Passive radiators are beginning to get much more attention in the home theater world, especially recently, than they have in the past.  I believe this is due to that more companies are designing and building better and more versatile PR's for DIY use.  Yet there is always one stipulation that tends to veer people away from PR designs, the cost of a PR.  Assuming a PR enclosure's response resembles that of a ported enclosure, we're looking at difference of $5 to nearly $100.  And if dual PR's are in your plans, you could be spending well over $200 for a mere "radiating diaphragm" that does pretty much same thing as a $5 port.  Now I won't get into whether or not PR's sound better or play louder and deeper than ports.   Everybody has their own theories on the matter and regardless of what you may tell them, they'll hold true that their PR sub sounds better than your ported sub.  As far as what I have read and understood, PR's have just a few (but important) advantages over ports.  One being that they allow a small sized enclosure to be tuned really low (near and below 20 Hz) without wasting up precious internal volume.  Also due to the high compliance of the PR, they help prevent total driver unloading below Fb.  IOW, you won't need any type of subsonic filter to prevent overdriving your sub from frequencies that are too loud and too low.  The PR design resembles a sealed enclosure in this sense.  PR's, if designed and used properly, exhibit much less turbulent noise than ports.  You don't get pipe resonances or "moving air"  noise that is common with ports, especially small diameter ports.  I'll be creating a page comparing Ports vs. PR's as soon as I get more personal data to back up my thinking.  I've got a bunch of PR projects I plan on putting together over the summer.  In the meantime I've recently learned of a radical technique that may take out one of the impeding factors from the equation of building you own PR system.  The cost.

To Cantilever or Not to Cantilever 

Below you will see 4 animations of PR's.  These PR's I am dealing here with are plain flat slabs of wood with surrounds.  They have no basket, no spider, no mounting gasket, no nothing.  I'm not sure if it is possible to buy these types of PR's commercially just yet.  The companies that make the full-blown PR's make these diaphragms, but whether or not they sell them separately, I don't know.  They are cheap though.  They basically are just wood and surround.  You gotta build the rest.  It has been rumored that by using a surround repair kit and a flat piece of wood, it might be possible to fabricate this type of PR.  I don't disagree.  Although I have not actually built one from scratch like that, I think it could be done.  The one major problem with this type of PR vs. the full-blown versions, is a problem called cantilevering.  That's where one side of the PR moves out while the other moves in.   It basically makes the PR useless in that it stops doing its job properly.  The air pressure on either side of the PR begin to cancel each other out, which we will just say is bad.   So the trick is to use a flat PR and modify it so that is won't cantilever.  One way that might help prevent cantilevering is to use TWO flat PR's with TWO surrounds that are inverted and back to back from each other.  I have modeled this in the animations below.  It would appear as though this idea will work.  I have not built a prototype but intend to very soon.  I have at home at least fourteen 12" flat PR's with large surrounds capable of at least 30mm peak to peak excursion.  Yet the useable excursion is actually around 15 mm before the diaphragm cantilevers out of control.  I have been contemplating putting 3 of these into an enclosure with my Shiva, but I knew the cantilevering would cause huge problems.  This new Dual Surround idea may solve that problem or at least reduce it.  I'll keep you posted of my progress.

For the record, this page has been up since the beginning of 2000 and was written without any prior knowledge of Earthquake having developed a similar passive radiator which uses this technique.  The information herein has no intentional references to Earthquake whatsoever and any similarities are simply coincidental.