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Say you have a scenario where there is a shockwave, and you sent a normal sound wave towards it. Could the sound wave travel through that shockwave? If so/not, why?

Would it depend on the strength of the shockwave? For example a sound wave might be able to travel through a weak shock but not a strong one.

(It also might depend on the strength of the sound wave itself)


I'm aware that shock waves themselves can pass through each other, (see the opposed wedges model here)

If that same thing applies to sound waves, that would indicate that they can pass through each other, but shock waves are a lot stronger than sound waves so I'm not sure this example would work.


Also, let's say for this example that the sound wave is approaching the shockwave from behind. In a real scenario that wouldn't be possible, but imagine somehow the sound wave could catch the shockwave and intersect it from behind.

I would also imagine some of the principles that apply to the sound wave intersecting the back of the shock would also apply for a sound wave intersecting the front.

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    $\begingroup$ Interesting if the sound wave would be refracted by the change in air density. $\endgroup$ Commented May 9 at 23:55
  • $\begingroup$ Towards it from the front, or from behind? ;) $\endgroup$ Commented May 10 at 16:31
  • $\begingroup$ @MichaelHall Good question; it’d be interesting to see both scenarios. Probably from the back, as I’d imagine that would be the most common scenario on a plane (the engines making sound waves behind the plane shock) $\endgroup$
    – Wyatt
    Commented May 10 at 16:43
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    $\begingroup$ I was joking. Because a sound wave couldn't catch a shock wave from behind. $\endgroup$ Commented May 10 at 17:38
  • $\begingroup$ @MichaelHall Oh right, thanks for pointing that out haha. Still would be interesting to see (in a theoretical scenario) what would happen if the sound wave could catch the shockwave from behind. $\endgroup$
    – Wyatt
    Commented May 10 at 18:39

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The illustration that you use is actually less general than the question you ask. The illustration is of a situation where all the shocks are caused various part of the same flying object. I will deal with that case first.

Consider an observer moving with the shock surface and observing the flow in their immediate vicinity. They can measure the orientation of the shock surface, the flow vectors on either side, and all thermodynamic properties including the speed of sound waves. For them, the flow is steady. They will always observe certain facts. The ones that relate to your question are these.

The components of the flow velocity that are tangential to the shock surface will not change. The observer can also move with them, and the flow will now appear one-dimensional. It will be a "normal shock". The approaching flow will always be supersonic, that is, faster than the pre-shock sound sound speed. A sound wave can therefore approach from in front, and will be absorbed into the shock. The flow speed behind the shock will always be reduced, but may be supersonic (weak shock) or subsonic (strong shock), that is, faster or slower than the post-shock sound speed, which is itself always less than the pre-shock sound speed. In the weak shock case, no sound wave can approach from behind against the supersonic flow. In the strong-shock case a sound wave can approach from behind and is absorbed.

Summing up for shocks from a single object, sound waves can always approach shocks from ahead and are absorbed. The can approach strong shocks from behind and are also absorbed. Compressive waves strengthen the shock, expansion waves weaken it. The key is to realize that there is always an observer who sees the situation as one-dimensional.

More generally, consider two vehicles flying close together but not at the same speed or even in the same direction. There are now many more possible cases. Broadly, the situation is that when one aircraft overtakes another, their waves will momentarily coalesce, but if they pass in opposite directions, the left-pointing V will pass through the right-pointing V.

The whole thing is pretty-well understood, but you need a college-level text to give you the details. It might be more fun to spend an afternoon at your nearest duck pond. Wave behavior is pretty universal in all media, but surface wave speeds in a shallow pond are only a few feet per second!

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