Doppler Effect, explained.
The Doppler effect is a change in observed wave frequency caused by relative motion between source and observer.
Why it happens
For a source approaching a stationary listener in still air, observed frequency is emitted frequency times 343/(343−source velocity). Earlier wavefronts remain centred on their emission positions.
The Doppler effect is a change in observed wave frequency caused by relative motion between source and observer.
Read the result
Positive velocity means motion toward the observer on the right. Compare emitted and observed frequency without interpreting circle size as loudness.
A worked example
A moving source with an unchanged note
Keep emitted frequency at 440 Hz with source velocity zero.
Raise approach velocity to about 114 m/s and inspect the observed frequency near 660 Hz.
Reverse velocity to obtain a lower received frequency while preserving the emitted note.
OPTIONAL DEEPER DETAILGo deeper: inside the model
Inside this model
For a source approaching a stationary listener in still air, observed frequency is emitted frequency times 343/(343−source velocity). Earlier wavefronts remain centred on their emission positions.
Where this idea is useful
A practical use
Separate a changing siren pitch from a claim that the source changed its emitted note.
A common misconception
“A higher received pitch means the source emitted a higher note.”
Relative motion can change the arrival spacing of wavefronts without changing the source’s emitted frequency.
What this explanation leaves out
- Classical sound propagation in still air is assumed. Source speeds stay subsonic; no actual audio, moving listener, wind or relativistic light shift is modelled.
Why is light not calculated with this same formula?
The board models sound relative to still air. Light’s relativistic frequency shift needs a different propagation account.
Which motion and reference medium explain the frequency you observed?
Associated thinkers
Associations marked provisional are awaiting source review.
Further reading
Explore the original research or the teaching reference behind this experiment.