In the early 19th century Christian Doppler, an Austrian physicist, proposed a theory regarding the properties of a moving source of sound. This is now known as Doppler's law or the Doppler effect. What does this law state?

The Doppler effect is the effect produced by a source of sound waves that is moving relative to an observer. There sound will have a higher pitch as it approaches the observer, and a lower pitch as iits moves away from the observer. The effect is commonly experienced as an ambulance passes.