Is a sonic boom a one-time bang or a continuous noise?

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 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.