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In the Forum: Horn-Loaded Speakers
In the Thread: Macondo’s Midbass Project – the grown up time.

Post Subject: Hard rear chamber/basket coverPosted by jessie.dazzle on: 8/31/2010

Here's how I'd make a hard rear cover/rear chamber.

I describe the process in detail, as it is a good investment of my time; I expect to have a similar condition and requirement for my own mid-bass horns (intended for use with AK-151s).

This approach consists of casting a rear cover for the basket, with small ports to allow precise tuning of the rear chamber volume.

So here's how I'd do it:

Have your carpenter cut two rings from 3/4" thick hardwood that slip snugly over the magnet cover of each driver. The ring should have a radial cut about the width of a saw blade to allow for slight expansion/contraction.

Place the least desirable AK series driver you own face down on a table.

Install the ring just below the back plate of the magnet cover and secure it in place using a hose clamp of a diameter large enough to fit around the perimeter of the ring. Cover the down-facing side of the ring with tape.

Neatly cover the basket from the rear with either tape or plastic wrap. Build a 1.5-inch-high "fence" around the periphery of the basket just above the mounting flange using thin but stiff cardboard. Build a similar fence around the binding posts. Wrap the magnet with tape, building up the thickness to around 1/16th of an inch (for clearance); be consistent... Do not wrap more tape around the top of the magnet than around the bottom. Using a brush, apply a thin coat of liquid bees wax (heat it up in the microwave) to all inner surfaces; the tape-covered back of the basket and the fences.

The covers will be made of synthetic/acrylic plaster; it is important to understand that despite the name, this is nothing like gypsum plaster... Once it takes a set, it is hard and tough, very much like hard plastic. This is what I used to make my mid-bass horns... Once set, it is indestructible. Get it from places that supply sculptors:

http://shop.propbuilder.co.uk/

http://shop.propbuilder.co.uk/jesmonite-11-c.asp

It will be expensive, but for the quantity needed, it is not worth trying to get it from the manufacturer. A very cheap and effective alternative would be to use hemp-reinforced natural gypsum plaster (plaster of Paris), mixed for a very hard set, and built up to around 1.5" thickness. Done right, it would not be fragile. There are other alternatives (fiber-reinforced polyester auto body filler, slightly thinned with polyester resin for example; do not breath it) but they are not so user-friendly. For this process I'll assume the acrylic synthetic plaster, as it is ideal.

Mix up a batch and fill in the volume behind the basket in stages, starting down at the perimeter of the driver, working up toward the center, inlaying fiber reinforcement at each stage. For this you can use just about anything from hemp (the real stuff, available from fine arts supply stores) to fiberglass matt, available at auto body repair supply shops... Even Pep Boys has it (avoid tightly-woven cloth). The fibers will need to be fully saturated with the acrylic resin before they are inlayed (this is why I don't recommend the tightly-woven cloth; it is not as easy to saturate with the acrylic resin). Cut the matt into manageably-small, odd-shaped pieces before saturating them. Wear surgeon's gloves when working with the resin and use a small brush to position the reinforcements. It is not toxic, but its a pain to clean off your hands if allowed to set.

Continue the process until the desired thickness is achieved, building a neat collar around the magnet up under the hardwood ring.

Here is some info on how to use the product:
http://www.propbuilder.co.uk/howto/jesmonitelaminate.html

Allow a minimum of 4 hours before removing the cured cover.

Repeat the process for the second driver.

If the lamination is sloppy, the edges can be trimmed using a sheetrock blade in a sabre saw, or simply ground down with an aggressive sanding disc mounted on a hand-held angle grinder (really aggressive, like 16-36 grit, with a stiff backing). All this can be avoided if the lamination is done with care.

Buy small air bleeders like this:
http://www.faster-motocross.com/images/15-1027lg.jpg

Available here:
http://motorcycle.motorcycle-superstore.com/motorcycle/Fork%20Bleeder

Alternatively, do a search on "fork bleeders" or "air bleeders" (avoid the push-button type bleeders)
http://www.google.fr/images?um=1&hl=en&safe=off&q=%22fork%20bleeders%22&ie=UTF-8&source=og&sa=N&tab=wi

As the orifice is quite small, you might need more than one per driver. Also, unlike the tape, the cover is hard this time, so I would order 8 just to be sure, in which case its worth searching for a good price.

Decide how many air bleeders you need, then drill a corresponding number of holes through the cover, centered in the areas of the cover that close off the openings in the driver basket. The diameter of the holes must be slightly smaller than the diameter of the threaded end of the air bleeder body. Install the air bleeder body, applying pressure as it is turned, thus creating threads in the cover; do not over tighten (just snug is enough). Remove the air bleeder body. Put a drop of Locktite Red or Blue (permanent and semi-permanent) on the threads of the air bleeder body and reinstall it; do not over tighten, the Locktite will create the necessary seal.

Install air bleeder adjustment screw.

Final assembly:

Place the drivers you will be mounting in the horn face down. Using RTV silicone, lay a neat bead around each opening in the baskets, leaving at least a half inch margin (the silicone will squish out, and you don't want it dripping down on the driver cones). Immediately place covers over backs of drivers and do not touch them for 24 hours. If in the future it becomes necessary to remove the covers from the baskets, the silicone bond is easily broken, and can be easily removed from the basket.

Install the hardwood ring so that it holds the chamber cover in place, and secure its position to the magnet using the hose clamp.

Mount drivers with covers to horns. You may want to make some tabs from metal that fit under the mounting bolts heads and reach out to further secure the cover.

The cover could have been made to fit under the bolts, but this would require some planning and the use of fairly thick metal sleeves in the holes of the cover. I do not like this approach, as I think it would unnecessarily stress the cover. In fact, given the rigidity of the assembly (basket and cover), I don't think anything more than the hardwood ring is necessary.

Tune resonance by adjusting the bleeder screws. Once desired resonance is found, remove screws, carefully noting the number of turns. Apply a thin coating of silicone to the threads of each bleeder screw, and reinstall them to their pervious depth by counting the number of turns. Allow silicone to set. Blow through the hole to verify that it was not blocked by silicone.

Alternatively you could lock the screws whith a small drop of super glue, or with enamel (non-water-based) paint. If using Locktite, make sure its the low-strength purple stuff, and use only a very small amount around the screw after it is installed and adjusted (do not apply it deep down into the threads).

jd*

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