Doppler Effect (Sound)

The Doppler effect for sound predicts how the pitch you hear changes when the source and observer move relative to the air. Motion toward each other compresses wavefronts so more crests arrive each second, raising the observed frequency; motion apart spreads them out, lowering the frequency. The plus/minus signs depend on whether the observer is moving toward the source and whether the source is moving toward the observer, because each type of motion changes the spacing or the arrival rate of wavefronts in a different way.

Use this formula for sound waves in a stationary medium, such as a moving ambulance siren, a passing train whistle, or any problem asking for the pitch heard by a moving listener or emitted by a moving source. It is especially useful in exam questions where either the source, the observer, or both are in motion along the same line.

• Using the same sign choice for source and observer without checking whether each one is moving toward or away from the other
• Putting the source speed in the numerator or the observer speed in the denominator
• Forgetting that the sign convention is defined relative to the line joining source and observer, not just the direction of motion in the lab frame