The Extra Stimulus Twin Iso 8
[Drawing of the front layout]
[Measurements of the front layout]
The major concept of this design was the idea of putting four woofers in an actual mechanical, and electrical, series - parallel arrangement.
Electrically wiring four similar woofers together in series - parallel has the effect of doubling, and halving various electrical parameters; ie, the DC resistance, impedance, inductance, etc. The effect of the series connection is canceled by the parallel connection, and the resultant electrical parameters are exactly the same as that of just one of the four woofers.
Similarly, putting two of the same type of woofers together in an isobaric pair, is the same as physically coupling the cones and motors in series. By doing this, the Vas (acoustic compliance; a value used to determine box volume) is cut in half. Putting two identical isobaric pairs, side by side, in the same baffle (and box) is the same as putting them physically in parallel. This doubles the effective Vas for the woofer system, which brings us right back to where we started.
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 thermal power rating of the woofer system is quadrupled and the effective cone radiating area is doubled.
If the MCM 55-1190 (8" 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.
Both cones (in each isobaric pair) are coupled by the smallest possible (least compliant) volume of air between them. This provides a constant pressure (isobaric) support for the entire surface of each of the four woofer's cones, which greatly reduces cone ripple, and the distortion, and loss of efficiency associated with it.
Putting four woofers in a dual isobaric arrangement also offers the benefit of averaging any variations in each of the woofers rated specifications.
All of the drivers in this system came from MCM Electronics (U.S.A.) (1-800-543-4330).
The woofers are four 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 no longer available from MCM.
It is distributed by MG Electronics
Model DM-250 6" Polymer Dome Mid-Range Squawker.
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 in to 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 various frequencies between 20 and 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. The parameters that I got for each of the left and right woofer sets were very close to each other. 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, included with this design description, 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 200 lbs. each.
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.
The overall sound of this system is remarkable. There is no doubt that they really do put out at 20 Hz. There is very little noticeable intermodulation distortion, because of the separation of the audio spectrum into 4 nearly equal bandwidths. The average, slow response SPL reading in a room is somewhere around 106 to 110 dB at about 100 watts - very clean. The only other time I've heard sound as good as these is in very good movie theaters. The average music program, on a compact disc, doesn't really even begin to challenge these speakers. A good, super dynamic, movie soundtrack, from a laser disc, is a much better test of their outstanding performance. I can't really comment on how much power they will handle, because my amp is only 150 watts / ch. and it breaks up way before these speakers do - not good for my tweeters!
I would look long and hard to find the very same midrange dome. Even though it's similar to others, it has a very high efficiency. I would be pressed to find a better woofer too. The MCM 55-1190 is a great deal for the price, And it just happens to have the right specs to make this system work. I think any good 8 ohm tweeter would work, and I might look for something different. The mid-bass is something I would definitely change. The five inch cone that I got has about the same effective radiating area as the three inch dome above it. It works a bit too hard. I would use an eight inch carbon fiber woofer with a low Qts and Vas for in a small, well damped subenclosure instead.