717 Approximately 7 ft3 internal tuned to 17hz. This design features a pair of CSS APR15's (passive radiators) dual opposing on each side of the cabinet and a single 15" active woofer. The APR15's are fully loaded with 900g of mass ea which comes included with each APR. A passive radiator design is a form of bass reflex, and it operates almost identically to a standard vented alignment. There are some minor differences between the two system type that aren’t really worth mentioning as they don’t make any real difference. The major factors at play here are zero port noise, zero port resonance, smaller overall enclosure, and increased cost. For the majority of sub builders the only time you would really need a PR system is when you intend to tune low in a small box. To most, the benefits do not outweigh the added cost.
There's a bit of a story to how this system came about. My woofer choice is very unconventional, a Power Acoustik Mofo152X. What!? isn’t that some chitzy car audio one note junk you say? Well maybe on the surface. It's actually a well-built decently priced woofer with a very usable parameter set for HT use.
Not to long ago I lived in Alaska, and anyone familiar with that situation knows that shipping prices can get into the realm of absurd. I really wanted to purchase a Dayton DVC385-88 or RSS390HF for this build but for some reason PE wanted to charge me over $100 just to ship ea sub! For the DVC this was almost doubling the price. Luckily for Alaskans Amazon still offers free shipping on almost everything. So, $140 shipped to me for a 15" with 15mm xmax, cast frame, and double stack magnet seemed like a pretty good gamble. The manufactures specs showed it would work well, and while my measured specs are different, these babies are still very much usable as a HT subwoofer.
The APR's just happened to go on sale for $60ea with something like $2 shipping to me a little over a year ago and I jumped on the deal. Making their cost almost equal to the shipping PE tried to charge me for a pair of 15" woofers.
The great thing about subs is cabinet designs can work with a multitude of woofers, and that is true of this design. Almost any decent 15" subwoofer will work in this box. Just fire up your favorite box-modeling program like WinISD, select passive radiator and input the APR15 specs, change the number of PR's to 2, add 900g weight, and change the cabinet to 7 ft3.
Above is the basic cabinet dimensions. Below is my bracing scheme from the top looking down into the sub. This follows the "Bill Fitzmaurice bracing idea" for the most part. I deviate with the spline braces running across the top and bottom, they may not be necessary but I feel better with them. It is essentially two # shaped braces connecting the front/back/sides at four points each, placed above and below the woofer and PR's. The top to bottom brace sits behind the woofer and connects to the + shaped splines. These braces are simply 1.5" wide strips of 3/4" plywood. The point is to take up a minimum of cabinet volume and material while still doing the job. The four # connecting points are actually better than a normal + window frame brace you may see in most builds, as the bracing in the corners of a window frame are not really doing anything.
View looking down before holes cut
View looking in from woofer cutout
I stuffed 3 standard sized poly filled pillows into the cabinet ($2 at walmart). 1 ea above and below the # braces and one behind the top/bottom brace. You should always add some kind of absorbent material into a cabinet in order to dampen any internal noises that may otherwise escape
Here is the nearfield (1") frequency response merged with both PR's. This is raw, unfiltered and unEQ'd. The hump you see at 50hz is inductance related. Virtually every large subwoofer that does not feature a low inductance motor design (demodulation rings for example) will have this same feature to the response. One can see many woofers measured at www.data-bass.com. That hump is easily EQ'd out. You can see the ultra-deep extension down to 15hz before response begins to fall off.
Just for comparisons sake here is the $1000 Reaction Audio PV15X from data-bass measurements. Note the 10db scale. Red is with EQ, Blue is raw. Looks pretty similar right :)
Here is the system impedance verifying the 17hz tune. This was also verified with the cone motion test, which is extremely easy to see on a PR system
This sub definitely gets the job done for both HT and music with prodigious amounts of ULF and plenty of punch. I have a pair of these subs paired with a Crown X3000 and miniDSP. I originally had a pair of NLA O Audio 500 watt plate amps which were a great match. Both of the amps died, I can see why O Audio went out of business...
For anyone interested in the Mofo 152X woofer the measured parameters are below. The blue cone X is disgusting looking, but I took care of that (Pic below).