Sealed Sonosub

The sound in our basement theatre needed better bass. I decided to make a sealed subwoofer, largely due to considerations of size and suitability for both music and films. After some thought about shape and size versus ease of build I settled on a sonotube-based design, the end result looks like this:

This speaker is based on the NHT1259 12" driver and is fed by a 300W Keiga plate amp. (model KG-5320). The box is made from a piece of 24" diameter concrete-column forming cardboard, often referred to as "sonotube". It is veneered with red oak and has five legs of 3" diameter cherry dowel. The top is oak-faced plywood and is removable so that alternate tops can be used depending on furniture layout.

There are several designs for sonotube-based subwoofers on the web. Most are the vented type, in fact this is the only sealed unit that I know of. I aimed for an internal volume of 3.5 cubic feet so as to match some of the characteristics suggested for the NHT1259 in sealed-box subwoofer designs. Given a 24" internal diameter for the tube, this meant a piece about 15" long (some internal volume is lost to bracing, top/bottom faceplates, stuffing and driver volume. With legs about 5" long, this makes an ideal height for an end table. The tube itself comes in 12' lengths from concrete supply contractors. Some places apparently sell off-cuts but I wasn't so lucky. I started making two identical units just in case something went awry.

The top and bottom caps are made from a double layer of 3/4" MDF. The layers are glued together with subfloor cement and further fastened with countersunk screws. With the end caps in place the cylinders are remarkably strong, so minimal internal bracing is needed, but five 2x2 struts were glued and screwed in place as shown below left:

Once the struts were in place the top cap was fitted (above centre) and a hole for the speaker was cut (above right). A smaller hole for speaker terminals was also cut at this time. Following a recipe found on the web I caked the inside of the cylinder with black automotive undercoat paint. This and liberal amounts of subfloor caulk seem to have sealed the joints pretty well.

The inside is stuffed with a couple of pounds of "acoustistuff" (below left). Two pounds may not sound like much but when it's all fluffed up it is a lot of fluff! The speaker driver is held down by eight clips that are bolted completely through the full double thickness of MDF.

I decided to keep the amplifier separate from the speaker and housed in the cabinets along with the rest of the electronics in our basement. That way there is only speaker wire running to the unit and no need for a nearby electrical outlet. With the stuffing in place and the driver firmly fastened down, the speaker looks like this:

This subwoofer was in use for about a year before I finally got round to making a top for it. The top is pretty simple - it is made from a double layer of 3/4" oak-faced plywood, as shown below left. It is squarish in shape with rounded corners. One of the corners is much less sharply curved, so as to fit in with the furniture layout that we had when I built it. That layout has changed, so there is some fine-tuning to do to the top. Still, it sounds and looks pretty good so I'm relunctant to mess with it for a while.

The stuff on this page is intended to be informative rather than impressive.
If you find it useful or have suggestions for improvements, please let me know.
All words, pictures and original concepts are © Dr. Sean Parkin (2006/12/21)