|Project by ferstler||posted 08-20-2012 12:43 PM||3017 views||16 times favorited||14 comments|
Some time back, I wrote a construction piece on two “cylinder-style” subwoofers that are now installed in my main music room. The article can be found at:
These units work very well, but even after I finished them I swapped out the Dayton Titanic drivers for Dayton Reference drivers. The Titanics were not quite right when it came to handling very low bass (pipe organ) and the Reference versions, although less potent in terms of maximum output, were cleaner down in the 20-25 Hz range. That room is strictly for music, with no home-theater abilities.
The question now was: what would I do with the two Titanic drivers? Solution, make two more subs for use in my home-theater in another part of the house. The Titanics are better at head-banging action-movie sound than much of the competition, and there is no need in there to go below 25 Hz. The result can be seen here, with each unit located in a room corner, flanking three bookcases I built recently out of a mix of redwood and mdf. A piece about the building of the bookcases can be found at:
The redwood was obtained from a friend of mine who had been using it for shelving purchased way back in 1980. He sold me 260 feet, at a buck a foot, and he still has another 600 feet that I may buy into down the line.
Note that the spacing between the two taller bookcases is more than wide enough to handle that 56-inch TV monitor, because down the line I hope to replace it with a 70 incher. The wife will have to agree with this first, of course.
Anyway, the new subs are 57 inches tall (the earlier ones in the music room were 68 inches), with the main functioning body made of a Sonotube concrete post mold 14 inches in diameter and 48 inches long. This adds up to about 4 cubic feet and the 4-inch diameter port tube at the bottom is sized to give the subs tuning at 22-23 Hz. (The bigger subs in the main room, with an interior displacement of 5 cubic feet, tune at about 18 Hz.) While the sizes are different, the main difference is that I used that recycled redwood for the top and bottom sections instead of mdf.
This required more work, because the redwood boards were only 5.5 inches wide, and so I had to join them edge to edge to give me the diameters (16 inches) needed for the visible sections. For the joining work I used Kreg pocket screws, obviously located so that they would not interfere with the need to cut holes in the tops for the woofers and in the bottoms for the long port tubes and electrical binding post clips. The end of each Sonotube contains an mdf “plug” inserted into the opening with the top and bottom redwood caps glued (Elmer’s carpenter glue) and screwed to them. The plugs are secured in place at the tops and bottoms by screws and PL constructive adlhesive. The bottom plates attached to the Sonotubes are held away from the bottom plates by three 2-inch dowel sections with drilled out centers and screwed in place by bolts threaded into t-nuts fitted between the stacked redwood bottom plates. There has to be spacing down there for the downward-aimed port tube to “breathe.”
On top, a thin redwood, driver-protecting plate is held off from the top Sonotube plate by three 2.75-inch dowels also drilled out, with machine screws tapped into t-nuts and with brass cap nuts on the very top. The upward-facing Titanic driver does not have interaction problems with that protective plate, because the spacing is not close enough to matter.
The cylinder interior is covered with .75-inch fiberglass batting (furnace-wrap) held in place with Scotch photo-mount spray-on glue.
Redwood has a natural red look, but to goose that a bit I stained the surfaces with Minwax red-oak stain (applied and removed quickly to keep things from getting too dark), and then the plates were given five coats of Minwax spray lacquer. The first three were high gloss, with the last two satin. Actually, I may go back and give the protective top plate a sixth coat, again with high gloss, which in this case may be better than satin. The plate is easy to remove and reinstall.
Each tube was painted flat black, and each is wrapped in conventional speaker grill cloth material. To cover up the ragged edges of the cloth at the top and bottom, a heavy felt band was wrapped around and secured in back with Gorilla tape. The backs of these subwoofers are not meant to be seen.
Sonotube is actually dense cardboard, 3/8-inch thick, and one would think that it was not particularly strong enough to make a subwoofer enclosure. However, remember that box-shaped subwoofers have walls that are trying to flex into a circular shape when strong bass notes happen. Sonotube is already circular and because it is obviously strong enough to hold a mass of wet concrete it is obviously not going to stretch when put under stress. The bottom plate, with the double redwood and mdf interior plug is 2 inches thick and the top plate is mostly filled up with a woofer driver, and so there is not much flexing going on with them. The result is an enclosure that is surprisingly inert. Companies like Hsu Research and SVS have used Sonotube material successfully in their subwoofer designs for years.
These two units are powered by the same stereo amp that I use to power the subwoofers in the main system: a 350-watt-per-channel Crown XLS1000 that I got from Parts Express for 300 bucks. A Hsu “Optimizer” equalizer adds some electronic bass boost below 35 Hz (subs like this begin to gradually roll off below that frequency), and the sub-EQ section of a Rane THX-44 equalizer is used to flatten some room modes between 90 and 30 Hz. The result is very good acoustic performance.