Somehow, I’m not sure how, it has taken me this long to report the news that I am building a giant subwoofer. I ordered the drivers all the way back in September and now I’m excited to finally be making some real progress on the J-Sub mk.III. Although this is the third subwoofer I’ve built, this is the first one that I get to keep, which makes it especially exciting. This time around I tried a few different things on the enclosure and overall I’m feeling very pleased with the results. First off, a few specs: The amp is 300W, class A/B which means it has a very classy THD of merely 0.08% which is unusually great for a sub amp (Keiga KG5230 Subwoofer Amplifier). I elected to mount it on the side of the box so that it’d be easy to reach in case I want to fiddle with the controls. The driver (SB34NRX75-6 datasheet) is a 12″ SB Acoustics, one of the few available which is intended for a sealed box (SB Acoustics SB34NRX756 12” Woofer). I took this approach because my goal here is accuracy and tightness, not loudness or low frequency extension. I’ve had subs before that went loud and low, but I’ve never had one that was super clean and punchy so I decided that’s what I wanted. This makes sense given that I’ll be using it to help master recordings. And there’s also the fact that super low frequencies are incredibly good at punching through any wall imaginable, which means I’d be wary of letting her rip. In college I built my roommate a passive/active sub with Peerless XLS (extra long excursion) drivers and 500W behind it. Our room was on the east side, 1st floor but you could hear that sub on the 4th floor west side… in a concrete cinder block building! That’s just too much.
Sealed boxes always demand a larger size than a ported box. My enclosure is four cubic feet, which is very big in person. I knew there would be a side wide enough to accomodate my 12″ driver, so I elected to proportion the internal dimensions according to the golden ratio–the height is 1.62 times the width, which is 1.62 times the depth of the inside volume. This ratio was famously used in the Egyptian pyramids, The Parthenon, works of art by Da Vinci and Dali, and also naturally occurs in both plants and animals. I’ve always been somewhat fascinated by the golden ratio and how it pops up so many places
For the enclosure I used 3/4″ MDF, which is the ideal material for speakerboxes thanks to its acoustically-dead properties. MDF is basically wood ground up into fine pulp then compressed back into a solid piece with high density and perfect uniformity. The downside is that it’s very heavy and it generates clouds of superfine dust whenever you cut it, which is very annoying and unhealthy to work around. I did all my cutting outdoors with a fan blowing the dust away from me. Even then I held my breath while cutting and walked away before inhaling again. That dust is no joke.
The MDF was then covered with 1/4″ oak veneered plywood. Since it’s plywood, it has a fantastic looking outer layer and then some unknown garbage wood sandwiched inside it. Initially I had felt pessimistic that I would be able to get good-looking edges since I thought using my high speed router would cause chipping and rip-outs on the thin oak layer. Fortunately I was completely wrong about this! Using a scrap piece as a test, I found out that my laminate cutting bit created perfectly flush edges with zero rips in the oak, and my 1/4″ roundover bit actually created a smooth edge with a nice grainy look. This was a very pleasant surprise. My one flaw was not having the 40+ inch clamps it would have taken to clamp the top and bottom laminate pieces. Because I didn’t have enough pressure on them, there is a small 32nd to 16th inch gap at the center where the sides meet the top/bottom. I can live with this.
A few coats of stain and a few coats of poly and she’ll be ready for action!
Tags: DIY, loudspeakers, Music, subwoofer, woodworking
