The Extra Stimulus Twin Iso 8

By James Lehman
11 Feb 1996

Introduction

The "Extra Stimulus Twin Iso 8" Fig. 1. Picture of the speaker is a four way ported system with four 8" woofers in dual isobaric pairs Fig. 2. Drawing of the frontplate. All of the drivers in this system came from MCM Electronics (1-800-543-4330). The woofers are 8" poly with butyl surrounds (MCM 55-1190). The mid bass is a 5" poly / paper open back (MCM 54-310). The midrange is a 3" poly dome (MCM 54-140). The tweeter is a 1" titanium dome (MCM 53-410). The midrange listed here is not pictured in the current MCM catalog 36, but is still available.

I would estimate the total cost of the stereo pair, all materials included, to be less than $600, and an awful lot of work.

The major concept of this design was the idea of putting four woofers in an actual mechanical, and electrical, series parallel arrangement. The benefits of this design are that 8 ohms impedance is maintained. The box volume required for all four of these woofers is the same as that required by just one of them. The power rating of the woofer system is quadrupled and the effective cone radiating area is doubled.

If the MCM 55-1190 (woofer) is put in a box of the right volume with a properly tuned port, it can respond down to about -3 dB at 19 Hz. This doesn't mean a whole lot coming from such a small cone, but if you double the cone area, it becomes quite a bit more effective. Putting four woofers in a dual isobaric arrangement also offers the benefit of averaging any variations in each of the woofers rated specifications.

During the design phase of this project, I had the added advantage of having some friends at Audio Technica. I was able to take the baffles into AT, with the four woofers mounted on them, and a small test box, and test them as one working woofer unit, to determine their actual T/S parameters. Before I took them in to be tested, I exercised them with about 18 volts rms of sine wave at around 30 Hz for 3 1/2 hours or so. The results that I got were a bit different than the parameters listed (for just one of these woofers) in the MCM catalog. I would assume that the difference is a result of the mass of air inside the chambers being added to the mass of the cones. With just a slight adjustment to the port length, I was able to get the low end response curve that I was looking for.

The picture of the response curve Fig. 3. The response curve, is a mathematically derived prediction, and not an actual measurement.

The estimated effective cabinet volume is 5.55 cu. ft.. The inside dimensions would indicate a volume of 7.02 cu. ft. (17.5" W x 36.5" H x 19" D), but estimated volumes of all of the stuff inside the box were subtracted from this figure. I used 5.55 cu. ft. as a good guess, and a 10" long, 4" id port to tune the box to 21.75 Hz. I figured the response curve for a box 20% larger, and 20% smaller, and it had little averse effect on the curve.

The boxes were made of 3/4" thick particle board with dado-rabbit joints in the corners. A second layer of 1/2" thick particle board was laminated onto the outside of this box - making the total thickness of the box walls 1 1/4". The woofers, mounted on the fronts of the baffles, are mounted onto the surface of the 3/4" thick inner box. Holes, big enough to fit around the woofers' outer flanges, were cut into the 1/2" thick front lamination before it was applied. A layer of wood veneer, formica, paint, or some other covering may be applied. I'd have to take a wild guess and say they weigh about 170 lbs. each, or more.

The mid bass is isolated from the woofer enclosure by placing a 6" id plastic tube 19" long between the back of the baffle (behind the mid bass) and the back of the box. It was glued in with epoxy and sealed with siliconised acrylic caulk. A small hole was drilled in the side of it, the wires were fed through and sealed, and then it was stuffed with poly fiber fill.

About the
author

Print this
article

Email this
article