I tried doing a search for this on the web, but couldn’t find anything concrete. You know when you’re in elementary school, and you make those little “telephone” things out of string and some plastic cups? Do those really work, or is it just something that works when you’re 5, and later magically stops? My sister and I were talking last night, which is a pretty rare thing cuz I’m never home and she’s never awake when I am, and we decided it’d be cool if we could talk our wall, because we both wanted to lay in bed, but neither of us wanted to go to sleep. Our rooms are seperated only by a thin wall, so we figured we could just drill a hole through it, thread a string through, attach some cups, and make a telephone out of it. This made a lot of sense last night at 4 am, but would it really work? And if it does, what kind of string do you use? Course, if it doesn’t work, then our next step is to just cut a hole out, so we can hear better without getting out of bed 
Guess you’ll just have to try it, but I believe it will work on a couple of conditions:
- That the string is pulled taut;
- That the hole in the wall is large enough so that the vibrations of the string are not excessively dampened by the drywall and insulation of the wall.
Of, course, you could just go to Radio Shack and get yourselves a couple of cheap walkie-talkies.
Well, what is happening with the ‘tin-can on a string phone’ (or in your case, plastic cup) is that the string is serving as the medium for the soundwave, not the air. Sound waves work by compressing the surronding medium around them, normally air, but string could be used. (This is why there is no sound in a vacuum.) Since the string is more dense than the air, the compression waves work better and degrade less, in essense requiring a smaller amplitude to send the wave over the same amount of distance. The smaller amplitude translates to you speaking softer. The cups serve as conduits that make sure the wave goes into the string, and then back out into the air.
So in theory, yes it works. I have never tried it, though. I did see a demonstration in my physics class the other day with something similar, it was two cones and a compression spring, but it accomplished the same effect. It’s just that the sping works even better than the string since it is designed to be compressed.
I hope I got that right, a doper with more knowledge in physics might want to check my work, though.
I’ve done plastic cups and paper cups with string as a kid. They work – you just have to remember to keep the string taut!
But neither of these is as impressive as:
Metal Cans strung together with Wire
If you’ve only tried plastic or paper cups with string you will be blown away by this. The fidelity is awesome. You loop the wire around nails or bolts inside the cans, and these pass through holes in the bases. Because it’s metal, you can pull it a lot tighter, and the wire transmits vibrations much more faithfully than string. It’s more trouble, but definitely worth while.
Once you’ve tried such a True Tin Can Telephone, you can appreciate that they actually used such things as commercial devices. I once saw a commercial “Tin-Can” Type “Telephone” on display at the Armory in Boston.
I second JonScribe’s idea of walkie-talkies. Cheap, portable, useful in other places, and you won’t have to put a hole in the wall.
The principle behind all of this is sound, but a string makes a pretty poor conductor of sound waves. The flexibility of the string, even if taut, doesn’t allow it to conduct the vibrations well. By the time you get far enough apart that you can’t hear the other person well, the vibrations will be pretty well damped out. A gedanken experiment to illustrate this: Imagine a long rope strung out across the ground. You grab one end, and shake it. I little shake causes a hump of rope to go across the ground a short ways. A big one will send it further. Betcha can’t send a hump more than 50-75 feet though. The same thing happens with the sound waves in the string. They just peter out pretty quick.
Obligatory Hijack: Back in college, I was a dispatcher at the campus police station. Next door was a McDonalds. One month their promotion o’the month was for the cookies. On the back of the box was instructions on how to make a tin can telephone using 2 cookie boxes and a string. I made one up, threw it in the bottom desk drawer, and labelled the drawer “Emergency Communications Equipment”.
Yes, but as Cecil wrote, it is important not to use them in conjunction with certain Yoga exercises.
That would be vulgar, & we wouldn’t want that, now would we? 