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Supersonic vehicles produce Mach cones behind them, which observers hear as sonic booms as they pass by. So if, for example, a jet goes supersonic as it heads toward you, but decelerates to subsonic before it passes you, the Mach cone will never pass your position, and you'll never hear the boom.

So how then was the sonic boom(s) from the SpaceX Falcon Heavy side boosters audible from the ground at all? Shouldn't the Mach cones have been directed behind the side boosters, up into the sky, meaning they'd never pass any point on the ground?

Note: There is a question that uses the Falcon Heavy boosters to ask about sonic booms in general, where even though the boosters are mentioned in the question it's actually just asking about general principles of sonic booms rather than about Mach cones in the Falcon Heavy specifically here.

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I don't have specific knowledge on this subject, I'm reasoning from general principles of mechanics here.

What happens as a device (rocket stage or aircraft) goes from supersonic to subsonic?

The Mach cone results from continuous production of more new shock front. It seems to me that as the aircraft slows down from supersonic to subsonic the shock wave that was last produced proceeds to overtake the aircraft.

Reasoning from general principles: before going subsonic the Mach cone has a sharp point: the aircraft that is continuously producing new shock front. After going subsonic new shock front is not added anymore, and I expect the Mach cone becomes more and more blunt, with the last produced shock wave now being in the lead. That is, I expect that only the shape of the propagating shock front will change. The Mach cone will continue to propagate until dissipated, but as the aircraft is no longer adding new shock wave the overall shape of the shock front changes accordingly.

(I've been using the expression "shock front" in this answer. My language is not precise here, but it's hard to see how to be more precise. I tend to use the expression "shock wave" for spherical propagation away from a source, and I tend to use "shock front" for a more compound propagation pattern, such as a Mach cone. These are not general conventions; I'm improvising here.)

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Consider a plane moving left-to-right at $v$ faster than the speed of sound. At each time $t$, a disturbance starts radiating from the point of the plane. Just later, while that last bit of disturbance has reached $c_s \Delta t$, the point of the plane has moved to $v \Delta t$, a larger distance, and a new sphere starts to radiate. The tangent surface of those spheres provides the shock cone as they radiate together.

Now consider the plane just disappearing as it gets to $x = 0$ in the middle of your mental image. It's no longer radiating new spheres.

But all the old spheres are still there, and the last few of them are still propagating past $x = 0$. There won't be a new shock, but the existing one will continue to move out and to the right. It'll still be heard in parts of $x > 0$.

A more formal way to see this: The normal to the shock cone points to the source of the most direct excitation, and because of the cone angle it's pointing to a place "behind" the point of closest approach. See figure below, taken from Wikipedia with added arrow.

enter image description here

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