I have a scene where an alarm is sounding in an airlock, as the pressure is lowered from normal atmospheric pressure to vacuum (takes a few minutes). How would this alarm sound to an unprotected person unlucky enough to be in the airlock?

What would this also sound like to someone inside a pressurized space suit?

A good answer will describe the sounds for both protected and unprotected ears, covering pitch changes, filtering effects, etc, as well as comment on at what point unprotected eardrums may rupture, and what that might sound like.

(in my case, the air is being recycled by noiseless pumps sucking the air into storage.. but I am curious what it might sound like if a valve was venting air into space.. would that add a noisy hiss?)

  • $\begingroup$ How fast does the air leave your airlock? or alternatively, what is the volume of the airlock and how large is the air outlet? $\endgroup$ – Mathaddict Apr 18 '19 at 18:00
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    $\begingroup$ I would need to double check some bell jar experiments before making a formal answer for the pitch interaction, but answers for the effect on the alarm would depend on its frequencies, and how it is produced/mounted. Explosive decompression vs slow drop in pressure is also an important factor. - Answers should also consider the sounds of the suit itself, and how active or passive it is. $\endgroup$ – TheLuckless Apr 18 '19 at 18:07
  • $\begingroup$ I assume that the sound changes would be the same regardless of what the alarm sounds like. Feel free to correct me if that's not the case. It's a decompression over some minutes. The suit noises and muffling effects I can guess at myself. $\endgroup$ – Innovine Apr 18 '19 at 18:38
  • $\begingroup$ You want to describe a normal airlock operation (i.e. a pump is evacuating air for storage), rather than quick decompression, when airlock is open to space? $\endgroup$ – Alexander Apr 18 '19 at 18:38
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    $\begingroup$ @Innovine "normal" airlock operation, in real life, goes in 2 stages. In stage 1, air pump evacuates most of the air from airlock. In stage 2, the rest of the air is getting vented to space. Due to acclimatization requirements, whole process (again, in real life) takes hours. Technically, it can be done in minutes. $\endgroup$ – Alexander Apr 18 '19 at 18:49

The sound intensity formula (I=ξ²ω²cρ) tells us that the volume of your alarm would drop in linear proportion to the decrease in atmospheric density. How this will sound will depend a lot on how you are depressuring the airlock.

The pitch comes from your medium so your alarm's pitch should remain unaffected because the gas mixture would not change.

Lastly, undistorted sound cannot be carried by more pressure than is available. At 1 atmosphere, we can perceive clear sound at volumes up to about 190bd. As the atmosphere dissipates, the alarm will begin to distort because sound is only clear when it's peak compression does not exceed twice the density of the medium. This distortion will start to take place sooner, and be more pronounced, the louder your alarm is.

These effects will be most noticeable if you are wearing a helmet or inside of a cockpit where the actual air pressure you are in is not changing. If your ears are exposed, the sound will much more quickly begin to sound distant and distorted as the air pressure in your ear drums exceeds the atmosphere around you. In a slow depressurization, your eardrums will 'pop' equalizing pressure several times as you loose pressure. In a rapid decompression, your eardrums could potentially rupture which would be both painful and deafening.

Some sound will also reach you traveling through physical contact with the hanger as TheLuckless pointed out, but there are so many unknown variables there I would not begin to be able to hypothesize what that would sound like exactly other than still being able to still hear a faint and badly distorted alarm as you approach vacuum that would disappear when you leave physical contact with the hanger if you have not already done so.

  • $\begingroup$ This site has $mathjax$ enabled. I would have just edited in the $s, but I am not sure whether the last variable in the formula should actually be $\rho$ (or $\varrho$ to be more different from $p$) or not. $\endgroup$ – Jan Hudec Apr 18 '19 at 19:24
  • $\begingroup$ Note that pumping out the air would decrease the temperature (adiabatic expansion) and that in turn would increase the speed of sound, so it would be a bit less than linear. $\endgroup$ – Jan Hudec Apr 18 '19 at 19:26
  • $\begingroup$ at 1atm, i think clear perception of sound isn't so important at 190db, since its also fatal... $\endgroup$ – Innovine Apr 18 '19 at 19:30
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    $\begingroup$ Suggested expansion to the answer: Accounting for sounds transmitted through the hull/suit - Sounds can still route their way through things with physical contact, and depend on the materials and sounds involved. $\endgroup$ – TheLuckless Apr 18 '19 at 19:40
  • $\begingroup$ @luckless thanks, but it's the alarm sound I want to get perfect. A few suit noises I can guess at :) $\endgroup$ – Innovine Apr 18 '19 at 22:02

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