Why does a siren drop in pitch as it passes? A moving source compresses its waves ahead and stretches them behind — raise the speed past the wave speed for a shock cone.
Doppler EffectLive
moving source · wavefront compression
Controls
A moving source squeezes its wavefronts ahead and stretches them behind — higher pitch approaching, lower pitch receding. Push past the wave speed for a sonic boom.
Reading this result: Approaching, the 440 Hz source is heard higher — about 1383 Hz; receding, it drops to about 262 Hz. The faster the source (Mach 0.68), the wider that split.
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How it works
This visualizes the Doppler effect: circular wavefronts emitted by a moving source bunch up in the direction of motion (higher frequency) and spread out behind (lower frequency). When the source outruns its own waves, they pile into a Mach cone — a sonic boom.
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