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81-10 Kit: Story and Photograph

Mon, 3 Dec 2001
From: Fred Reed
To: Huw Powell

It took a while, but I thought you might be interested in a story and picture of a takeoff on your 81-10 model from parts I got this spring. It's my first speaker project, but I threw every tweak in the book I could find at it (just for fun).

Fred Reed's 81-10 kit project!
(photo: Fred Reed)

It's hard to tell from the perspective in the picture, but the enclosure's only parallel walls are the top and bottom. The sides have a gentle slope outward toward the bottom and the back has a pretty substantial slope to it. The front is vertical. The external dimensions of the speaker are approximately:

41" tall

(33" to center of woofer - ear height in my favorite listening chair)

11" deep and 12" wide at the top

15" x 15" at the base.

The arrangement of drivers is based on two ideas. One is borrowed from the Sound Clearing House's tower speaker and some other measurements reported on the web.

That idea is to put the woofer above the tweeter so that from positions roughly even with the woofer and above (as in standing), the distance to the driver acoustic centers remains more equal (as the woofer is somewhat inset) and therefore more likely to remain in phase than if the tweeter was on top (and closer to begin with). I have to admit this doesn't take into account the effect of the crossover, though.

(ed. note: this idea may also be seen in the Genesis 2 loudspeaker)

The other idea is based on the Roy Allison rule of putting the woofer (in this case the PR) as close to the ground as possible. The woofer and tweeter are also placed in unequal (and hopefully not related) distances from the edges, and the tweeter is closer to the inside edge (speakers are mirror images of each other) by an amount that I eyeballed to make it roughly appear to be directly under the woofer dust cap when viewed from the listening angle (60 deg).

The enclosure is a 3/4" plywood box surrounded by 3/4" MDF, with 5 layers of roofing felt sandwiched in between (the constrained layer idea). However, I used drywall screws to assemble the whole thing, and I suspect that they act as "sound shorts" between the inner and outer layers, defeating the layer idea somewhat anyway.

As you can see in the picture, instead of the bevels on the 81-10, I came up with the idea of using reversed clamshell molding, with the thicker edge trimmed and angled so that it would fit tight against the rest of the baffle. The thought here was that it was the only pre-made moulding I could think of that had a turn radius big enough to operate at the wavelengths I understand are important (i.e., a radius of at least a couple inches). Of course, it's not a full turn, but at least the first couple inches a wave coming out from the driver sees is a gentle turn rather than a sharp corner. Hard to say how well it works acoustically, but I think it looks nicer than I expected after I stained them.

The inside of the enclosure has relatively few braces (one between the woofer and tweeter because so little material there, and one between the sides near the bottom where the surface area is so big). Instead of stuffing, I used eggcrate foam in the top areas near the drivers, with two layers nearest the woofer. My theory was stuffing a bass reflex didn't do much good, but that any absorption would help. I also gopped a mix of sand and glue to the inside surfaces to add mass damping as well.

I oversized the box a bit, then filled the bottom with about 25lb of sand in each speaker for stablity and low frequency damping. I actually think I might have still left the box volume a bit too big (over estimating how much glop and sand I would add later), which might have an audible effect. Those mothers are now so heavy that I'd better never have to move them.

I used mortite to mount the drivers and squeezed a bunch of it around on the driver frames as well to add damping. Both drivers are set into the baffle and the tweeter is actually set in its own little enclosure hollowed out into the layers of MDF and plywood.

At first, I thought they sounded a little thin on bass, but that seems to have gone away with break-in and new CD player cables (I thought people were just making that stuff up about cables...). Using a test disk and my ear (no SPL meter yet), I'd say it has effectively full response down to 27-28 hz and sounds great with good recordings that make use of that range. My older/cheaper "rock" music bass sounds weak though and I don't know if it's:

a) I'm just finding out that good speakers make bad recordings stick out,
b) the speakers are just weak in the higher bass frequencies,
c) I have a room problem and it's getting sucked out by cancellation, or
d) I just miss the bass boom of my old bad speakers.

(ed. note: I suspect a, c, and d!)

Voices sound really good and with good recordings the soundstage is really nice--which I hope is a result of good luck in placing the drivers and corner moulding.

In general, they seem better for "quieter" music--easy listening stuff, jazz, etc., which is somewhat unfortunate because I was really hoping for a kick-a** rock speaker. But, like I said, maybe it's because the rock recordings tend to not be as good. I have a couple CDs from Little Feat that come out sounding real nice.

The only thing that seems to be lacking is a bit of high end - the "air" thing - which I'm not sure how I could have screwed up with all my tweeking. I'm beginning to think it might just be my hearing going bad, although I have heard some store speakers that seem to have more (e.g., Vandersteens). I have heard, though, that some "high end" speakers actually over-do that stuff to make them sound expensive. My speakers are actually very easy and non-tiring to listen to. You have to really crank it up before they sound "loud"--which I take to be a good thing. Perhaps that "air" would get tiring after a while.

Well, that's about it. Hope you enjoyed hearing about all the fun I had messing around with your parts.

Fred Reed

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