Doppler Effect Calculator
Calculate the observed frequency shift caused by the Doppler effect. Set source frequency, wave speed, and the velocities and directions of both source and observer. No signup, runs entirely in your browser.
Doppler Effect Examples
Police siren
A siren at 700 Hz, moving toward you at 30 m/s: the observed frequency is 700 × (343/313) ≈ 767 Hz — noticeably higher pitch. As it passes and moves away, it drops to ~648 Hz.
Astronomy
Astronomers use the Doppler shift of spectral lines to measure the radial velocity of stars and galaxies. A red shift of hydrogen-alpha line indicates recession.
Radar speed guns
Police radar emits a known frequency and measures the Doppler shift of the return echo to calculate vehicle speed. The same principle applies to medical ultrasound Doppler imaging.
Stationary observer
If the observer is stationary (v_obs = 0 m/s), set observer velocity to 0. The formula simplifies to f_obs = f₀ × v / (v ∓ v_src).
Frequently Asked Questions
What is the Doppler effect?
The Doppler effect is the change in observed frequency of a wave when the source or observer is moving relative to each other. As they approach, the observed frequency is higher (blue shift); as they recede, it is lower (red shift).
What is the Doppler formula?
f_obs = f₀ × (v ± v_obs) / (v ∓ v_src), where v is the wave speed, v_obs is the observer velocity, and v_src is the source velocity. The signs depend on whether the source/observer are approaching or receding.
What speed of sound should I use?
In dry air at 20°C, sound travels at approximately 343 m/s. The speed increases with temperature: roughly 331 + 0.6 × T(°C) m/s. For water use ~1480 m/s.
What is blue shift vs red shift?
Blue shift means the observed frequency is higher than the source frequency — occurs when source and observer approach. Red shift means the observed frequency is lower — occurs when they move apart. These terms originate from light but apply to all waves.
Can this be used for light (relativistic Doppler)?
No. This calculator uses the classical Doppler formula, which is valid for sound and other mechanical waves. The relativistic Doppler effect for light requires a different formula that accounts for time dilation.
Is my data stored?
No. All calculations run locally in your browser. No data is sent to any server.