GoodSoundClub - Romy the Cat's Audio Site

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

Post Subject: Mid-bass horn; thoughts on materialsPosted by jessie.dazzle on: 6/16/2010

Romy wrote:

"...The material itself – what to use. I am wide open from any type of wood to synthetic panels (like cement panels). Perhaps it might be a combination of organic and synthetic layers..."

I realize you won't be doing the work yourself, but this approach to the project may be of interest.

The process will be a lot easier and the results more precise if you start with a good CAD model of the horn including its supporting structure. From there you can have full-scale plots printed to use when cutting the wood for the structure. Get several copies and mount them directly to the sheets of plywood (use 3M Super 77 spray glue; don't wast time with anything else).

For the curved wall(s), I would start by making an external webbed structure from 3/4" plywood, having both longitudinal (along the horn's firing axis), and transverse sections. I would put the max distance between sections at no more than 12" on center. This structure will depict the final inner surface of the horn, less thickness of the material it will support, so obviously you'll want to deduct wall thickness (no problem in the CAD model).

With the structure built, on its convex side, fit 1/8" thick sheets of coarse-grained plywood (cheaper than fine-grained), screwed and glued, followed by laminating successive sheets of the same thin plywood. For the final lamination, as it will be the part that is seen, use 1/8" fine-grained plywood; it will be a lot easier to finish. The webbed structure can be designed so that it bolts to the structures of the horn's other walls. For the flat walls, I would build them in the same way, but as thay are flat and more prone to resonate, I'd make their supporting structure even more robust or increase the number of laminations.

If necessary, resonances can addressed by isolating the horn's walls from each other; In this case, avoid bolting them togther. My upper-bass horns have felt "gaskets" between all joint planes; my mid-bass horns incorporate provisoins for the same. I have to say, unless you have to get the horn through a door, I would not slice it into sections cut across its firing axis, as this will entail more work.

If you know someone with the skill to work with plaster (old-school plaster, not drywall), this would be another option; it is quick and materials are very inexpensive, allowing you to make the horn as thick as you want, but in general plaster is less likely to resonate. Also, "radiused" corners and transitions involving compound curves are no problem.

jd*

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