Let’s say a fighter jet is traveling faster than the speed of sound at 500 feet above the ground. Will there be a continuous sonic boom audible for all ground observers as the plane rushes by overhead or will a sonic boom only occur as the plane transitions from subsonic to supersonic.
The former. See this image to understand what’s happening.
As Q.E.D. says, the shock front is continuous; however, the observer on the ground is going to hear the single wavefront as it passes by (or perhaps multiple wavefronts, depending on how the aeroshape interacts), not a continuous blast.
Stranger
So if one could time it perfectly, and followed the airplane making the sonic boom at the same speed, what happens then? Besides the following airplane making its own sonic boom, of course.
I think there’d be a continuous roar audible from somewhere relative to the following plane. But, whether you could put a microphone on the outside surface of the following plane I’m not so sure of. Hearing the roar pass as a boom from a stationary location has little or no relative speed between the listener and the air that carries the sound. The following airplane setup doesn’t provide that.
From what I remember, the sonic boom ‘wavefront’ is nearly vertical at the time of the transition, and more diagonal, trailing behind the aircraft, if it is cruising at a speed much faster than the speed of sound.
Does this suggest that the sonic boom would be more pronounced for listeners on the ground if it came from a plane just crossing the speed of sound, because the plane was nearly overhead? (Assuming that the plane maintains constant altitude)
I don’t think there would be a continuous roar if you could follow the shock front at the same speed. The boom is not simply caused by high or low pressure, but by the sudden (and large) change in pressure as the shock front passes you.
I’m not sure how well this analogy actually holds up, but imagine you’re sitting in a tiny boat when a big boat goes by. The big boat’s wake is a shock front which tosses you up and down as it passes you. However, if you match the big boat’s speed, you can sit peacefully right up against its wake, and even cross it forward or back slowly without too violent a disturbance. I wonder if supersonic jets in proximity to one another experience this sort of “surfing” on one another’s shock fronts. In a boat, you can feel the resistance to crossing the wake, and then once you punch through you get a little shove to help you through, because sitting on top of the wake is not stable.
Allow these fine folks to demonstrate.
ETA: That doesn’t answer your question, but it’s a cool video, regardless.
Reasonably well, actually. A boat’s wake is basically the same sort of phenomenon as a sonic boom, just with a different kind of wave. A boat which is traveling at faster than the speed of water waves produces a wake continuously, for as long as it’s traveling faster than the wave speed, but the wake only hits you once as it passes you. Likewise, an airplane traveling at faster than the speed of sound continually produces a sonic boom, but you only hear it once as it passes.
Depending on A/C type you will hear 2 sonic booms close together. One for the wing, one for the tail. I heard that on 9/11 and thought it was 2 aircraft.
Can I assume you are talking about one of the planes that hit the WTC? Is it understood that it was travelling near the speed of sound near/upon impact?
Correct me if I’m wrong, but neither of those planes attained supersonic speeds. Unless a sonic boom also occurs at subsonic speeds?
An aircraft will start developing shockwaves when there are localised areas where the speed is faster than the speed of sound. For example, air travelling over the top surface of the wing is moving at a faster speed relative to the wing than the aircraft is travelling relative to the main air parcel. Shockwaves therefore can start forming from the wings at speeds well below the speed of sound. Design features such as wing sweep and relatively thin wings with minimum curvature are attempts at delaying the onset of shockwaves so as to allow the airframe to travel faster.
Fighter jets were rushing all over the sky that day. I assume that’s what they’re talking about, not the jets that crashed.
**Magiver ** notes he is from Dayton. That’s where Wright-Patterson AFB is located, and a little far to be near the flight paths of either of the planes that hit the WTC. (Of course, he could have been on holiday.)
When the space shuttle lands there is a double sonic boom. It is audible in the LA area for landings at Edwards.
Nope and nope. Both aircraft were flying moderately slowly as I understand it–at about 300 knots or so. Oh, that’s fast enough to turn an aluminum can with wings into confetti when it hits a steel structure, obviously, but sonic booms only occur when the aircraft flies faster than the local speed of sound.
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Or when their parts of one move faster than sound…
The Soviet TU-95 “Bear” turbo-prob aircraft was said to have engines so powerful that the tips of the props created a “sonic-boom”-like racket. 
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Lightning follows a somewhat irregular path. I read somewhere that researchers thought lightning bolts tended to connect the most nearly contiguous ionized paths caused by cosmic rays shooting through the atmosphere (but I think this was only speculation).
When a length of a bolt is at right angles to the line from it to you, you hear a bang.
When a length of the bolt is more nearly aligned between its location and you, that is when it points at you or away from you, you hear a sort of a paper-tearing roar.
I think the paper-tearing roar might be a good example of what the jet should sound like in this thought experiment.