Doppler Pitch Shift

This formula gives the frequency an observer hears when a sound source and/or the observer are moving relative to the air. It works because motion changes how tightly the wavefronts are spaced at the observer: moving toward each other compresses the waves and raises the pitch, while moving apart stretches them and lowers it. The signs reflect the chosen convention: motion of the observer toward the source increases the heard frequency, and motion of the source toward the observer also increases it by shortening the wavelength in front of the source.

Use this for sound in air when you need the heard pitch from a moving source, a moving listener, or both—for example, trains, sirens, drones, or musical instruments on moving platforms. It is most appropriate when speeds are well below the speed of sound so the simple Doppler model applies.

• Using the source speed in the numerator or the observer speed in the denominator
• Forgetting that the sign depends on whether the source or observer is moving toward or away from each other
• Applying the formula to supersonic motion or non-air media without checking the model assumptions