Why a siren drops in pitch as it passes: the Doppler effect
30seconds
An ambulance siren sounds high as it comes at you and drops the moment it passes, yet the driver hears one steady note. The trick is in how crests per second reach your ear.
Drag a moving sound source across a live canvas, watch the wave crests bunch up ahead and stretch out behind, and push the speed past Mach 1 to see a sonic-boom cone form. Includes the one-line formula and the 1845 train experiment that proved it.
Deep dive · 3 min
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