The Doppler Effect
The Doppler effect is an effect of the relative motion between a sound source and an observer.
Doppler shift is a shift in pitch. As sound source and observer move closer to one another, the time required by each cycle to reach the ear is decreased. Therefore frequency is increased, thus raising the pitch. As sound source and observer are moving apart, frequency is decreased, and pitch is lowered. A third factor is the medium in which the sound travels. The Doppler shift will be greater when the sound source is moving at a given speed relative to the medium, than when the observer is moving at the same speed. The reason for this is described by the principle of relativity.
A sonic boom is a special case of the Doppler effect. In a sonic boom, air pressure variations are tightly packed together, due to the increasing reduction in distance and decreasing relative velocity of an approaching sound source, typically a supersonic jet aircraft. When the sound source is still quite distant, the sound will actually be heard in reverse, since, when the source is approaching faster than the speed of sound, the sound produced later will reach the observer before the sound produced earlier. As a jet approaches, its relative velocity decreases, since its altitude becomes the predominant factor in the vector, and its altitude is more or less stable. For this reason, directly below a jet flying at supersonic speed, sound waves converging from an extensive span of its flight attain their highest concentration, and a wavefront of very high pressure is built up. This produces the distinctive loud crack of the sonic boom as it reaches the observer.