The Doppler effect calculator computes the observed frequency when a source or observer is moving relative to the wave medium. For sound, the classical formula applies. For light, the relativistic Doppler formula accounts for the fact that light speed is constant in all frames.
Wave Type
Sound in air at 20°C ≈ 343 m/s
+ = toward observer
+ = toward source
+ = source approaching, − = source receding
Results
Step-by-Step
Formula Reference
How to Use the Doppler Effect Calculator
The Doppler effect occurs whenever there is relative motion between a wave source and an observer. This calculator handles both the classical formula for sound and the relativistic formula for light.
Step 1: Choose Wave Type
Select Sound for classical Doppler calculations (ambulances, trains, musical instruments) or Light for relativistic calculations (stars, galaxies, radar guns).
Step 2: Enter Source Frequency
Enter the original frequency of the source in Hz. For example, 440 Hz is concert A (A4). For light, the visible spectrum is approximately 430–770 THz (use Hz with scientific notation or convert to nm first).
Step 3: Sound — Velocities
Enter wave speed (343 m/s for sound in air at 20°C), source velocity (positive = moving toward observer), and observer velocity (positive = moving toward source). A fire truck moving at 30 m/s toward you with a 700 Hz siren: f_obs = 700 × (343+0)/(343-30) = 768 Hz.
Step 4: Light — Relative Velocity
Enter the relative velocity between source and observer. Positive values mean approaching (blueshift), negative values mean receding (redshift). For v = +3×10⁷ m/s (10% of c): the observed frequency increases by about 5.4%.
FAQ
What is the Doppler effect?
The Doppler effect is the change in frequency of a wave when the source or observer is moving. When the source moves toward you, waves compress and frequency increases (higher pitch). When moving away, waves stretch and frequency decreases (lower pitch).
What is the Doppler effect formula for sound?
The classical Doppler formula is f_obs = f_source × (v_wave + v_observer) / (v_wave - v_source), where velocities toward each other are positive. The speed of sound in air at 20°C is approximately 343 m/s.
How does light's Doppler effect differ from sound?
Light uses the relativistic Doppler formula: f_obs = f_source × √((1 + v/c)/(1 - v/c)) for approach, where v is the relative velocity and c = 3×10⁸ m/s. Approaching light appears blueshifted (higher frequency); receding appears redshifted.
Is this calculator free?
Yes, completely free with no signup required. All calculations run in your browser.
What is a real-world example of the Doppler effect?
A classic example is an ambulance siren: as it approaches, you hear a higher pitch; as it passes and moves away, the pitch drops. Astronomers use the Doppler shift of light to determine how fast distant stars and galaxies are moving toward or away from Earth.