A source of sound sends out one wavefront every period. Set it moving and watch where the wavefronts crowd together and where they spread out, then read what each listener measures.
Motion slowed 100× · the tone plays at the true frequency · drag A or B to move them
running
What moves
Sound in air
340 Hz
136 m/s
136 m/s
Speed of sound v = 340 m/s, the same for every wavefront whatever the source is doing.
What each listener measures
A—
wavefronts per second—
wavelength reaching it—
wave speed relative to it—
B—
wavefronts per second—
wavelength reaching it—
Listen
Plays the frequency the chosen listener is hearing at that moment. A listener on the track hears the pitch drop all at once as the source passes; one standing beside it hears it slide.