The Extra Stimulus Twin Iso 8

By James Lehman
11 Feb 1996

Inside the boxes

This Fig. 4. Inside the box is a view of the bottom of the box, without the cover on it. The reason that the bottom is removable is to accommodate the replacement of the rear mounted woofers - as they would not fit through the holes in the front of the box. There is no other way to get inside the box, because the rear woofers are screwed in from the back of the baffle, the mid bass is in its own enclosure and the midrange and tweeter are in small isolated chambers as well.

The fiber fill has not yet been added to this box, nor has the 1/2" of particle board been laminated onto the outside of the box. You can see the bottom of the two rear mounted woofers, a couple of the 3/4" thick laminated (isobaric chamber) wood layers, the crossover, the port tube (4" id white) and the mid bass tube (6" id green). The plastic tubing used in this project is very heavy grade stuff - 3/8" to 1/2" wall thickness.

Once I am completely satisfied I do not need to get inside the box for a while, I will seal the gaps around the bottom panel with caulk. If needed, I could still get into it with a razor knife and a screw driver.

The baffles
Fig. 5. The baffles and Fig. 6. The baffles with drivers are pictures of the backs of the baffles in progress. The isobaric woofer chambers are made by stacking four layers of 3/4" thick particle board onto the back of a 3/4" thick baffle - making two tunnels the same size as the woofer holes, 3 3/4" deep.

Each layer is a cutout of the same figure-8 shape. The inside of the holes is 7 1/8" in diameter and the outside of the circles is 10" in diameter. The centers of the circles are 9" apart. The same hole pattern is cut into the baffles. They are glued together with wood glue, and clamped to dry. After they have dried, a generous layer of clear siliconised acrylic caulk is smeared all around the inside and outside of the chambers. The surfaces that the woofers mount against (front and back) are scraped smooth with a putty knife before the caulk sets.

In Fig. 5. The baffles, I used some other kind of white caulk, before I found the good clear stuff, which is water washable, will not irritate bare skin, and can be rubbed into the wood by hand.

A small hole must be drilled through the walls of the chambers to feed the wires in to the terminals on the front mounted woofers. The air gaps around the wires leading into the front woofers' terminals must be sealed. I used a silicone liquid gasket compound, used to seal engines - the blue stinky stuff.

I used the same blue stuff to seal the front mounted woofers and mid bass into the baffle. I applied a bead around the holes, and let it cure until it seemed dry to the touch, but still very pliable (one or two hours), before I mounted the drivers, so that they would not be permanently glued into place. The rear mounted woofers did not require any special treatment to make a good seal, because the woofer, itself, has a dense, closed cell foam gasket on the front of its flange.

The seal between the isobaric pairs can be tested by pushing on the cone of the rear woofer to displace it forward. The front woofer should move forward with it, and stay there until the rear woofer is let go.

I used a similar wood stacking technique to make small chambers behind the dome midranges and tweeters. The only differences are that there aren't as many layers of wood thickness and the last layer of wood in the stack has no holes cut in it. Both of these drivers have sealed backs, and require no box behind them at all. The reason that I made small enclosures behind them is that only a small, drilled hole (big enough to feed a twin lead wire through) needs to be sealed to maintain airtight box integrity. (Even a ported box requires air tight properties to work right.) It eliminates the need to goop up the backs of these drivers with caulk, and makes them easy to remove.

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