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Imagine, if you will, a world where aural communication is impossible. Everything is done through gesture - sign language and written language are common, but the spoken word is not used, and using any sound for communication is not possible.

Now, the obvious solution would be to simply make everyone deaf, but that doesn't really sound like fun. I'd like environmental factors to make communication through atmospheric compression (sound waves) impossible.

The environment needs to support human life.

What atmospheric conditions could cause disruption to compression waves to the point where they could not be used for communication?

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    $\begingroup$ I'm pretty sure that breaks physics. $\endgroup$ – AndreiROM Mar 24 '16 at 21:40
  • $\begingroup$ @AndreiROM I'm not sure. I know a vacuum makes soundwaves impossible...and life impossible. Also extremely high temperatures might create enough background "noise" that meaningfully compressing atmosphere to communicate becomes impossible. (Does sound propagate in plasma? hmmm) $\endgroup$ – Josiah Mar 24 '16 at 21:46
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    $\begingroup$ I'm pretty sure people couldn't live in plasma :-P $\endgroup$ – AndreiROM Mar 24 '16 at 21:48
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    $\begingroup$ Yes, sound does propagate in plasma. No, people can't survive in plasma. en.wikipedia.org/wiki/Speed_of_sound#Speed_of_sound_in_plasma Which begs the question, if a tree falls in a plasma..... $\endgroup$ – John Robinson Mar 24 '16 at 21:56
  • $\begingroup$ Is it speech, or audible communication you don't want? Your title says speech, but your post seems focused on any audible communication. I only ask because there exist plenty of non-speech audible languages. (Whistled languages being the most common.) $\endgroup$ – nitsua60 Mar 24 '16 at 21:59
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There are two ways I can see which would wreck the ability for humans to hear speech:

White Noise:

This is along the same lines as John Robinson's answer, but on a smaller scale. Rather than a lot of wind, the wind would only have to be chaotic enough to destroy sound waves.

Rather than areas without wind, you would have periods of time without wind, though they would be extremely brief (ie, every other microsecond). This could possibly be justified by the presence of microscopic, ferrous particles suspended in the atmosphere all over the planet. The dust could vibrate as a result of fluctuations in the planet's magnetic field (some process occurring in the world's core, unique to this planet?). This would create a constant buzz or hum which would disrupt all spoken language.

Active Noise Cancellation:

The same concept as ANC headphones-- incoming sound waves are duplicated, but 180 degrees out phase. The two contradictory waves meet, and cancel one another out.

In the case of an entire world, this could be managed with nanoscale machines (or the ferrous particles being manipulated by a controlled magnetic field) spread throughout the atmosphere, either intentionally or as a remnant of some ancient technology.

Alternatively, there could be microscopic flora or fauna throughout the atmosphere which actively counteracts any sound waves which would disturb their position as they float. Since every individual microorganism is counteracting the sound which it encounters, this wouldn't just cancel human speech, but all sound below a certain volume.

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The only thing I can think of would be to have the world be incredibly windy. If it's windy enough, the wind will cause too much interference with any spoken words and people won't be able to hear what you say.

The problem with that, of course, is that (mathematically speaking) there has to be a place on the planet where there's no wind, so it kind of breaks down, but you can hand-wave it by saying there's no wind at the poles or something. This is a result of the single greatest-named theorem in all of mathematics, the Hairy Ball Theorem.

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  • $\begingroup$ That's an amazing theorem. +1 for that. Although I think that as long as there's vertical motion in the lowest layer of the atmosphere, there could be wind everywhere and we could put the 0 wind zones in the ozone layer or something. $\endgroup$ – Josiah Mar 24 '16 at 21:51
  • $\begingroup$ Perhaps. Though then you'd have to explain what the mysterious columns of vertical wind are. I smell a mythology/religion in the making.... $\endgroup$ – John Robinson Mar 24 '16 at 21:58
  • $\begingroup$ Well, hot air rises, but a mythology is more fun. $\endgroup$ – Josiah Mar 24 '16 at 22:17
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    $\begingroup$ I think your self-admitted problem isn't actually a problem, for two reasons. (1) the existence of only a few isolated spots where speech would be useful makes it unlikely 'enough' to presume some other communication mode is dominant. (2) the hairy ball theorem doesn't say that the point(s) of divergence have to be stationary; if the only place(s) where speech is viable move unpredictably and rapidly, that weighs even more on the side of developing non-aural communication. $\endgroup$ – nitsua60 Mar 25 '16 at 0:20
  • $\begingroup$ That wind would itself produce loud noise. Maybe that loud noise could make the zero-points still useless (not sure of it though). $\endgroup$ – PatJ Mar 25 '16 at 13:39
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Sound waves are propagated through a medium, wherever there is a medium, there will be sound. However if you can create some kind of phenomena on the planet that generates a sound noise level high enough so any practical communication would be impossible, it would work.

If the planet produces white noise at around 100dB or produces a single sound frequency at the max atmospheric pressure, that would hinder the use of sound to communicate.

The explanation for the latter proposal is that in order for the waves to be called sound, they need to oscillate, just like an AC wave. If there is already a one frequency that touches the minimum pressure, any other higher frequency would need to go to a negative pressure in order to manifest as sound, which is impossible. Imagine creating a wave that touches the top of an aquarium, and the lower part touches the bottom part of the aquarium, if you wanted to add a smaller wave within that big wave, you would need the water to go above and below the aquarium walls. In fact, water can go higher and over the open aquarium, just like pressure, but a vacuum is the lowest pressure you can obtain.

However, I am not sure what that would do to humans, since we know that prolonged exposure to loud sounds is very unhealthy, and I am pretty sure sound at the maximum atmospheric pressure would be similar to periodically generating shockwaves, which would kill any living organism caught in it.

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  • $\begingroup$ I explained more in depth why we need the max pressure. If it reaches close to the max pressure, it would be possible too, since any sound that wouldn't go in negative pressure would be too small anyway. We need to calculate what the ears can hear and correlate that to the minimum pressure where sound becomes impossible to hear. $\endgroup$ – Bloc97 Mar 24 '16 at 21:58
  • $\begingroup$ Apparently the smallest sound is REALLY small. According to the article, it's about 0dB, or 0.000002 Pascal or 10^-8 mm. $\endgroup$ – Josiah Mar 24 '16 at 22:18
  • $\begingroup$ But human hearing works with the logaritmic scale, and 100dB sounds 100 more powerful than 1dB, which means that you won't hear that small sound. I will look towards this question more in-depth when I get home. Maybe look at how humans perceive hearing? The dBA scale came to mind. $\endgroup$ – Bloc97 Mar 24 '16 at 22:24
  • $\begingroup$ Should have been more clear. The article I cited is based on what humans can actually perceive, at 1kHz. $\endgroup$ – Josiah Mar 24 '16 at 22:29
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This was a major plot point in Ocean on Top, one of the lesser-known works by Hel Clement from 1967.

His people lived under pressure in a oxygen-carrying fluid at the bottom of the ocean. Human vocal apparatus would not work. In fact (spoiler) they did not breathe tidally at all. The lungs did not pump in and out; rather, oxygen diffuses at high speed without the fluid moving.

I really suggest finding that novel, for his remarkable thoughts on how this affected the civilization.

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