Doppler Effect Calculator

This calculator works out the Doppler effect for sound, finding the frequency an observer actually hears when the source of a sound, the observer, or both are moving. The Doppler effect is the familiar change in pitch you hear as a siren or a racing car passes by: the pitch is higher as it approaches and drops as it recedes. It happens because the motion squashes the sound waves together ahead of a moving source, raising the frequency, and stretches them out behind, lowering it. The same effect applies when the listener moves. Beyond everyday experience, the Doppler effect is the basis of radar speed cameras, weather radar, medical ultrasound that measures blood flow, and, with light instead of sound, the redshift that reveals the expanding universe. This tool handles the sound version. You enter the frequency emitted by the source, the speed of sound in the medium, usually about 343 metres per second in air, and the speeds of the observer and source, taking motion toward the other as positive. The calculator returns the observed frequency, the shift from the original, and the wavelengths involved. The results update as you type, so you can see the pitch rise as something approaches and fall as it moves away. Use it for physics homework, for understanding radar and ultrasound, or to explain the siren effect. The sign convention is important: enter speeds as positive when the observer and source move toward each other, which raises the pitch, and negative when they move apart, which lowers it. The effect grows as the speeds approach the speed of sound.

Observed = source x (v + observer speed) / (v - source speed). Speeds positive when moving toward each other. Approaching raises pitch; receding lowers it.