I built a huge OB sub using 4 drivers. The amp and drivers are from Rythmik but it’s not a servo system. These were non-servo drivers that Brian had on sale. Drivers are 4 ohm and are wired in series-parallel, which wouldn’t work for servo anyway.
Sorry no picture in this post, somehow I can’t figure out the “insert picture” ritual. My design is a long folded baffle, so the drivers line up one behind the other while the baffle zigzags between them. Pairs of drivers move toward each other on-axis, facing front-to-back, to minimize vibration.
I had the idea that a coffee table sub would resolve the placement issues of a small room, because it will always be away from walls. I think that could work pretty well but you need to know a little about the nature of OB sound in the room. There’s a very noticeable difference to sound intensity depending on how near you are to the speaker, and the character of the sound changes as well.
With a coffee table, the listener is very near the speaker and you’ll want to hold down the volume to avoid overwhelming that front-center location. From that sweet spot the sound quality can be tight and deep, but the intensity and depth fade if you’re further away. I also find that bass is heavy and blurred within a few feet of the boundary walls, and it’s almost nonexistent near the center of the room. Also the frequency response will change depending on placement, so you should ideally have some way to EQ, and more than the 1-band on the amp.
My room is L-shaped and I ended up placing the sub upright at the bend of the room, against a wall. This way it faces forward toward one leg of the room, and backward toward the other leg. This probably minimizes dipole cancellation because the front output can escape pretty well from the back. Using a tone generator I get considerable output down to 14 Hz, and an alarming earthquake simulation at 8 Hz (huge cone motion, nothing audible from the speak but my sliding closet doors nearly jumped off their tracks).
Here are some conclusions based on this project:
One: there is a special quality to OB bass and I think the difference has to do with delayed energy from inside a speaker box. The OB bass has a pulsating texture that massages the eardrums, and this texture gets squashed inside a typical box. If you get your setup right it’s pretty great.
Two: people say it’s hard to get deep output because of dipole cancellation. Nonsense, you need to try harder. In hindsight I could have used a driver with half the x-max, or else I need to move into a larger space.
Three: the big 12” woof is surprisingly heavy, and my sub has four of these plus the amp. The weight is ridiculous, and some of you people are talking about hanging things from the ceiling. Don’t do 4 in one box and don’t hang more than 1 from the ceiling.
Four: dual subs (separate boxes) cost more because you need two amps, but the benefit is you can balance the output. Placing one sub at the left wall, the bass output will be lopsided. Note that this is especially true for OB. The ideal placement for one OB sub would be front-center, and that’s probably hard to reconcile with your wall boundaries.
Five: if you want to keep it simple, do a box sub in the corner. Much easier to make that work. Tried and true.
=TJ
