Doppler Effect Calculator (Sound)
By Santaji GadepatilinUpdated October 27, 2026
When a sound source moves towards you, its pitch rises; when it moves away, the pitch falls. The Doppler effect is used in radar, medical ultrasound and Doppler ultrasonic flow meters.
How to use the Doppler effect calculator
Enter the source frequency, the speed of sound (343 m/s in air at 20 °C) and the speeds of the observer and source. Use positive speeds when moving towards each other and negative when moving apart.
Doppler effect formula
f′ = f × (v + vo) ÷ (v − vs)
Siren passing at 72 km/h (700 Hz)
| Situation | Frequency heard |
|---|---|
| Approaching | 743 Hz |
| At the moment it passes | 700 Hz |
| Moving away | 661 Hz |
The drop of about 80 Hz — roughly a whole tone — is what you hear as the siren goes by.
Speed of sound in different media
| Medium | Speed |
|---|---|
| Air, 0 °C | 331 m/s |
| Air, 20 °C | 343 m/s |
| Water | about 1,480 m/s |
| Steel | about 5,900 m/s |
Speed of sound in air rises about 0.6 m/s for each °C.
Uses of the Doppler effect
- Radar speed guns measure the frequency shift of reflected radio waves.
- Doppler ultrasound measures blood flow in arteries and in pregnancy scans.
- Weather radar detects wind speeds inside storms.
- Astronomy measures how fast stars and galaxies move towards or away from us.
Sonic booms
If a source moves faster than sound, the formula breaks down — waves pile up into a shock wave heard as a sonic boom.
Frequently asked questions
What is the Doppler effect?
The change in frequency heard when a sound source and listener move relative to each other — higher when approaching, lower when moving apart.
Why does a siren drop in pitch as it passes?
While the ambulance approaches, sound waves are squeezed together (higher pitch); after it passes, they are stretched out (lower pitch).
How do I calculate a receding source?
Enter the source speed as a negative number. A source moving away at 72 km/h makes a 700 Hz siren sound about 660 Hz.
Does the Doppler effect apply to light?
Yes. Light from galaxies moving away is shifted towards red (redshift), which is how astronomers know the universe is expanding. Light uses a relativistic formula.