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Science Fiction has long written about this communication technique in vacuum: Astronauts touch their helmet against another astronaut's helmet, so that sound will transmit through that physical connection and they can to talk to each other without needing electronic communication devices (Example screencap from The Expanse). But is there a source that real astronauts have ever done this?

Googling "astronauts touching helmets" and "astronaut helmet-to-helmet" mostly yields verifications that it is indeed possible. The only real world mention I could find is from the US Navy diving manual: "The two divers need only touch helmets to talk with each other".

Edit: To clarify why I asked: Multiple Redditors claimed that this has happened in real life, but of course this is all hearsay: Comment 1, Comment 2

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    $\begingroup$ Unless there was an emergency situation, such a communication method would not be required inside a space craft during a mission. This then reduces the question to how many times have multiple astronauts conducted an EVA, or EVA conditions such as an open cargo area on the Space Shuttle, at the same time & were they in close proximity to one another be able to communicate in such a manner. $\endgroup$
    – Fred
    Jun 25, 2022 at 3:12
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    $\begingroup$ @Fred there would be no reason to use this method unless the EVA radio comm failed, and at least on US missions, I don't think it ever has. Also on most US EVAs the helmets have thermal / protective fabric or other structures over a large part of the helmet which might prevent this from working. My guess is that this has never been done on a US EVA but it would be an enormous task to prove that... $\endgroup$ Jun 26, 2022 at 13:05
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    $\begingroup$ Was this solid-borne sound transfer ever tested in a vacuum chamber on Earth? $\endgroup$
    – Uwe
    Jun 26, 2022 at 17:37
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    $\begingroup$ If this has ever been tested it would have been as a backup communication method, I suspect. Proving a negative is hard, however... $\endgroup$
    – PearsonArtPhoto
    Jun 27, 2022 at 15:32
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    $\begingroup$ @Uwe vacuum might not be necessary for the test. If they can't hear each other before touching and suddenly they can hear each other when touching, then it works and air vs vacuum outside doesn't matter. The only time that vacuum would be necessary is if they could already hear each other before touching. $\endgroup$
    – uhoh
    Jun 27, 2022 at 17:32

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In space, (probably) no-one can hear you scream through glass (very well)

This is an interesting "Have astronauts ever...?" question, and I can't answer it directly.

I can't answer if it's been tried, but it might give us some insight to calculate the acoustic losses due to the severe acoustic impedance difference between air and glass.

The same way that there's a roughly only 4% reflection of visible light at each air/glass interface (Fresnel reflection) due to the closeness of the ratio of electromagnetic impedance of free space and glass. We usually write that ratio as just the ratio of index of refraction; air at about 1.0 and glass at about 1.45.

The acoustic impedance of a medium is proportional to $\rho c$ where $\rho$ is the density and $c$ is the speed of sound of that medium.

If we write $r = \rho c$, then the acoustic power transmission coefficient of sound at an interface is given by

$$T = \frac{4 r_2 r_1}{(r_2 + r_1)^2}.$$

Let's use 350 and 5000 m/s for the sped of sound in air and glass, and 0.0013 and 2.2 g/cm^3 for the densities. Since we are always taking ratios, the units don't matter. That leaves us with

$$r_{air} = 0.45$$ $$r_{glass} = 11000$$

$$T_{air/glass} = 0.000165$$

So that's an attenuation in power of about 6,000 at each interface, and remember there are two interfaces because we need to go back to air again.

So we're talking $1/6000^2$ or a 76 dB drop if I've done my decibels right.

And remember, the area of contact is really, really small! This is not like talking through a plate-glass window, it's talking through, perhaps a few millimeter hole, filled with glass, in an otherwise acoustically opaque wall.

I included "(probably)" and "(very well)" in my tl;dr at the top because this is only a spherical cow approach. When we speak inside a hard glass fishbowl, certain frequencies may set up resonances in the glass, and for those frequencies there may be better transmission to an identical, mechanically coupled resonant fishbowl on our companion's head.

And if we couple lip-reading and hand gestures with that potential muffled transmission, communication might be enhanced.


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