Subject overview
Doppler Effect
Calculate emitted or observed sound frequency and source or observer velocity. Follow SI conversions and explore wavefronts for collinear subsonic motion in a stationary medium.
01 Acoustic Doppler Effect
Variables
- fsemitted frequency
- Hzhertz
- foobserved frequency
- Hzhertz
- vssource velocity toward observer
- m/smeters per second
- voobserver velocity toward source
- m/smeters per second
- csound speed in the medium
- m/smeters per second
- λwavelength toward observer
- mmeter
Key Formulas
Requires algebra and the relation between frequency, period and wavelength. Successive wavefronts are emitted one source period apart. During that period the wave travels cTₛ and the source advances vₛTₛ, so the wavefront spacing toward the observer is (c − vₛ)Tₛ. The observer encounters those fronts at relative speed c + vₒ.
Positive vₛ means the source moves toward the observer; positive vₒ means the observer moves toward the source. Negative values mean moving away. Both velocities are relative to the medium.
Uniform stationary medium, constant sound speed and collinear motion with |vₛ| < c and |vₒ| < c, before source and observer pass. No wind, sonic booms, reflection or relativistic light Doppler effect.
The sound speed is a medium parameter, not a missing target quantity. The default 343 m/s is an approximate value for air near 20 °C; enter the appropriate value for your medium. This model supports inverse solutions for either frequency or either velocity. Three of those four inputs are required, including explicit zeroes for stationary objects.
Calculate
Type prefixes directly: 1.5k, 250m or 20u. Results: 3 significant figures.
Positive vₛ means the source moves toward the observer; positive vₒ means the observer moves toward the source. Negative values mean moving away. Both velocities are relative to the medium.
Using c = 343 m/s (air near 20 °C).
Enter three of fₛ, fₒ, vₛ and vₒ; use 0 for a stationary source or observer.
Uniform stationary medium, constant sound speed and collinear motion with |vₛ| < c and |vₒ| < c, before source and observer pass. No wind, sonic booms, reflection or relativistic light Doppler effect.