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The RS15-ISO Tube

By Brian L. Mills
04 May 1998

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

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