Doppler Effect Calculator

The Doppler effect is the change in observed frequency caused by relative motion between a wave source and an observer. When they move toward each other, wavefronts reach the observer more frequently and the measured frequency rises. When they separate, arrivals are farther apart in time and frequency falls. This calculator uses the classical relation f observed = f source × (c + vo) / (c - vs). Here c is wave speed in the medium, vo is observer speed toward the source, and vs is source speed toward the observer. Signs encode direction in this form of the equation. Enter a positive observer speed when the observer moves toward the source and a positive source speed when the source moves toward the observer. Enter a negative value for either object when that object's motion increases their separation. The source and observer speeds must be measured relative to the medium, not simply relative to one another. For sound, still air is the medium reference; wind changes propagation and must be treated consistently. At about 20 degrees Celsius in dry air, sound speed is approximately 343 meters per second. It changes with temperature, humidity, gas composition, and altitude. Sound travels much faster in liquids and solids, so enter the wave speed suitable for the medium. The classical expression requires source speed below wave speed in the approach direction. As the source nears the wave speed, compression becomes extreme and ordinary assumptions break down; at and above that speed shock-wave analysis is needed. The equation is well suited to sound from sirens, horns, moving machinery, and classroom wave problems. Medical ultrasound and radar also use Doppler principles, but practical systems often involve reflected waves, angles, and specialized signal processing. Light in vacuum does not require a medium and must be analyzed with the relativistic Doppler formula when relative speeds are significant. This tool does not include relativistic corrections, reflection doubling, or an angle between velocity and line of sight. Use only radial velocity components for oblique motion, and rely on calibrated instrumentation and the correct domain model for medical, navigation, enforcement, or safety-critical interpretation.