SpeakerBuilding.com - The RS15-ISO Tube

The RS15-ISO Tube

By Brian L. Mills
04 May 1998

Printed from SpeakerBuilding.com, 04 Feb 2012 08:46
URL: http:///content/diy/1029/

Goal

My goal was to design and build a subwoofer for my home theater system while using a pair of Radio Shack (model RS40-1301) speakers. I realize that these aren't the best speakers to use for anything, but I had them laying around. I wanted to keep the enclosure size small, about 3-4 cubic feet and I wanted to use a cylindrical design. More on that later. Also, I wanted a good transient response for the high impact during home theater subwoofer special effects as well as rich bass notes for the wide range of music I listen to. Most of all, I wanted a smooth sound that wasn't real boomy but would shake the floor during the Jurassic Park T-rex scene.

Design Process

In order to design these speakers, I used several sources, including the SpeakerBuilding Page, the Subwoofer DIY Page, The Audio Corner and the Speaker Building Net. JL Audio is another great site filled with informative tutorials about the different box types and which type of speaker mates well with certain box types. From reading other's projects and sampling several different types of loudspeaker design software (BlauBox or BoxPlot) I got a feel for the type of speaker to build and the general size I would need for these speakers. I chose an clam-shell isobarik configuration in a sealed enclosure. This gave me the smallest box size, since size was a major stumbling block. Still, in order to get a box small enough, I used a Qtc of 0.997 and box size of 4.06 cubic feet. This gave a fairly flat response with a ripple of 0.69 dB, f3 of 34Hz and an effeciency of 92 dB Fig. 1. BOXPLOT response.

This was low enough to feel most subwoofer special effects, eventhough I would be missing the lowest octave of pedal tones in pipe organ music. Next I needed to decide what the box shape should be. Many box types having rectangular sides have trouble with wall resonances and require bracing. In order to reduce the amount of wall vibration, I took my design from a combination two existing designs. Hsu Research makes a subwoofer using a ported cylinder enclosure. Since the cross-section is a circle, it's very difficult to create vibrations due to pressure from the speaker. Russ Button also used this design, but improved the concept by increasing the weight of the enclosure. Since the original design was too light and wasn't stiff enough, Russ doubled the thickness of the cap and baffle to increase the stiffness and decrease the end vibrations. I then used both Ray's ideas and the concept from Hsu Research to design the RS15-ISO TUBE.

Fig. 2. Cabinet drawing Fig. 3. Cross-sectional 3D-model

Construction

Construction for this speaker was really simple. After cutting out the cap, support, baffle and base using a jig on a router table, the assembly went very smooth with no complications. The hardest part was cutting near perfect circles out of particle board. I chose not to use MDF for cost reasons, but instead elected to use 5/8" particle board laminated together to form a thickness of 1 1/4". The tube is a piece of 20" DIA. Sono-Tube that I bought from a local concrete supply store. The legs are 1" dowel rod. Most of the work was spent making sure that the diameter of the circles were very close to the internal diameter of the tube so that there was very little gap during assembly. I would guess that there is less that 1/16" eccentricity and concentricity. To support the legs between the baffle and the base, the bottom layer of the baffle and the top layer of the base were drilled with 1" holes. This was done before the baffle and base were laminated together so that the legs could be sunk into the baffle and base to offer more support. The image below shows the order of assembly. We first started with the baffle SUPPORT. It was located into the correct position then glued and screwed in place from the outside of the tube. The heads of the screws were countersunk everywhere we used screws. The support was then sealed with silicone sealant at the edge on the inside of the enclosure, and not the side that would meet the baffle. Next the CAP was put on. On the inside face of the cap the -18dB passive cross-overs were mounted so that they would be out of the way. The cap was also sealed onto the tube from the inside to make sure there weren't any air leaks. Next, the speakers were mounted to the BAFFLE. The speakers are held in place by 8 bolts that go through the baffle and both speakers. I didn't seal the speakers into the baffle because I felt that I had a good air-tight seal. I filled the enclosure about 75% full of dacron pillow stuffing between quilt batting to help increase the apparant volume. The baffle and speakers were then assembled into the tube. The baffle was pre-drilled with mating holes so that the baffle would screw into the support. If I ever needed to take the speakers out, I could. Next, the LEGS were glued into the base, but not into the baffle. The enclosure was heavy enough that I was certain that the tube wouldn't bounce on the legs. This allowed me to get the speakers back out from the baffle if ever I blew a speaker. The last step was to place the base on the floor and set the tube on top, making sure the line up the legs with the holes in the baffle.

Finishing Touches

The last thing to do was to make the sub look inconspicuous and functional as a tall end table. I bought a 3/4" thick by 24" diameter piece of oak plywood and veneered the edge to cover the laminates. I stained the oak table top to match the rest of our livingroom furniture and finished it with a light coat of polyurethene. Masking the speakers, I first used a sanding sealer on the tube, legs and base to fill up any pores. After a light sanding, I spray painted the tube a hunter green color and the legs and base black. This was the primer for the final painting. My wife found a fleck paint that made things look like stone. This was the final coat on the tube (in green) and on the legs and base (in black) for a finished look. Finally, the oak table top was glued to the exposed top of the cap to complete the look.

I haven't yet measured the actual response of the sub, that's still to come. But so far, I'm very happy with the actual sound. It's tight and full of deep bass with little coloration, exactly what I was missing to complete my home theater system.

A view of the final product Fig. 4. Picture of the sub.

Brian L. Mills

Figures

Fig. 1. BOXPLOT response

Fig. 2. Cabinet drawing

Fig. 3. Cross-sectional 3D-model

Fig. 4. Picture of the sub