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Doppler with moving observer
Intermediate
An approaching observer detects more wavefronts per second.
For an observer moving at v_o toward the source f' = f(v + v_o)/v. The observer meets more wavefronts per second.
Driving at 20 m/s toward a 500 Hz source: 529 Hz.
Not symmetrical with a moving source at high speed.
In more detail
For a moving observer and a stationary source, the observed frequency is f' = f (v + v_o)/v when approaching. The observer meets more wave crests per second.
Real-world uses
- Passengers in a train hear the horn change pitch as they pass the crossing.
- Sensors on moving vehicles detect frequency shifts from fixed signals.
- A person running toward a loudspeaker hears more wavefronts per second, so the sound seems higher in pitch.
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Frequently asked questions
Is the observer case the same as the source case?
Not exactly. The formulas differ, because the medium is stationary in one case.
Why does the medium matter?
Only the relative motion between the observer and the medium changes the observed frequency.
What if the observer moves away?
The observed frequency is lower, because the wavefronts reach the observer less often.
Related topics in Waves & Sound
Sonar and echoes · Doppler effect · Doppler with moving source · Doppler in light · Shock waves and sonic boom · Mach number