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Chief narrators: Pete Riggle & Stephæn

The Monster in the Attic

An Introduction by Pete Riggle:
Hello again dear reader. Good friend and partner in audio misdemeanors Stephæn has warned me that once again we are going to press—so to speak—with another article about our audio high jinks and shenanigans. This time we are talking about a fiendish horn subwoofer that we installed in the loft of our audio playhouse otherwise known as the Garden of Earthly Delights (GOED). As you may recall, our first major audio transgression was a pair of loudspeakers we built around the Great Plains Audio 604 8H-II coaxial drivers derived from Bill Hanuschak’s GPA Altec 604 series of coaxial speakers. These were reported on as Stephæn’s Dream Speakers

Next we did a pair of hefty horn loudspeakers greatly influenced by Steve Schell and Rich Drysdale of Cogent True to Life loudspeakers fame and by Bill Woods of Acoustic Horn. These were reported on in Part I of this series. We called these loudspeakers the Po’ BoyZ. Given all that, the next commonsensical element in the pursuit of Hornographic Ecstasy had to be a subwoofer horn to get the bottom two octaves not covered by our earlier preceding endeavor. Just as a fun aside, check out the image of the aluminum Institute of Hornographic Research plaque lovingly machined for us by Kara Chaffee of deHavilland Electric Amplifier Company. 

Stephæn received a matching plaque for his birthday this year. Not sure where his will be mounted yet but mine graces the entry to the GOED. Kara did these with the same CNC milling machine that she uses to produce the chassis components for her various vacuum tube amplifier, line stage and tape preamplifier products. While I’m standing on this soapbox, I want to put in a pitch for Kara’s products. These are some of the best amplifiers and preamplifiers ever produced. We are talking about both sonic grace and robustness of construction. And we can stop complaining about all that stuff made in China.  Kara’s amps are made in the USA. Just give them a listen and the buy-American among you will not be tempted to look further. Kara’s new 50A power amps inspired by the legendary Fisher 50 A monoblocks have, to my ears, improved every system in which they have been tried. I’ve heard them in quite a few systems now so allow me to let the cat out of the bag. If you ever wanted a tape system but noticed how tapes can be a little screechy—not to mention expensive—you’ll want to consider Kara’s new tape playback preamplifier. You can hook it up to a $100 used Teac tape transport to get a tape system which is sonically smooth as silk and affordable.

Okay, back to the horny monster in the loft. For a lifetime I had lived with various conciliatory means of deep bass reproduction and never had been fulfilled. It is really difficult for a 10- or 12-inch direct radiator speaker or even several of them working in harness to light up a room with nimble, tuneful and authoritative bass. After getting the Po’ BoyZ operating, Stephæn and I began to think about the fact that there was a loft at the end of (and above) the GOED. This loft spans the 25-foot wide end of the listening space and is 12 feet deep from front to rear. The floor of the loft is about 9 feet above the floor of the GOED. At the front of the loft the ceiling is about 7 feet above the loft floor. At the rear of the loft the ceiling is about 4 feet above the loft floor. Can you say potential? The loft was mostly storing empty boxes and such. What if? What if! So I began doing the calculations.

Design: If you want to do horn calculations, read Bruce Edgar’s "Show Horn" article from the old Speaker Builder magazine. You can find a copy online. For a bass horn you will want to do a pure exponential horn, the simplest member of the hyperbolic exponential family. A fundamental term in the calculations for the geometry of exponential or hyperbolic exponential horns is a parameter called the flare frequency. Simply put, the flare frequency is the frequency at which horn theory predicts the horn’s output falling to zero. A generalized set of horn response curves shown in Bruce’s article give information that leads to the conclusion that the horn will be down 3dB at a frequency that is one third higher than the flare frequency. Originally I intended 30Hz as the 3dB down point. I was concerned about the horn getting too long and having excessive delay. Steve Schell of Cogent loudspeakers and the Lansing Heritage site dissuaded me from 30Hz as the half power frequency, arguing for 20Hz instead. I took Steve’s guidance and have not been disenchanted by the results. So 15Hz was selected for the flare frequency.

One of the parameters important in designing a bass horn is how much of the four pi steradians of space around the horn mouth will load the horn. If the horn mouth rests 1000 feet above the Bonneville Salt Flats, it is safe to assume it is loading all of the space around it. If the horn mouth rests on the Bonneville Salt Flats, it loads only the upper half space. If a horn is at the intersection of a wall and a floor, it loads one fourth of the total space around it. If the horn is at the intersection of two walls and a floor, it loads one eighth of the total space around it. Horn theory tells us how long the horn has to be and how much area the mouth must have to load all of the space around it. If we plan to place the horn in the center of the floor, we need only half the mouth area of a full-space horn. At a wall intersection we need only one fourth, in a corner just one eighth. The equations led us to a horn that is 29 feet long from throat to mouth, with a mouth 9.5 feet wide and 6 feet high. That’s a little large for a typical big city apartment but just fine for the Monster in the Loft. The horn pretty much loads into a corner of the room so this is a one-eighth space horn. If the horn had to work in free space, the mouth dimensions would be about 28 feet wide by 18 feet high - almost three times what we specified. Whew!

