretch of road.
- Choose “Moving observer”: the source stands still and the observer drives past it at 60 m/s. The tone falls from 690 to 510 Hz; far away it would be 705 and 495 Hz. A source moving at the same speed would give 727 and 511 Hz – source and observer motion are not interchangeable.
Model limitations
The source and observer move uniformly in still air, without wind, reflections or attenuation with distance. Source speeds up to 240 m/s are exaggerated for clarity but stay below the speed of sound, so no shock wave forms. The circles mark every 200th period, not the real wavelength.