The Doppler effect is the shift in a wave's observed frequency when its source moves. Drag the sliders below to change the source speed and the emitted frequency, and watch the frequency ahead of the source rise while the frequency behind it falls, in real time.
An ambulance racing toward you screams at a high pitch, then the note falls the instant it flashes past. This simulation isolates why. Drag Source speed vs upward and watch the two readouts split apart: Frequency ahead climbs above the value you set with Emitted frequency f, while Frequency behind sinks below it. The source is chasing its own sound, crowding wave crests together in front and stretching them apart behind, so it is the wavelength that shifts — not the sound's speed, which stays pinned at 343 m/s by the air itself.
Here is the misconception this kills: many students imagine the pitch sliding down smoothly as the ambulance recedes into the distance. It does not. While the source approaches, ahead-frequency holds high the entire time; the drop happens abruptly as it passes, switching from the f_ahead = f·c/(c − vs) regime to the f_behind = f·c/(c + vs) regime. Loudness fades with distance, but loudness is not pitch.
Push vs toward the 330 m/s cap — just short of the 343 m/s speed of sound — and the ahead-frequency shoots up as c − vs narrows to a sliver, a hint of the shock wave that forms once a source reaches the speed of sound, where this model breaks. Put real numbers behind the shift with the Doppler effect calculator, then chase another wave through the lab collection.
The Doppler effect is the change in the observed frequency of a wave when the source and observer move relative to each other. A source moving toward you sounds higher-pitched; moving away it sounds lower, even though the emitted frequency never changes.
As the source approaches it crowds its own sound waves together in front, shortening the wavelength and raising the pitch; behind it the waves stretch out, lowering the pitch. The pitch stays high the whole time it approaches and drops abruptly at the moment it passes — not gradually as it recedes.
No. The speed of sound is fixed by the air itself, about 343 m/s, whatever the source does. The source's motion changes the wavelength, and therefore the frequency you hear, but not the speed the sound travels through the medium.
For a stationary observer, the frequency ahead of an approaching source is f' = f·c/(c − vs), and behind a receding source it is f' = f·c/(c + vs), where c is the speed of sound and vs the source speed. Approaching raises the frequency; receding lowers it.