I copied my posts from the old forum on this design, in case it disappears:
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OK, here's the idea. I have a nasty cold right now so I'm not up to making a drawing and finding a place to post it - I'll take the easy way out with an ASCII diagram.
Basic idea is two panels about 2 ft wide and 4 ft tall - these are often available pre-cut at lumberyards. (Or buy a full sheet and heve them quarter it!) Then get some boards about like 2x4s. This is the hard part, finding dimensionally stable boards that have flat finished surfaces and a precise width. You'd like them to be around 3.5-4" wide, and at least 1" thick. You cut to size and glue them on edge in this pattern to one of the plywood sheets:
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(port) | (port)
------- | -------
| | |
| | |
| | |
| | |
| | |
| WOOF | WOOF |
| | |
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TWEET
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| | |
| WOOF | WOOF |
| | |
| | |
| | |
| | |
| | |
------- | -------
(port) | (port)
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Cut holes for the woofers and tweeter in the other sheet and glue it on top like a sandwitch.
The "tweeter" is now installed in a short transmission line - actually a dual line, both sides being about 12" long. Stuff it with Acousti-Stuff or long-fiber wool or whatever you like. George Augsberger has published design methods for these, both in Audio Xpress and in the AES Journal, which will help choose the area of the line and the stuffing density.
Each of the four woofers has its own chamber, with a large and long port (gives more unconstricted bass). The port dimensions depend on the chamber volume, which in turn depends on what you can find for the 2x4s. "Boxplot" and other speaker design freeware can work it out. If you will be using an SET, be sure to increase the QT of the speaker by 30% to account for the low damping factor. Line the area behind the woofers with a good absorptive material, like 1" acoustical fiberglas - but keep it a couple inches away from the port. You may not need to line the edges or the front. Wire the woofers in series-parallel to maintain the 8 ohms impedance.
A first order crossover at about 800Hz will work well, because the tweeter is a fullrange with plenty of excursion, and the drivers are very close together in terms of wavelength. Plus, the drivers are flat for a couple octaves on each side of the crossover so you don't have to account for phase and amplitude anomalies. In some back issue of VALVE I published an adjustable passive line-level crossover which would probably work pretty well if you want to biamplify; I would thing the tweeter amp should be about the same power as the woofer amp, or at least half that much power. I'd bring out the woofer array and tweeter wires and use an outboard crossover, to allow for bi-amping at a later date.
When I find my notes (they are in a file somewhere in the office here...) I'll try to fill in some blanks about the chamber tuning and crossover details.
For refinement, you might want to put a "belt" of felt across the middle section, probably 1/4 to 1/2" thick - go full width, and at least 12" tall. Wouldn't hurt to do the whole surface, but that might get costly. I always planned to search out some grille cloth the same color as the living room walls and cover the whole thing so it sort of disappears.
