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ISS astronaut Randy Bresnik recently posted a video on social media from his spacewalk, shot with a GoPro.

You can watch his clip here (Embedding the clip didn't work):

https://twitter.com/AstroKomrade/status/935278434158333952

So there is obviously sounds coming from his movements and his suit and so forth.

But why is there a constant noise in the audio? To me it sounds like a really bad recording. Is that just common with GoPros or what is the thing here? Any clever ideas?

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  • $\begingroup$ Combination of poor mic placement and noise from the suit movement, Nasa is working on it nasa.gov/topics/moonmars/features/hatsman.html\ $\endgroup$ – Martin Beckett Nov 29 '17 at 0:02
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    $\begingroup$ I've added the sound tag. Somewhat related sounds discussed here and here for example. $\endgroup$ – uhoh Nov 29 '17 at 0:17
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    $\begingroup$ The noise should be structure-borne noise. It is not conducted by air, but within solids. The life support system of the suit generates some noise and it is conducted by the rigid parts of the suit to the mounting of the cam and within the cam to the microphone of the cam. $\endgroup$ – Uwe Nov 29 '17 at 10:21
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    $\begingroup$ And would you expect uniform noise from that? It is weird because I would assume that the smaller the acoustic system, the lower the average randomness of noise. Yeah life support would be plausible, but what is producing random noise in that? Are there fans or something? Or a steady flow of gas? I would assume that not. $\endgroup$ – lthz Nov 29 '17 at 10:46
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    $\begingroup$ @lthz: There are fans for a circulation of the atmosphere in the suit and there is a water pump for cooling water circulation. Oxygen is added from the tanks when suit pressure is too low, that is a source of noise too. $\endgroup$ – Uwe Nov 29 '17 at 13:15
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Short answer:

These "sounds" consist of vibrations produced by the astronaut's movements and his suit systems and transferred through his space suit, camera enclosure and PCB the microphone is soldered to plus electrical and digital noise added by the sound processing pipeline.

Long answer:

Microphone is an electromechanical device, that converts vibrations into electricity. Different types of microphones contain different mechanical parts, that can easily perceive air density oscillations. Usually it is a sort of thin foil membrane fixed around edges and vibrating in the middle. Since its so lightweight, it picks all sorts of vibrations that reach it - taps of your fingers touching the enclosure and buttons, your steps, ultrasounds that you can't even hear - everything (that's why on Earth studio microphones are frequently vibroisolated using so called shock mounts). You don't need air to conduct them, because dense materials usually conduct vibrations even better than air (think about echolocation on the submarines). Also membrane sensitivity should be somewhat stronger in space, where the amplitude of membrane's vibrations isn't limited by the pull of gravity.

This membrane is one half of variable capacitor or a coil, which produces small oscillations of current in the input circuit of the microphone pre-amplifier, thus converting them into electricity. You need pre-amp, because those oscillations are really small, in the millivolts range. Pre-amp produces amplified signal suitable for analog-to-digital conversion and recording, but all the pipeline parts - amplification, ADC and sound compression - add their own noise to the microphone signal. That's why you hear something even when there are no obvious vibrations produced by the astronaut himself.

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  • $\begingroup$ It's a helpful answer. Are microphones really less sensitive on Earth because of gravity? $\endgroup$ – uhoh Nov 30 '17 at 1:30
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    $\begingroup$ I don't know if practical measurements were ever made. Idea here was that membrane basically is a pendulum. When you apply constant force to a pendulum, you change it's maximum deviation and period (depends on how force vectors add up, but generally period and deviation should become shorter). On second thought, this effect may be not as noticeable as dampening effect of the surrounding air, but should be there anyways. $\endgroup$ – ZuOverture Nov 30 '17 at 5:05
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    $\begingroup$ Well a pendulum has a characteristic frequency, it's a fairly high-Q resonator. This is why it's used in clocks. A microphone on the other hand is the exact opposite - you do not want it to have a resonant frequency at all. In fact you would like it to have a response that is mostly flat and independent of frequency (except maybe for some roll-off). $\endgroup$ – uhoh Nov 30 '17 at 5:43
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    $\begingroup$ Do not take it literally. Membrane is different from a pendulum, but there are certain similarities: equilibrium state, profile of forces that return it to equilibrium, size dependence, etc. So you should expect similar effects. $\endgroup$ – ZuOverture Nov 30 '17 at 6:01
  • $\begingroup$ Wow interesting answer, thanks. I think the pendulum idea is flawed: The gravity is for a pendulum what the tension is for the membrane. The most gravity can do to a membrane is applying a constant that shifts its amplitude, it will still oscillate as before. $\endgroup$ – lthz Nov 30 '17 at 7:57

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