Fine Tunes #24

Okay, back to equipment racks and resonance tuning.

Let's say your components are snugged into one of those ubiquitous Target racks I used to use myself, or a similar round- or square-section hollow-tube rack with MDF shelves supported at their corners by tabs or push-in paddles. Shut your system down, remove the component on the top shelf, and rap the shelf with your knuckles. Not very reassuring, is it? Not only is the shelf resonant, but being supported at only its four corners makes it rattle'n'jump when you rap it.

Back in February's "Fine Tunes," I suggested making your own constrained-layer-damped shelves by bonding several 1/4" sheets of MDF, or alternating sheets of wood and MDF. Well, this month we're going One Step Beyond.

Take exact measurements of your current shelves, how deep they're set into the retaining tabs or clips, and how much room you have between shelves. Then run down to your local Home Improvement Hutch and pick up two 1/8"-thick sheets of Masonite. Two 4' by 8' by 1/8" sheets should cost no more than $2.50 each, and most places will cut the boards for free or for a nominal charge. If you do your sums right, you can make use of the entire sheet without wasting an inch. Pick up a few cheap clamps, a coarse wood rasp, and some Elmer's Wood Glue if you're not a Tool Time type.

Haul your booty home and clear a work area. You'll want a large, flat surface to set up on; a baking marble would do nicely (but don't tell the wife!). You'll find the Masonite has a rough, fibrous "backside" (don't we all?), while the other side is smooth and ready to paint. Rasp down the fuzz on the backs of all the cut panels to prepare for gluing. Use a stiff cardboard paddle to spread the Elmer's evenly over both sides to be glued, press them together, and wipe off the edge ooze. Don't be shy: Try two, three, or even more sheets for greater damping effect. Clamp the sheets together, or weight them down with a couple of bricks or concrete blocks, and let dry for 24 hours. During that time, think creatively about how to explain away the missing baking marble.

Next day, unclamp your new equipment shelf and give it a right rap. I guarantee you'll be startled at how dead it sounds, how nonresonant. Eat your heart out, Norm Abrams! You can finish the shelves however you'd like. If you're a bachelor dude and décor is the last thing on your mind, use 'em as is. If you're style-conscious and wish to impress friends'n'family, clean up the edges and paint them whatever color suits your creative soul. The point is, you can make them as nicely finished as you like, and it still won't cost you more than a couple of bucks.

As also mentioned in the February "Tunes," consider installing a piece of mild-steel sheet metal to the underside of Your New Shelf to further improve damping characteristics and to add a modicum of RFI/EMI shielding. That should run you less than $10. Or find a scrap-metal dealer or a local metal fabricator. Bet they have a scrap bin full of pieces of just the right size---for free!

By the way, if you don't mind opening the ol' wallet, you can have 1"-thick glass sheets cut to size, then glue the corners onto the retaining tabs of the rack to keep them from resonating. Yes, these have to be thick, which means they'll be expensive, but they should be surprisingly inert.

While the shelves are drying, turn your attention to the rack itself. Back in February I suggested using steam-cleaned "pool" sand to fill hollow-tubed stands. Tuning maven Victor Tiscareno of AudioPrism/Red Rose Music had an even better idea. "No matter how many sandboxes, inner tubes, or even Iso Bearings you use, if the stand is rickety, you're still in trouble."

His solution? Pick up a three- or four-dollar can of nontoxic expanding insulation foam, pull the footer spikes from This Old Rack, and shoot it up! As for the smaller-gauge cross-members, drill 1/8" holes and blow the foam in through a straw! What a terrific (and cheap) way to stop your rack from ringin'.

What might be the results of all these heroic efforts? Listen for a lower noise floor, tighter bass, cleaner midrange, more open highs, and better imaging.

Reader Leif Christensen, from Norway, recently sent an e-mail with another do-it-yourself shelving solution for heavier components: "Make an approximately 4"-deep tray of whatever size best fits your rack. Use MDF acrylic, Lexan, or even aluminum. For each tray, buy three wheelbarrow-wheel inner tubes. Drill three holes for the valves in the bottom of each tray. You may need the small plastic adapters so the valves can reach through the bottom plate of the tray. Place one tube in each front corner and one in the back at the center. Then place the component on a shelf, and bleed the inner tubes down to level them and adjust the cutoff frequency of the isolation. (The less air, the lower the isolation filter frequency.)

"I use this technique with great success for my Basis Ovation turntable, Aesthetix Io phono front-end, and Audio Research VT100 Mk.II amplifiers. I've tried all sorts of leaking sinks, footers, and points, but they all seem to have some negative effect. This doesn't. The inner tubes I get in Norway can take a 30-40kg load each without leaking significantly for a whole year. And as a bonus, it's cheap! Good luck and keep up the good work."

Many thanks to Leif and all the other readers who have e-mailed their suggestions and experiences with resonance tuning.

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