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I noticed in couple of videos of big explosions muffled sound right after the explosion flashes but before the shockwave. What is this sound?

Here are some examples from the net:

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  • $\begingroup$ I think it would be at least interesting to look at how the sound for those videos originated: in particular how well it was synced with the video an whether it was dubbed, which it may have been for some of them. $\endgroup$
    – user107153
    Commented Sep 28, 2016 at 14:30
  • $\begingroup$ I agree but I doubt that this is some video to sound synchronization effect. $\endgroup$
    – Todd
    Commented Sep 28, 2016 at 15:03
  • $\begingroup$ In the first at least the initial thump is the detonation cord: you can see the flash in a couple of frames at the bottom of the picture. I have no trust in the sound of the second at all. Have not looked further, sorry. $\endgroup$
    – user107153
    Commented Sep 28, 2016 at 16:45
  • $\begingroup$ I agree with @tfb, you should not hear anything due to the explosion before the shock wave (unless some small projectile ejected during the explosion propagated ahead of the main shock, which would produce its own "little" shock). The piston (i.e., heated and compressed air in explosions) travels faster than sound, thus sound waves should not out-run it. If you hear something before the shock, it came from something else... $\endgroup$ Commented Sep 28, 2016 at 19:23
  • $\begingroup$ Ok, I am removing the second link because it is unreliable indeed and putting the recent SpaceX explosion on its place, where is the detonation cord in this video? $\endgroup$
    – Todd
    Commented Sep 28, 2016 at 22:19

2 Answers 2

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A pressure wave can travel through solids at a greater speed than through air. And this means a "pre sound" can reach you before the shock wave does - as the motion of the ground will in turn induce a sound wave in the adjacent air.

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  • $\begingroup$ Yes, this matches with the research and observations I made. I came to the conclusion that this sound is from the P-wave of the earthquake made by a big explosion which travels approximately 2 to 8 km/s in the Earth's crust and can be heard only in big enough explosions. $\endgroup$
    – Todd
    Commented Sep 29, 2016 at 9:03
  • $\begingroup$ I thought this as well, but my initial assumption was that the amplitude would be too small for the speaker to respond but perhaps I was wrong. $\endgroup$ Commented Sep 30, 2016 at 12:53
  • $\begingroup$ The amplitude depends from the size of the explosion, that's why you can't hear it from a small explosion because the amplitude/energy propagated through the earth is too small. $\endgroup$
    – Todd
    Commented Oct 2, 2016 at 10:21
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Technically a shockwave is a propagating disturbance that travels faster than soundwaves in a particular medium. If the wave travels slower than sound it isn't a shockwave. The answer to your question though is that the energy and displacement of pressure that are the "shockwave" you're probably thinking of, propagate through air at a slow speed than sound waves do.

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  • $\begingroup$ No, I know what a shockwave is, and it is clearly visible where it is and how it is propagating from most of the videos. This sound I hear is before the shockwave. $\endgroup$
    – Todd
    Commented Sep 28, 2016 at 14:12

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