✓ Sources checked · 10 September 2026 · View verification
The Doppler effect changes the observed frequency when source or observer motion changes the rate at which wave phases are encountered. For sound, motion relative to the medium matters.
By Matan · Updated
A moving source changes wavefront spacing
For a source moving through still air, successive fronts are closer together ahead and farther apart behind. A moving observer instead changes the rate of crossing existing fronts. These are distinct acoustic cases, even when their low-speed shifts are similar. [1]
Worked approach and recession
A 600 Hz source moves at 20 m/s through still air with sound speed 340 m/s. For a stationary observer directly ahead, fapproach = 600 × 340/(340 − 20) = 637.5 Hz. Directly behind, frecede = 600 × 340/(340 + 20) = 566.7 Hz. Neither calculation changes the siren’s emitted 600 Hz frequency.
Same relative speed, different exact formula
If the source is stationary and the observer moves toward it at 20 m/s, fobs = 600 × (340 + 20)/340 = 635.3 Hz. The difference from 637.5 Hz shows why source and observer speeds must be defined rather than replaced by one unsigned relative speed.
A shift is not a direct distance measurement
One-way radial Doppler data constrain radial velocity under the assumed model, not distance or full motion on their own. Weather radar mainly observes moving scatterers such as precipitation, not simply individual air molecules. Vacuum light requires the relativistic relation rather than this sound formula.
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How this article was checked
Sources checked; calculations checked where shown. Main claims and model assumptions were checked against the references below. Example arithmetic and units were checked separately. Numerical examples are illustrative calculations, not experimental measurements.
Source checking and editing assisted by AI. No independent human scientific review is recorded for this revision. This record applies to the version dated 10 September 2026.
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Sources and further reading
Cite this article
Matan. “Doppler effect.” Your Physicist. Originally published 2023-03-20; updated 2026-09-10. https://your-physicist.com/doppler-effect/
Revision history
10 September 2026: Added approach/recession and observer-motion calculations and corrected direct-distance and radar-scatterer claims. Added references and a transparent verification record.
