And simulations:
#1. As designed above you have at least 113L or 4 cubic feet with 15% fill. If you line the walls of the cabinet with 1-2" fiberglass that will do the trick. This is plenty large and as you will see later, we can push the design smaller with some changes with minimal tradeoffs. In all of these situations (except the last), I'm modeling the design with the BASH 500W amp and the subsonic filter set to 18.7Hz and a Q = 1.0. The resistors on the BASH signal processing board need to be R17 = 20K, R18 = 75K.
Also note I'm only modeling these with a single PR-15. You technically don't need two of them for this design. One of them has sufficient stroke to handle the job. The advantage of running two is that you get a little less cabinet vibration. All of these simulations are with a full 500W driving the system. The SPL in Room & Cabinet window shows an estimation of the in-room response. None of these simulation show power, BL or Cms compression so at maximum output, you will get a little less total output than shown in the simulation.
Here we go a little smaller, 100L or about 3 cubic feet. This is smaller than the cabinet shown in the drawing. I'm showing it as a reference for people who want to trim down some of these dimensions. Notice that we don't loose headroom up high, just a couple Hz extension as the tuning frequency is a little higher and the box smaller. This is using all the same parameters and the same power as above including the 18.7 Hz subsonic filter with a Q = 1.0.
Here is 85L or 3 cubic feet. Once again, all the same parameters but I've added some mass to the PR to keep the tuning frequency low.
And going even smaller, 70L or 2.5 cubic feet. I've added yet more mass to keep the tuning frequency at 20Hz but in the smaller box the system becomes less and less efficient @ low frequencies as you would expect. This is with the same subsonic filter as before, 18.7Hz corner and a Q = 1.0.
Here we have the same cabinet as above (2.5 cubic feet) but I've changed the subsonic filter. We use the same corner frequency, 18.7Hz but we change the Q to 1.3, essentially using the subsonic filter to provide some boost down on the low-end. The resistors on the BASH 500W amp are R17 = 15K, R18 = 100K.
You might be looking at these and say "Hey... that looks just like the 4 cubic foot cabinet!" and you would be right. It isn't free though. We use the higher Q of the subsonic filter, effectively increasing the gain (hence power) at low frequency. The amp will clip sooner than it would for the larger box given a low-frequency test signal. In real-life use though, you will find this works great. The trade-off is smaller box size for less overall headroom at really low frequencies. A good trade for most people and for normal music or reasonable HT playback, you get the same in-room response as we did with the much bigger box.
That is it for now. Go build them! These are great little subs for an unbeatable price.
Shiva-X & Single PR-15" = $265
Shiva-X & Two PR-15"s = $300
Parts Express has the BASH 500W amp on sale for $185 with free shipping. For less than $500 you can make a sub that substantially outperforms anything you could buy in this price range.
Kevin Haskins
Exodus Audio
