🚨 Doppler Effect Calculator
Find the frequency an observer hears when the source or listener is moving, the approaching/receding toggles handle the tricky signs for you.
Observed frequency f′
482.173 Hz
Higher pitch (blue-shifted)
f′ = f·(v ± v_observer)/(v ∓ v_source), signs handled by the approaching/receding toggles. 🔒 Computed in your browser.
How the doppler effect calculator works
The Doppler effect shifts the observed frequency: f′ = f·(v ± v_observer)/(v ∓ v_source), where v is the wave (sound) speed. Approaching raises the pitch, receding lowers it. Set each speed and its direction, and the tool applies the correct signs and reports the observed frequency and whether it is higher or lower.
The sign convention is exactly what students and chatbots get wrong. Here the approaching/receding buttons set the signs, so you can’t apply them backwards. If the source reaches the wave speed, a shock wave forms and ordinary Doppler no longer applies.
Frequently asked questions
What is the Doppler effect?
The change in observed frequency (pitch) when a source or observer moves relative to the medium. Approaching raises the frequency; receding lowers it, the reason a siren drops in pitch as it passes.
What is the Doppler formula?
f′ = f·(v ± v_observer)/(v ∓ v_source), with v the speed of sound. The signs depend on direction; this tool sets them from the approaching/receding toggles.
Why does the pitch drop as a car passes?
While approaching, the sound waves are compressed (higher frequency); once it passes and recedes, they are stretched (lower frequency), a sudden drop as it goes by.
What speed of sound should I use?
About 343 m/s in air at 20 °C (the default). It varies with temperature and medium; enter the appropriate value.
What happens at the speed of sound?
When the source reaches the wave speed, the waves pile up into a shock wave (a sonic boom) and the simple Doppler formula no longer applies.
Worked example: an ambulance siren
A 700 Hz siren approaches a stationary listener at 30 m/s (sound speed 343 m/s). f′ = 700 × 343/(343 − 30) ≈ 767 Hz while approaching, dropping to 700 × 343/(343 + 30) ≈ 644 Hz once it has passed, a clearly audible fall.
Is there a Doppler effect for light?
Yes, but the formula differs because light needs no medium and special relativity applies. The redshift of receding galaxies is the optical Doppler effect. This calculator handles the classical sound (mechanical-wave) case.
Does it matter whether the source or the observer moves?
For sound it does: the formula treats a moving source and a moving observer slightly differently, because the medium (air) provides a fixed reference frame. The results are close at low speeds but not identical.
What units does the calculator use?
Frequencies in hertz and speeds in metres per second, matching the speed of sound you enter. The observed frequency comes out in the same unit as the source frequency, so you can work in kHz if you prefer.