I'm doing research for a sci-fi story involving a primitive human civilization on another planet. My idea is that they use big "acoustic long-range communication tubes" to communicate between cities or outposts. Ever see those parabolic "Acoustic Mirrors" at a park? My idea is a pair of large sound mirrors at the end of a big tube. The tube ensures that sound stays confined along the line of travel instead of dispersing into the air. The parabolic mirror focuses the sound so that it becomes audible to a person who's head lies at the focal point. The end points obviously have a door in the wall so that people can enter and exit, and it does NOT have to be air-tight because the surrounding walls will reflect sound waves well enough. Also, the diameter of the tube at the end points is obviously the same as the diameter of the parabolic dish, but in the middle the tube might be narrower. I need to know all the acoustic equations necessary in order to investigate if such a thing is viable. In particular, I want to answer the following specific questions: 1. What min or max diameter of the tube is necessary to capture all frequencies of normal human voice? I don't want this to be a "whisper dish" like at the parks. Why do you have to whisper there anyway? 2. If a speaker was to speak as loud as comfortably possible, what is the max length of the tube before it becomes inaudible at the receiving end? Note, this must also take into account the amplification/focusing of the dish, so the equation must relate length and diameter somehow. 3. Would rain drops on a part (or all) of the tube interfere with communication? This might depend on the composition, so for right now I'm saying its made of stone, possibly with a metal interior. 4. Would it be all right to narrow the tube in the middle for most of its length? This would save on construction costs, but would it adversely affect sound in any way? I also have a weird idea of choking the tube near the ends, like a rocket engine, but maybe this is just invalid reasoning by analogy. 5. Would it be all right to have some openings or windows on the sides or roofs at the ends? There has to be some way to illuminate the interior and replenish oxygen, especially if using a torch inside. 6. If both people speak at the same time, will the sounds interfere or be changed by the time both messages arrive at the other ends? Hopefully those won't involve too much calculation, but you can just leave me the equations and I'll figure it out on my own. And yes, I searched google and wikipedia but it's not easy when you don't know much about the fundamentals of sound in the first place. Can someone also clear up one more thing? I remember years ago reading that sound diminishes as an inverse cube, but tonight I read it's an inverse square if not reflected by anything (which actually doesn't apply in my sound tube). So now I have no idea. Okay well that should be enough for now. I hope you find this as intriguing as I do. Otherwise, sorry for such a long post. -DrZ.
