Why does the sound barrier boom?

The popular image is of an aircraft punching through a wall of sound with a single bang. The reality is a continuous phenomenon. Every supersonic aircraft drags a cone of shock waves behind it, and the boom is the sound of that cone sweeping across the ground. A listener hears one bang as the cone passes, but a listener 10 km further along the track hears it a few seconds later, and so on for the whole supersonic flight. The crew hears nothing, because the cone trails behind them; a fighter at Mach 1.5 lays down a boom carpet roughly 30 to 50 km wide beneath its path.

At subsonic speeds pressure disturbances run ahead of the aircraft and spread smoothly. Above Mach 1 they cannot outrun their source, so they stack up into shock waves, thin surfaces where pressure jumps almost instantly. Far from the aircraft the many individual shocks from nose, canopy, wings and tail merge into two: a leading shock where pressure rises sharply and a trailing shock where it snaps back after a dip. Plotted against time this is an N shape, and the ear responds to the two abrupt edges with a double bang about a tenth of a second apart for a fighter, longer for an airliner. Concorde's boom was measured at roughly 100 Pa of overpressure at the ground, enough to rattle windows.

The moment of acceleration through Mach 1 can produce a focused boom. As the aircraft accelerates, shocks generated at slightly different speeds arrive at the ground simultaneously in a narrow zone, and the overpressure there can be several times the steady-state value. That is the origin of the idea that the barrier itself booms. Weather adds another twist: temperature and wind gradients refract the shock, so a boom can be heard well off track or, at low supersonic Mach numbers, not reach the ground at all, because the shock bends upward before it arrives.

Because the boom follows the aircraft everywhere, most countries prohibited civil supersonic flight over land in the 1970s, which confined Concorde to ocean routes. NASA's X-59 experiment reverses the question: by stretching the fuselage and positioning engine and wing so that the individual shocks never coalesce into a sharp N-wave, the aim is a soft thump of about 75 perceived decibels, comparable to a car door closing at a distance.