Breaking the Sound Barrier
When an aircraft exceeds the local speed of sound, it outruns the pressure waves it generates. These waves pile up into shock waves — extremely thin regions (a few micrometers) where pressure, density, and temperature jump discontinuously. The conical shock system that envelops the aircraft sweeps across the ground, and observers within the 'boom carpet' hear a startling double bang — the sonic boom. Chuck Yeager first experienced this crossing Mach 1 in the Bell X-1 on October 14, 1947.
The N-Wave
The ground-level pressure signature of a conventional sonic boom resembles the letter N: a sudden overpressure (bow shock), followed by a gradual pressure decrease through the aircraft's flow field, ending with a sudden underpressure (tail shock). The peak overpressure depends on Mach number, altitude, aircraft weight and length, and atmospheric conditions. Typical values range from 0.5 psf for a high-altitude fighter to 2+ psf for Concorde — enough to rattle windows and startle people.
Boom Carpet and Propagation
The Mach cone intersects the ground in a hyperbolic strip — the boom carpet — whose width is approximately 2h·tan(arcsin(1/M)). For a Mach 1.6 aircraft at 15 km, this carpet is about 45 km wide. Atmospheric refraction bends shock rays away from the surface at the carpet edges, creating a sharp cutoff. Temperature gradients, wind, and turbulence cause focusing and defocusing that produces significant overpressure variations along the carpet.
Low-Boom Design
NASA's X-59 QueSST program and private ventures like Boom Supersonic are pioneering shaped sonic boom technology. By carefully distributing the aircraft's volume and lift along its length, designers can prevent the bow and tail shocks from coalescing into a sharp N-wave. Instead, the pressure disturbance arrives as a series of weak shocks perceived as a quiet thump — potentially opening the door to overland supersonic commercial flight for the first time since Concorde's retirement.