Ernst Mach was born in Austria in 1838. He achieved a modest reputation in his lifetime as a psychologist with somewhat zany views as to what life was all about, but he is better known to posterity for his other major area of interest.
He was a competent physicist who made himself an expert on the propagation of sound waves, and he was in due course to give his name to the way in which we measure supersonic speeds.
In Mach's time, supersonic flight was still a distant dream. Indeed in those days it was thought impossible; the speed of sound was looked on as a "barrier" that could not be crossed, because it was believed that the stresses and strains would be such as to cause an aircraft to disintegrate. This, as we know, did not turn out to be the case.
Supersonic speeds are described by their "Mach number", broadly defined as the ratio of the speed of the aeroplane to the speed of sound in air of the same temperature and density. An aircraft at sea level, for example, has a speed of Mach 1 if it is travelling at 760 m.p.h., the approximate speed of sound in those conditions, and a speed of Mach 2 if it travels at 1,520 m.p.h. Mach 2 for an aeroplane at 40,000 ft, however, is reached at about 1,320 m.p.h.
As an aeroplane moves through the air, its motion causes pressure waves to radiate outwards from it at the speed of sound. At subsonic speeds, these waves surge ahead of it, faster than the aeroplane itself. The air ahead is "warned", so to speak, of the imminent arrival of the aircraft; it moves aside and allows the aeroplane 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", as it were, 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 atmospheric pressure.
Shock waves radiate outwards from a supersonic aeroplane 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 startled listeners on the ground detect as two loud bangs.