What Sound Is

Author: N.H. Crowhurst

Air can move in different ways. One way can be illustrated by an oscillating electric fan. It blows the air at a comparatively slow speed, but moves quite a quantity of it. The slowness with which the air is moving is apparent because the draft of air reaches you a little while after the fan has stopped blowing in your direction. Here a large quantity of air is moved at a speed of only a few feet per second (much slower than sound travels). The air movement is large, and there is hardly any compression.

When a jet aircraft breaks through the sound barrier, however, it encounters different (and, at first, strange) kinds of air movement. The aircraft is now traveling faster than sound, and the air in front of it is no longer moving freely, but is "piled up" at high pressure on the front surfaces of the plane. As the aircraft goes through space, more air piles up in front of it, while some escapes at the side, producing the well-known "shock wave." The air in front of the plane hardly moves (relative to the aircraft), but is in a state of high compression - just the opposite condition from the air moved by the fan. However, the aircraft must move faster than sound (about 750 miles an hour) to cause this effect.

Both these forms of air movement do not "carry" very far: air movement from a fan soon gets "lost," and the pressure buildup in front of the aircraft in supersonic flight is not very deep - at most a few feet.