The Doppler effect is the change in a wave's frequency when the source or observer moves. For sound, f' = f (v + v_o) / (v − v_s), with motion toward the other party counted as positive. This calculator returns the observed frequency, the shift, the percentage change, and whether the pitch rises or falls.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the classical sound-Doppler relation, recomputed in code.
The formula & signs
When the source moves toward you, the denominator shrinks and the pitch rises; when it moves away, the denominator grows and the pitch falls. An observer moving toward the source adds to the numerator, also raising the pitch. The default sound speed is 343 m/s (dry air, 20 °C); it rises with temperature and is much higher in water or solids.
Worked examples
1000 Hz siren approaching at 30 m/s:
Same siren receding at 30 m/s:
Observer moving toward a 440 Hz source at 20 m/s:
The approach-then-recede jump is what you hear as a siren passes: from about 1096 Hz down to 920 Hz in this example — a drop of roughly 176 Hz, clearly audible as the pitch suddenly falls.
Frequently Asked Questions
f' = f(v + v_o)/(v − v_s) for sound, with toward-motion positive.
Waves bunch up approaching (higher) and stretch receding (lower), so pitch drops as it passes.
≈ 1096 Hz. Receding, ≈ 920 Hz.
≈ 343 m/s in air at 20 °C; higher when warmer or in water (~1480 m/s).
No — light uses the relativistic Doppler formula (astronomical redshift).