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As I understand it, when an object pushes past the sound barrier, a sonic boom doesn't happen just once, but rather, continually (correct me if I'm wrong). So why is it that there seems to be only a confined, cracking bang instead of a gradually crescendoing booooooooo...m sound, building in intensity until the object passes and then gradually decreasing, sounding more like a long continual "tearing" sound?

Conceptually, I'm visualizing it like a boat moving through water, eventually moving fast enough that it pushes beyond its interference rippling, creating a wake behind it, the wake being one continuous wave. Imagining an observer up ahead seeing the boat coming, they would also be able to see the boat's wake coming, increasing in size and intensity, and then diminishing once the boat had passed, until it gradually disappeared.

So why is it that a sonic boom seems to be a digital/discrete/binary occurrence rather than an analog of continuous sound?

Bonus question (can post separately if necessary):

If a sonic boom is relative to the listener, does that mean that there's an infinite number of boom-boom-boom-boom's happening as it moves? E.g., if a supersonic plane flew past and I heard its boom directly above me, wouldn't my friend 100m further up ahead also hear "my" boom, as well, though diminished? Would they not get their own fresh boom when the plane passed over them? How far away would a listener have to be in order to hear their own "fresh" boom (...not an actual question, just conceptual to paint a picture of how I'm currently (mis)understanding it)?

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Keeping with the boat wake analogy (which is a good one) you can see the wake from far off, but you cannot "hear" the wake until it reaches you. Your eardrums are like a duck floating on the surface of the lake. When the boat passes by, it is only when the wake passes under the duck that it begins to bob up and down. Since the wake of a boat is in the form of discrete lines, the duck bobs in short pulses rather than a continuous up-and-down motion.

You can actually see a sonic boom with a special technique called Schlieren photography that allows for the photographing of pressure changes. In the photograph of a T-38 fighter jet trainer below, you can see that the pressure waves of a sonic boom always exist while the aircraft is going faster than the speed of sound. But, it is only when those pressure waves hit your ears that you hear the crack. The sound is a crack because the pressure waves are so narrow.

Photograph of a fighter jet's sonic boom shock wave

In this picture, the jet is flying to the left while the shock waves extend vertically and travel with the jet (actually, the shock wave forms a cone with the plane at the tip). There are multiple shock waves due to multiple leading edges on the plane (nose, wing edges, tail sections, etc.). The black line to the right of the plane is jet exhaust.

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  • $\begingroup$ So is the "crack" sound technically happening continuously then, as the jet flies? I.e. it only sounds like a crack to an observer because they're stationary and the wave merely passes over them before it's quickly gone (and also fast enough that it sounds like a crack)? I guess what I'm wondering is if there was a cable dangling from the jet and someone attached to it, would they hear a continuous "crack" as it passed over everything (setting aside the fact that they'd be breaking/already past the sound barrier themselves)? Very cool picture, by the way. That helps visualize things better. $\endgroup$ – Coldblackice Sep 28 at 11:12
  • $\begingroup$ @Coldblackice You are right on both counts. $\endgroup$ – Mark H Sep 28 at 11:26

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