What is the Doppler effect?

The Doppler effect is the change in frequency perceived by an observer when the source of a wave and the observer are moving relative to each other. When they approach each other the frequency increases; when they recede it decreases.

What sign convention does this calculator use?

Observer velocity is positive when moving toward the source and negative when moving away. Source velocity is positive when moving toward the observer and negative when moving away. This follows the standard textbook convention f_obs = f × (v + v_obs) / (v + v_src) with the appropriate sign choice.

What speed of sound should I use?

In dry air at 20 °C the speed is approximately 343 m/s. It rises roughly 0.6 m/s per degree Celsius. In water it is about 1480 m/s. You can type any value to match your medium.

What happens when the source moves faster than sound?

When the source velocity equals or exceeds the speed of sound the classic Doppler formula breaks down — a shock wave (sonic boom) forms instead. The calculator flags this condition and does not return a meaningful frequency.

Does this work for light (redshift)?

No. Light obeys the relativistic Doppler formula. This calculator is for mechanical waves (sound) only, where the medium matters.

Doppler Shift Calculator (Sound)

Name: Doppler Shift Calculator (Sound)
Availability: InStock
Author: Nham Vu

Calculate the observed frequency of sound when the source or observer is moving, using the Doppler effect formula.

Inputs

Source Frequency (Hz)

The frequency emitted by the source (Hz)

Speed of Sound (m/s)

343 m/s in air at 20 °C; adjust for temperature or medium

Observer Velocity (m/s)

Speed of the observer relative to the medium

Source Velocity (m/s)

Speed of the source relative to the medium

Results

Enter values and press Calculate

Doppler Formula

f_obs = f_src × (v + v_obs) / (v + v_src)

v — speed of sound in the medium
v_obs — observer velocity (+toward source, −away)
v_src — source velocity (+away from observer, −toward)

Quick Presets

Summary

Calculate the observed frequency of sound when the source or observer is moving, using the Doppler effect formula.

How it works

Enter the source (emitted) frequency in Hz.
Enter the speed of sound in the medium (default 343 m/s for air at 20 °C).
Set the observer velocity — positive means moving toward the source.
Set the source velocity — positive means moving toward the observer.
The observed frequency is calculated using f_obs = f_src × (v + v_obs) / (v + v_src), where approach is positive for the observer and negative for the source.
Results update instantly; copy or use the displayed values.

Use cases

Physics homework and exam preparation on wave mechanics.
Understanding how a passing ambulance siren changes pitch.
Acoustic engineering and transducer testing scenarios.
Teaching the Doppler effect in classroom demonstrations.
Estimating frequency shifts in sonar and ultrasound contexts.
Exploring edge cases such as supersonic sources.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu