My 4-way horn stack had its humble beginnings at the end of the summer of 1999. That was when I built a Decware 'Wicked One' subwoofer. I had been attracted to the design by the claim that it was a horn subwoofer (which it really isn't, if you ask me). In retrospect, I realize that it is pretty silly to build a box and stick any old woofer in it, especially a bandpass box which really needs to be tuned to the actual woofers used, but I didn't know this at the time. Anyway, I used two Peerless 850146 10" woofers in it. I constructed it out of 0.75" MDF and made the top (one of the 36" x 36" sides) removable, attaching it with screws and using a garage door seal stapled to the tops of the internals of the box as a gasket. Initial impressions of the sub were mixed. This was due to my inexperience with audio electronics in general, and now I get a good laugh thinking about a lot of the puzzles that box presented me with and some of the things I did with it. As it turned out, the amount of bass produced was very dependent on its location within the room - its low cutoff varied up to almost an octave depending on the specifics of its placement. I was getting an introduction to the concept of room modes, as I was initially using it in a small, concrete room (my dorm room). In retrospect, I'm glad I never really extracted its full potential from it while I had it in my dorm room - the people below me pounded on their ceiling enough as it was.
The second piece of my horn stack, a 75hz midbass horn, was designed during the spring of 2000 while I was interning and had a bit of free time on my hands. It was a pure exponential horn with the length set at 1/4 wavelength of 75hz and the mouth set to 1/4 size. I approximated the curve with three flat sections. The driver I used was an EVM12L, a 'classic' midbass horn driver. Working from these constraints, the rest of the horn parameters were found using Marshall Leach's horn model. I didn't start building the until the summer of 2000, and it took me about the whole summer. I didn't know what I had right away - I just knew it got pretty loud with a single-ended 2A3 amp and a portable cd player. When I moved back up to school, the first order of business was to get the sound system working (of course!), so I stuck the horn on the floor against the wall and put the Wicked One next to it pointing into the corner. For highs I used an MCM 55-1595 5" 'fullrange' driver on a 250hz tractrix horn set on top of the sub. Not too bad... But better things were coming.
In the middle of the fall 2000 semester, I got my hot little hands on an honest-to-goodness compression driver - an old JBL 2470. Watch out neighborhood - here comes John's sound-reinforcement system! Anyway, it took me a while to really make use of this find (I got it on Ebay for $35 while they normally sell for about $150). It languished on top of the Wicked One while I tried to figure out what to do for a horn. Eventually, I decided to make a 10 sided 500hz tractrix horn out of 1/16" mahogany veneer. This was really a small prototype for another project I was doing for someone else, but it worked out just fine for me - the veneer all came from a free sample sheet courtesy of Certainly Wood. Once this was done, things really started to get good. At this point, I had also decided that it was a bad idea to have the midbass horn on the floor - I decided it would work better at ear height (and it did). So I sort of levered it up onto the top of the Wicked One and shoved it all against the wall. I hung the compression driver over the top lip of the midbass horn just to have some sound again. A few weeks later, I decided I'd see what all the 'time-aligned' hype was about, and I constructed a little tripod to support the compression driver above the midbass woofer's voice-coil (approximately). Well, there was something to the hype after all... As it turns out, this setup was probably not aligned exactly right, but it was much better than having the tweeter about four feet in front of the midbass driver. At this point, the horn stack looked like this. Notice my dorm room-esque attempt at decorating nine square feet of MDF (the side of the sub). The electronics running the whole thing included a singled-ended 2A3 amp (the big mess of wires on the left of the rack) for the compression driver, one channel of an Adcom GFA535 driving the midbass horn, one channel of an Onkyo HT receiver for each of the Peerless woofers, a Bottlehead Foreplay serving as the preamp, and an active crossover built from John Pomann's kit. The cd player was an Onkyo six-disc mass-market thing.
After living with the speaker like this for a while and getting more experience using my computer measurement setup, I decided it would be nice to have some highs. The 2470 by itself didn't really extend much past 10khz, and it was already down around 10dB there. To this end, I decided to pick up a JBL 075 bullet tweeter off of Ebay. No deals on this one - I got it for the standard $150 with a mounting ring included. (I've since sold it for the same price - not a bad deal.) I extended my frame of 2x2's up a bit to hold the 075 in place above the 2470. I had lots of different ideas on more attractive and acoustically better ways to do this, but those ideas would have all taken time, and I wanted to listen! (That's always the downfall of my projects' cosmetics.) Even so, it sounded pretty good. Following this addition, I pulled the stack out into the room a bit and shoved it against the back wall. This brought a little further improvement in the sound, probably due to moving away from the side wall and reducing the level of early reflections. At this point (early in 2001), all the components were finalized and in their places, and I started devoting more time to refining the crossover. The tweeter ended up needing a lot of attention to really get it to extend up to around 20khz. Left to its own devices, its frequency response rolled off a little too soon on the high end for my taste, although I have since heard there is a large peak in the ultrasonic range. The story goes that the 075 was originally used as an ultrasonic transducer to count box cars. With a bit of fiddling with the crossover, it turned out pretty nice, though. Further tweaks to the crossover included adding some parametric equalization to the sub. I got this circuit from Rod Elliot's website, and it really helped smooth out the low end frequency response, at least in the listening position where I was measuring. I'm not really sure if this would have been very useful if I didn't have the capability of measuring the results, though. The final response of the system measured at the listening position ended up looking like this with this crossover circuit. I subsequently found out that the nastiness in the 1khz to 4khz range was the result of an imperfect transition from the 2470's throat to the horn. After some work with a chainsaw file and some wetsanding, the response smoothed out drastically in this region. The addition of a little ring of felt stuck a little ways out from the throat of the horn removed one last dip in the response. Unfortunately I discovered all this after I had disassembled the stack. I really liked how the whole thing turned out. I would describe the sound overall as very clean, quick, and dynamic if pressed for audiophile-speak descriptions. I especially liked how it handled bass drums in classical music - there was just an impact without anything extra. I think the midbass horn was probably the best part of the whole system, and there were even several things that could have made it much better (a better structure or panel damping to reduce vibration, a notch filter for a peak at it's high end, some sort of phase plug or at least a round throat, and adding large, round edges to the mouth in an effort to reduce diffraction).
I started dismantling this system at the beginning of the summer in 2001. I needed the 2470 to experiment with my next project, and I sold the 075 in order to buy other parts. The midbass horn got the woofer removed so I could set it on my floor and try not to put my foot through the cone while the horn itself got trundled down to the basement to wait for me to get a bigger house. The Wicked One was the last thing to leave my room. I removed the woofers and slid the box downstairs. It served as a convenient place to set beers while playing darts for a while, but now it has been passed on to someone else.
Copyright 1999-2004, John H Sheerin
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