Doppler Effect Calculator
Hear the shift. Calculate how motion changes the perceived frequency of sound or light, from passing sirens to redshift in astronomy.
f = f₀ × (v ± v₀) / (v ∓ vₛ)
Air at 20°C: 343 m/s
About the Doppler Effect:
The Doppler Effect is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. Common examples include the changing pitch of a siren as it passes by.
How the Doppler Effect Calculator Works
Select whether you're calculating for sound waves or light waves. Enter the source frequency (or wavelength for light), the speed of the source, the speed of the observer, and the wave speed in the medium.
Specify the direction of motion: approaching or receding. For sound, also indicate if the source, observer, or both are moving. The calculator applies the appropriate Doppler formula with correct sign conventions.
Results show the observed frequency (or wavelength), the frequency shift, and the percentage change. For light, it also calculates redshift or blueshift. A diagram illustrates the wave compression or expansion.
When You'd Actually Use This
Astronomy redshift measurements
Calculate galaxy recession velocities from spectral line shifts. Determine how fast distant galaxies move away from us due to cosmic expansion.
Radar speed detection
Understand how police radar guns work. The frequency shift of reflected radio waves reveals vehicle speed for traffic enforcement.
Medical ultrasound imaging
Calculate blood flow velocity from ultrasound frequency shifts. Doppler echocardiography measures heart valve function and blood circulation.
Weather radar analysis
Measure wind speeds in storms. Doppler weather radar detects precipitation motion to identify rotation in severe thunderstorms.
Train whistle pitch changes
Calculate the pitch shift of a passing train's horn. Classic physics example showing frequency increase as it approaches, decrease as it recedes.
Satellite communication
Compensate for Doppler shift in satellite signals. Low Earth orbit satellites move fast enough to cause noticeable frequency shifts.
What to Know Before Using
Sound requires a medium, light doesn't.Sound Doppler depends on motion relative to air. Light Doppler depends only on relative motion between source and observer.
Approaching means higher frequency.Waves get compressed when source approaches, raising frequency (blueshift for light). Receding stretches waves, lowering frequency (redshift).
Relativistic effects matter for light.At speeds approaching light speed, use relativistic Doppler formula. Classical formula works for everyday speeds but fails near c.
Sign convention is critical.Velocity toward the other is positive for observer, negative for source (in common convention). The calculator handles this automatically.
Pro tip: For sound, motion of source and observer have different effects. A moving source changes wavelength. A moving observer changes how fast wave crests are encountered. Both change observed frequency but through different mechanisms.
Common Questions
Why does the pitch change suddenly as a vehicle passes?
The frequency changes continuously, but the rate of change is fastest at closest approach. Your ear perceives this as a sudden drop.
What's the difference between redshift and blueshift?
Redshift: object moving away, wavelength increases (shifts toward red). Blueshift: object approaching, wavelength decreases (shifts toward blue).
Can Doppler effect occur with any wave?
Yes, any wave phenomenon shows Doppler effect: sound, light, water waves, even matter waves in quantum mechanics.
What happens at the speed of sound?
When source moves at sound speed, waves pile up creating a shock wave (sonic boom). The Doppler formula breaks down at supersonic speeds.
How do astronomers measure redshift?
Compare observed spectral lines to laboratory wavelengths. The shift z = (λ_observed - λ_rest) / λ_rest gives the redshift parameter.
Does wind affect Doppler shift?
Wind changes the effective speed of sound relative to ground, affecting the Doppler shift. The calculator assumes still air unless you adjust wave speed.
What's the maximum Doppler shift?
For sound, theoretically infinite as source approaches sound speed. For light, limited by relativity - frequency can't exceed infinite energy requirements.
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