After a bit of deliberation, I decided that I would tread in some new territory and try to design a passive radiator system. I've gotten into a habit of building sealed, ported, ported, sealed speakers and just for the thrills, a sealed sub. It's getting old and not much of a challenge. I did some research on the web and figured out in which direction I would venture. The passive radiator idea is basically a ported enclosure, only it trades a column of air (the port) for an equally massive diaphragm. Simple enough. But where to start? I did some further research on the web and found a spreadsheet that took simple info about the system and the parameters of the radiator. In a millisecond you have a starting point for determining how much mass to add to the radiators. I also found the equations for figuring out the mass requirements at another website-http://www. diysubwoofers.org/prd/-which confirmed my earlier results.

Then came a three hour session with WinISD. If you don't have this program in your arsenal yet, shame on you! Get it and you will never regret it. I plugged and chugged the numbers and did a lot of tweaking to find a response curve I could live with. It's a good thing the sub I had been modeling liked it too, or else there were going to be issues. It has an F3 of around 40 Hz and a F10 around 31 Hz. The rolloff is steep but it all works out well in the end.

The amount of added mass turned out to be 46 to 50 grams per radiator. The passives have an M5 threaded hole to adjust mass by way of a bolt, nuts, and washers. Just make sure you add weight in this range. It's the best compromise between system power handling and frequency response I have found so far. Once you are satisfied with the mass tuning, grab a bottle of Loctite and secure the threads. This is a vibration rich environment, and without threadlocker the whole package will eventually come loose and start buzzing like mad.

The gross volume I ended up with is .88 cubic feet. By the time you figure in the volume displaced by the active driver and the passive radiators, I estimate the net volume somewhere in the vicinity of .80 cubic feet. Not overly compact, but not huge by any means. I like to have something to prop up my feet under my desk so this might be just the ticket. The 13" cube dimension came as a bit of a surprise when I was figuring out the cut list. I am hoping that if someone replicates this sub they aren't afraid of the number 13. Ah, sounds like I'm onto something here! A person who is afraid of the number 13 is a triskaidekaphobic. A bit of a mouthful so I shortened it to Triska. A name is born!