Boom is really a misleading word for it because it implies something like an explosion – when really it’s more of a constant roar. It just sounds like a boom from land, because unlike normal sound, which would reach you from the plane constantly as it flies by, the boom is like the wake of a boat, only hitting you once for each pass.
I think part of the reason this isn’t well known is because of all the photos of jets breaking the sound barrier.
http://res.cloudinary.com/dk-find-out/image/upload/q_80,w_1440/A-iStock_000017498245Large_pxojdi.jpg
That plane wasn’t going supersonic though. The cone happens at transonic speeds, so when the plane is accelerating, as it nears Mach 1 a shock cone can appear. You can tell in the video that it is very close to Mach 1 because there is no sonic boom and the sound from the plane appears just before it passes.