Is a sonic boom a one-time bang or a continuous noise? Is a sonic boom a one-time bang, caused when an object initially goes supersonic, or is it a continuous noise emitted by the object as it's travelling?
To be more specific, will an aircraft which is continuously accelerating cause a boom only over the point where it first transitions to supersonic, or does it cause a 'boom' to be heard over the whole length of it's supersonic journey by all of the observers along it's route (the same way that a fast boat will create a bow wave which follows it wherever it goes)?
 A: A sonic boom is a continuous event just so long as the aircraft initially producing the sonic boom stays above Mach 1.  Pressure and temperature affect the actual speed of Mach 1 which at sea level is at least 750mph. A sonic boom only appears to be a momentary event as the individual who hears it is stationary position.  The boom itself travels along the path that the aircraft while travelling supersonic.  Now if the aircraft was at high altitude and then suddenly turned downward pointing directly at an individual on the ground while travelling supersonic, the sonic boom would be more continuous and not just be a passing event.  There is an inventor in the US who has created a device that produces a continuous sonic boom from a stationary position. A very real sonic boom. Said device can be focused to point the sustained sonic boom (shock waves) in a particular direction. I hope this was helpful.
A: I've received an answer/explanation from Jim Wild at Lancaster University. I'll add it here in case anyone's interested. Full credit to him :)

Jim Wild: 
  But basically, no it's not just a single one-time bang.  The pressure wave (which we perceive as a boom) is generated continuously as long as the aircraft is moving supersonically.  This is why supersonic flights are usually prohibited over land - there isn't just one bang, it would "follow" the aircraft and be heard by lots of people!
Back when Concorde was flying, had you placed a chain of listening posts across the Atlantic, you would have been able to detect the aircraft moving across the ocean as it overflew each station, even if it only "broke" (i.e. accelerated through) the barrier once just after leaving the coast.
See: http://www.bbc.co.uk/news/science-environment-17701155

A: A sonic boom is a continuous high pressure sound wave following the aircraft. If a plane is flying over a long path say 100 mile, an observer is standing at (say) 10th mile will hear this boom as the plane will pass by and the observer standing at 100 mile will hear this boom after some time as the plane will pass by him.
A: 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.
A: "As long as an airplane travels at Mach 1 or faster, it will generate a continuous sonic boom. All those in a narrow path below the airplane's flight path will be able to hear the sonic boom as it passes overhead. This path is known as the “boom carpet."
https://wonderopolis.org/wonder/what-is-a-sonic-boom
Wikipedia has an excellent entry showing more technical detail:
https://en.wikipedia.org/wiki/Sonic_boom
Nasa Sonic Boom Fact Sheet
https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-016-DFRC.html
