Doppler Frequency Calculator

Calculator

Choose a mode and enter inputs.

Results appear above this form after submission.

Mode

Pick the scenario that matches your case.

Source Frequency

Enter the emitted/transmitted frequency.

Speed Unit

Used for sound mode speeds.

Medium

Sets wave speed v (m/s).

Air Temperature (°C)

Optional: v ≈ 331.3 + 0.606T.

Observer Speed

Speed of observer relative to medium.

Observer Direction

Sets sign of v_o.

Source Speed

Speed of source relative to medium.

Source Direction

Sets sign of v_s.

Clear

Example Data Table

Mode	Source Frequency	Motion Inputs	Observed Frequency (approx.)
Sound	1000 Hz	Air 20°C, observer 15 m/s toward, source 25 m/s toward	~1125.66 Hz
Sound	440 Hz	Air 20°C, observer 0, source 30 m/s away	~403.26 Hz
Light	5.00×10¹⁴ Hz	v = 30,000 m/s approaching	~5.0005×10¹⁴ Hz
Radar	10 GHz	Target 60 m/s toward	~10.0000040 GHz

Examples are rounded for readability and may vary slightly.

Formula Used

Sound (Classical)

Valid when speeds are well below wave speed.

f′ = f × (v + v_o) / (v − v_s)

v is wave speed in the medium.

v_o toward source is positive.

v_s toward observer is positive.

Light (Relativistic)

Uses β = v/c to include time dilation.

Approaching: f′ = f × √((1+β)/(1−β))

Receding: f′ = f × √((1−β)/(1+β))

c is the speed of light in vacuum.

Radar (Two-way)

Monostatic approximation for small v/c.

Δf = 2 f v / c

Toward: f′ = f + Δf.

Away: f′ = f − Δf.

How to Use This Calculator

Select a mode: sound, light, or radar.
Enter the source frequency and choose its unit.
Fill motion inputs and select toward/away directions.
Press Calculate to show results above the form.
Use Download CSV or Download PDF to save outputs.

If the result looks odd, re-check directions and units.

Notes and Practical Tips

For sound, keep speeds below the wave speed for best accuracy.
For air, temperature changes the wave speed and the shift.
For light, use the relativistic mode when v is significant.
For radar, the two-way shift doubles the one-way effect.

Doppler Frequency Guide

1) What the Doppler frequency means

The Doppler effect describes how an observed frequency changes when a source and an observer move relative to each other. If they move closer, the observed frequency rises; if they separate, it falls. This calculator reports observed frequency f′ and shift Δf.

2) Sound data you should know

For everyday acoustics, wave speed depends on the medium. Typical values are about 343 m/s in air near 20°C, around 1480 m/s in water, and roughly 5960 m/s in steel. In air, a common approximation is v ≈ 331.3 + 0.606T (°C).

3) Direction and sign conventions

Many mistakes come from mixing up “toward” and “away.” In the sound equation, v_o is positive when the observer moves toward the source, and v_s is positive when the source moves toward the observer. A flipped direction reverses the shift.

4) Why high source speeds can break the model

Classical sound Doppler assumes speeds well below the wave speed in the medium. If the source speed approaches v, the denominator (v − v_s) becomes small and predicted frequency can grow sharply. At and above the wave speed, shock effects require other models.

5) Light mode uses relativity

For electromagnetic waves, the calculator uses the relativistic Doppler factor with β = v/c. Time dilation changes the result, especially at high speeds. The approaching and receding cases use different square‑root ratios.

6) Radar: why the shift is doubled

In monostatic radar, the signal shifts once on the way to the target and again on the return trip. That is why the two‑way approximation uses Δf = 2 f v / c. A 10 GHz carrier and a 60 m/s target speed create only a few kilohertz of shift.

7) Unit handling and scale tips

Frequency inputs can span from audible ranges (20–20,000 Hz) to microwave carriers (GHz) and optical bands (hundreds of terahertz). This tool accepts Hz, kHz, MHz, and GHz to reduce typing errors. Keep motion speeds realistic for the selected mode.

8) Practical uses and interpretation

Doppler frequency is used in medical ultrasound (blood flow), weather radar (storm velocity), astronomy (redshift/blueshift), and speed measurement. Compare scenarios using Δf and percent shift, then export tables to document your assumptions and results. When motion is angled, only the line‑of‑sight component affects frequency; projecting speed onto that axis keeps comparisons fair across different trajectories. For sound tests, note medium and temperature so others can reproduce results.

FAQs

1) Which mode should I choose?

Use Sound for waves traveling in a medium like air or water. Use Light for electromagnetic waves when relativity matters. Use Radar for reflected signals where the two-way shift model is appropriate.

2) What does a negative frequency shift mean?

A negative Δf means the observed frequency is lower than the source frequency. This typically indicates the source and observer are moving away from each other along the line of sight.

3) How does temperature affect sound results?

Higher air temperature increases the speed of sound, which changes the Doppler factor. With faster wave speed, the same motion speeds usually produce a smaller percent shift than colder air.

4) Why does radar show a “two-way” shift?

The transmitted wave is shifted at the moving target, then the reflected wave is shifted again when received. This effectively doubles the one-way Doppler shift for the same radial speed.

5) Can I use km/h or mph for sound speeds?

Yes. Pick your unit, and the calculator converts speeds internally to m/s. Wave speed v is handled in m/s inside the formulas.

6) What if my motion is not directly toward or away?

This calculator assumes purely radial motion. If motion is at an angle, use the radial component v_radial = v × cos(θ), then enter that value with the correct direction.

7) Are results exact?

Results depend on the model and your inputs. Sound mode is classical and best for low speeds. Light mode is relativistic. Radar mode uses a common small v/c approximation.