"To be swift is less than to be wise," said Homer - but somehow most of us seem to think the former more exciting. In any event, man has always been obsessed with speed, and strives with a strange compulsion to reach the elusive goal of sublime, majestic instancy.
Not so long ago, however, it was assumed that there was an insurmountable obstacle in the way of this ambition. There was, it was thought, a "sound barrier", which was talked about as though it were something physical which could not be approached, and certainly not exceeded, without destruction. But this, as we know, did not turn out to be the case; Charles Yeager broke the sound barrier for the first time in October 1947, and all that was left was the sonic boom.
As an aircraft moves through the air, its motion causes pressure waves to radiate outwards at the speed of sound. If it is travelling at subsonic speeds, these waves surge ahead of it, faster than the aircraft itself. The air ahead is "warned", so to speak, of the imminent arrival of the aircraft; it moves aside and enables the aircraft to pass without a fuss. But when supersonic flight arrived in 1947 - the sonic boom was born.
An aircraft moving "faster" than the speed of sound is travelling faster than any pressure waves it may produce. A supersonic aircraft "surprises" the air ahead of it; the molecules cannot get out of the way quickly enough, and are compressed.
The result is a narrow zone of compressed air - a shock wave, characterised by a small, but very abrupt, change in pressure.
Shock waves radiate outwards from a supersonic aircraft in the shape of cones - one trailing backwards from the nose, another from the tail. Where these cones intersect the ground below, two powerful "pressure ripples" sweep along - shock waves similar in character to those produced by a very sharp explosion, and which we hear as two loud bangs.
A large supersonic aircraft can produce a sonic boom along a track up to 50 miles wide. The shock waves which cause it may result in an instantaneous change in atmospheric pressure of the order of half a hectopascal at ground level - a small value in absolute terms, but enough to produce a noise level about a million times greater than that of ordinary conversation. Moreover, much of the energy of a sonic boom is at a frequency below the threshold of human hearing; the low frequency pressure variations often cause vibrations to windows, doors and walls, and may occasionally cause damage by cracking plaster or glass.