Reducing an accelerating SST aircraft to point size, the sound (aerodynamic and mechanical) it creates are confined within a perfect cone. At Mach I, the angle of the cone margins to the line of flight is 45 degrees (the sound radiating laterally from the line of flight the same distance as the plane moves forwards). As the aircraft approaches an observer, the accumulating sound energy in the anterior sound cone displays rising sound frequency (Doppler Effect). with the wavelength decreasing proportionately.
Theoretically, at Mach I, the frequency should reach infinity, but this is impossible because the lower limit of the inversely related wavelength is restricted by the dimensions of the space occupied by the sound-transmitting adjacent air nuclei. As the aircraft passes the observer at Mach I, the frequency of the pent up Doppler-related sound energy in the anterior sound cone explodes into the ultimate example of low frequency sound – one single massive vibration, like the clap of a supersonic thunderbolt striking near the observer.
Noise interferes with the development of laminar flow. The violent reverberation of molecules with the intensifying noise in the anterior sound cone as Mach I is approached renders the anterior sound cone air refractory to laminar flow, converting the air into the equivalent of a gaseous gel, with greatly increased resistance to penetration by the leading edges. This resistance to thus gaseous gel effect is responsible for the increased air pressure anterior to the leading edges (the sound barrier effect so noticeable to earlier, less aerodynamic aircraft). The release of this pressure band as an SST aircraft passes rapidly through the sound barrier, potentially might result in a second source of a sonic boom, unrelated to the Doppler effect.