Why an ambulance's pitch drops as it passes
440 HzSOURCE
440 HzOBSERVED
0.00MACH
Source (drag in Free mode)
Observer (drag anywhere)
Wavefronts at speed of sound
Mach cone when supersonic
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Sound is a wave that travels at finite speed.
Watch the cyan rings — those are wavefronts crawling outward at the speed of sound (c). Right now the source is stationary, so the rings are evenly spaced and the observer hears the source frequency unchanged.