Doppler Effect Calculator

The Doppler effect is the change in observed frequency caused by relative motion between a wave source and an observer. For sound, an approaching source squeezes wavefronts together, so the observer hears a higher frequency. A receding source spreads wavefronts apart, so the observer hears a lower frequency. The same idea applies when the observer moves through the wavefronts instead of the source moving through the medium.

This calculator uses the standard one-dimensional sound-wave Doppler equation. It assumes the source, observer, and wave travel along the same line, and it treats the wave speed as fixed by the medium. Positive observer speed means the observer is moving toward the source. Positive source speed means the source is moving toward the observer. With those sign conventions, the observed frequency is the emitted frequency multiplied by (wave speed plus observer speed) divided by (wave speed minus source speed).

The tool is useful for physics homework, acoustics examples, siren and train-horn demonstrations, and quick checks of pitch shift. It reports the observed frequency, the absolute frequency change, and the percent shift relative to the emitted frequency. This page is for classical sound in a medium, not relativistic light. Redshift and blueshift for light need a different formula because light does not travel through air as a sound wave does, and speeds near the speed of light require special relativity.