Doppler Effect Calculator

Motion between a wave source and its observer changes the perceived frequency. An approaching ambulance compresses wavefronts — more arrive per second, so the siren sounds sharper; once it passes, the stretched waves drop the pitch noticeably. Light does the same: blueshift approaching, redshift receding.

f′ = f × (v + vo) / (v − vs), where v is wave speed, vo the observer speed (positive moving toward the source) and vs the source speed (positive moving toward the observer). Only relative components along the line of sight matter — a siren passing sideways produces no shift at the instant of closest approach.

The shift itself exists at any distance, but hearing it requires the geometry to change: the classic two-tone siren drop happens during the pass-by. Race fans hear it as cars sweep through a corner — roughly a full semitone drop for F1 cars passing at 80 m/s, about what this calculator shows.