This was inspired by the "Ultimate Subwoofer" proposed by Toby. Props to him. I was going to annotate this but it's different enough to warrant its own page.
As mentioned before, the main purpose of any speaker, especially subwoofers, is to move large amounts of air. A typical speaker does this by moving back and forth, alternately pushing air forward, and then moving back to create a vacuum.
There is another technology that has been in use for years for moving things back and forth. It's called a pneumatic ram. Pneumatic rams are capable of moving at extremely high speed with extremely high power, so: use one (or an array) to make a massive subwoofer.
Basically, make a tube with a sort of piston in it, which can move back and forth. Attach one or more pneumatic rams to this piston (on one or both sides depending on what sort of response is desired), which are hooked up to a computer-controlled compressed air supply. In response to the sound frequencies being fed into it, the pneumatic rams will move the piston back and forth, which will push a large amount of air back and forth in the tube. Hook this up to a cone to amplify the air movement, and you've got yourself a very large, very powerful subwoofer capable of frequency response down to 0 Hz. Upper frequency limit would be a product of the power of the rams, the mass of the moving parts, and the precision of the controlled air supply. A multiple of smaller rams on both sides of the piston would probably work better than a single large one.
The main problem is the compressed air supply for the rams--this would have to be supplied by and active compressor, which would have to be soundproofed as they tend to be rather noisy. But this is perfectly possible to do, and only the most hardcore of people would use this system for anything less than a theatre sound system.-- 5th Earth, Apr 09 2004 Sounds like it would work. I used to work maintenance in a place that used compressed air rams and if you built this you would be getting yourself into a whole heap of up-keep.-- 2 fries shy of a happy meal, Apr 09 2004 Hmm, I would have to challenge the claim that this could possibly go down to zero hertz. You still have the problem of lower bound frequency being limited by the distance of travel by the piston. That distance can not be made arbitrarily large practically, so the frequency produced can not be arbitrarily low.
All the same, this looks workable for producing some appreciably low frequencies. It would likely find its niche in very large scale sound systems where output power is the critical factor and a cone based subwoofer simply could not deliver the needed power.-- BigBrother, Apr 12 2004 I have trouble believing a pneumatic ram would exceed a few dozen Hertz regardless of the throw.-- phoenix, Apr 12 2004 that's why it's called a sub-woofer.-- my-nep, Apr 12 2004 Well, yeah, but how low can you hear?-- phoenix, Apr 12 2004 Not as low as you can feel...-- Mr Burns, Apr 12 2004 I can whistle at zero hertz.-- Worldgineer, Apr 12 2004 I can't feel zero hurts.-- FarmerJohn, Apr 12 2004 Eons ago, our band used to have a set of Martin bass bins that worked this way.-- waugsqueke, Apr 12 2004 I'll take 2 big ones and a little one- they'd go great in my video room(+++)-- croissantz, Aug 11 2004 random, halfbakery