Next came the question of cost. It had to be low given the pairing to the Po’ BoyZ. For starters, we could cut material costs in half by using the loft floor as horn floor and the loft walls as one of the two horn walls. So we did that. The plan had the horn starting near the right end of the 25-foot long rear wall of the loft. The horn extended from right to left along the rear wall of the loft, bent 90° at the left rear corner of the loft and continued expanding toward the front of the loft. The horn's internal dimensions ended up being about 5.5 inches wide by 12 inches high at the throat, 2 feet wide by 3 feet high at the beginning of the turn at the corner, 4 feet wide by 3 feet high at the exit of the corner turn and 6 feet high by 9.5 feet wide at the mouth. My initial take on how many sheets of 4’ x 8’ material would be needed to form the wave guide was seven. By the time I got it all figured out and diagram cutting was completed, I had nine. 

Now here was the marvelous part. I went looking for sheet material and found that ¾" thick flooring grade oriented strand board is remarkably heavy and stiff and at the time cost $11 per sheet. We were talking about $99 in sheet goods. I figured on using 1.5" square cleats wherever panels joined and stiffening the exterior at critical locations (particularly near the mouth where panel widths become large) with 2" x 4" nominal lumber. Total lumber costs not including sheet goods but glue, screws and latex caulk for the cracks came to about $100.

As it turned out, we had on hand a pair of Altec Biflex 415 C drivers. These are Alnico 15-inch units. For this particular driver the inner section of the cone decouples from the outer at maybe 1000Hz, allowing the inner section to extend response out to maybe 7kHz. This does not seem a likely candidate for a horn woofer but it was on hand and maybe we did not need to worry about the unnecessarily extended high response. This turned out to be a good call. I’d purchased the 415 C drivers some time back at $225/pr. It was time to put them to use. Also, a Parts Express plate amplifier sitting idle was commissioned to power the amp. Wires from the left and right mono amplifiers were routed to the plate amp, combining the channels for a single channel to the Monster in the Attic. Current versions of this amp sell for about $110. 16AWG patio cable from WalMart became the speaker cable. I combined the green and white leads as the ground wire and used the black for hot. A 40-foot run of this cable sells for $6. Including taxes and shipping, materials for the entire project came to less than $600.

I spent a lot of time figuring and making precise cutting diagrams. More time was taken building a 1/10 scale model of the Monster, cutting scaled-down panels out of file folder card stock and taping them together on a downscaled floor and two intersecting walls of Kraft cardboard taken from a cardboard box. I thought the scale model particularly important so that I could be absolutely sure the cutting diagrams would be correct. You see, I planned on having my friends and family build this beast on a single Saturday, each assigned a specific job. To bring off a trick like this without losing face and morale with compounded errors, the plans had to be perfect.

Construction and sociality:
Back around the year 2000 I was on my own in the audio hobby, with just one good audiophile friend—blind piano tuner and technician Bill Van Winkle—to share my interest with. I think it was in 2001 that I went to VSAC (Vacuum State of the Art), an audio confab invented by Dan Schmalle aka Doc Bottlehead. VSAC 2001 was great. I met the late and great Terry Cain, proprietor of Cain & Cain loudspeakers; and Leslie Cain, Terry’s wife, the other Cain in Cain & Cain. Leslie is a fine—really fine—artist who creates magnificent images of the wheat-tlands and byways of Southeastern Washington. The mural on the walls of the GOED is Leslie’s work in progress.

Later I introduced Bill to Terry. Terry introduced Bill and me to Stephæn. Stephæn introduced us all to Jeff Day. Bill’s intrepid old friend Ron Barbee retired from management of the "Fix it Now" group at the Energy Northwest nuclear power plant and joined the sociality. And so the Bad Boys (and girls) Benevolent Association somehow came to form. At VSAC 2004 Bill and I ran into Harry Zweben and Kent Layden, partners in the erstwhile Two Bald Guys Audio - two bald guys with one good ear. But that’s another story. When Kara Chaffee of deHavilland Electric Amplifier moved from Northern California to Vancouver Washington for financial and climatic advantages (Vancouver = Gloom Capitol of the Northwest), we gained another brilliant member for the Bad BoyZ n GirlZ Benevolent Association. 

It stands to reason that there’s no reason to have a Benevolent Association if there ain't gonna be acts of benevolence. What greater act of audio benevolence than to congregate for a social day of building the Monster in the Attic? So on the appointed Saturday, the kinfolk gathered for a barn-raising kind of event. There were Stephæn, Bill Van Winkle, Harry Zweben, Kara Chaffee, Ron Barbee, old traveling friend Joe Burdick, Ken Hannifin and Ken’s wife. Plus Leslie Cain, Leslie’s published poet friend Janice, Clark Blumenstein—one-time apprentice of Terry Cain and now proprietor of Blumenstein Ultra-Fi, manufacturer of custom loudspeakers—Clark’s attractive female apprentice, my son Joel and loads of camp followers including Uncle Bob my wife’s brother who sat in the shade directing all outdoor activities.

Chalk lines had been snapped in the loft before we convened. We got started about 10:00 in the morning, broke for pizza lunch at 2:00 in the afternoon and had the Monster in the Attic playing at 8:00 that evening! Then we all came up on deck for a deserved steak & tandoori dinner (thanks for the chicken and marinade, Leslie). Stephæn and friend Joe took care of the cooking. Joe also was water bearer over the course of the day (it was hot June weather), installed the two 15-inch drivers at the throat of the horn and along with Harry ran the wiring. Kara was saw mistress overlooking the layout of each and every panel.