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A big balloon filled with air explodes on the moon. Suppose we could bear the zero pressure on the ou there. Then the balloon explodes. Could we hear this explosion, from very nearby? A huge amount of air is after all released after the explosion.

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Sound needs a medium. The sound of an explosion in normal atmosphere is the displacement of air , the change in pressure at the front of the explosion.

Certainly in your scenario you will not hear anything until the escaping gas reaches you. The question is whether acoustic frequency waves will exist in the air as it dissipates. My intuition is that since the explosion is outwards, there is no inner pressure wave propagating to generate acoustic waves and so just continuously rarefied air will pass you.

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  • $\begingroup$ Can the reasoning that you have given in your second paragraph be also applied to the " Consider an open door , you open it and close it quickly again and again , generating a flow of air , which can be seen by keeping a paper near by or even felt by your eyes " but we don't hear a sound . I was just thinking about this in the morning and I came to a quite similar conclusions you have given . Only wanted to enquire whether that will work in this case too ? $\endgroup$ – Shashaank May 13 '17 at 14:58
  • $\begingroup$ @Shashaank Well, with the door it is a matter of frequency. 20 Herz is the lowest , whcih means opening and closing 20 times a second which cannot be done with a door, and it would be a very low sound. No, with the explosion it is different $\endgroup$ – anna v May 13 '17 at 17:12
  • $\begingroup$ Thanks again ! I thought about the frequency being less than 20Hz but I think that is not the main point. I think that even if one somehow manages to get a frequency of above 20Hz , there will still be no sound. Because when you pull the door the air there gets displaced. There is a whole movement of air particles from one place to another. Another part of air , say from the room outside comes to take it's place. The displaced air does not come back. There isn't any oscillation of that part of air. But not sure whether this will be the case if the frequency of pulling and pushing .... $\endgroup$ – Shashaank May 13 '17 at 18:19
  • $\begingroup$ The door is increased beyond a limit , because the air outside might not be able to occupy the new place that quickly ! $\endgroup$ – Shashaank May 13 '17 at 18:20
  • $\begingroup$ @Shashaank you will need the velocity of sound, the air is replaced with the velocity of sound. $\endgroup$ – anna v May 13 '17 at 18:22

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