How this calculator works
Enter velocities relative to a stationary medium along the source-receiver line. Positive source velocity means motion towards the receiver; positive receiver velocity means motion towards the source. Negative values mean recession. This classical moving-medium formula is suitable for subsonic sound, not the relativistic Doppler shift of light.
Formula
fobserved = femitted (c+vr)/(c-vs). Rearrange for any one unknown. Equal frequencies do not uniquely determine c.
Worked example
A 100 Hz source approaching at 30 m/s in a medium with c = 300 m/s is heard at 111.111 Hz by a stationary receiver. A receiver receding at 30 m/s from a stationary source hears 90 Hz.
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Frequently asked questions
Which velocity sign should I use?
Use positive for motion towards the other object, negative for motion away, measured relative to the medium.
Can I use this for light?
No. Light uses a relativistic frequency shift rather than velocities relative to an acoustic medium.
What happens at sonic speed?
The ordinary subsonic wavefront geometry breaks down. Sonic and supersonic inputs are rejected.