What to expect on my 48" MLTL with mahogany

[cpmiller]

I built a pair of GM's 48" MLTL's for the JX92 along with my brother this past Summer. We built prototypes using cheap plywood to practice our cabinet building skills and tune the design before committing to version that can be placed in view by my wife.

After some tuning and adjustment we've become very happy with the sound and have been buildling cabinets out of African mahogany. Probably not a very green choice on our part. We've been working on the boards for over a month and are close to gluing the cabinets.

Recently I was researching solid wood for speaker cabinets and discovered that most opinions by folks on the forum are strongly against solid wood cabinets for loudspeakers.

My question to the forum, now that the only practical recourse is to follow through on the build and learn from it, is there anything we might do to compensate at this stage of the build to reduce the negative impact of the material properties of solid African mahogany vs. BB ply?

BTW, one of the reasons we didn't go with BB ply for the finished version is we have no experience with veneer and I was intimidated to try and learn how to work with it. That, and the fact that the milling and joining of good hardware is somewhat familiar. I have nothing against BB ply and I notice our original ply seems less "lively" than the mahogany.

Thanks,
Chris

[planet10]

The biggest problem with solid wood is that it "moves" with the weather (largely the humidity) and ends up cracking leaving you with a leaky box that has lost some of its structural integrity.

dave

[cpmiller]

Thanks Dave. Structural integrity and cabinet resonance seemed like the two most oft quoted concerns working with solid wood, your reply is fully in line with that.

I'm wonder if we apply silicon caulk the internal seams if that would help maintains the integrity of the cabinet better - functionally at least? Naturally that won't hold the wood in place.

We're using rabbet joints for a little added strength. We used screws on the prototype but did not plan to use them on the finished versions to improve the appearance. My brother picked out the wood glue. One of the Gorillia glue products that he's used on some mahogony guitar cabinets. Those cabinets seem to have held up well so far. We live in Vermont.

Your comments are appreciated.

Thanks,
Chris

[planet10]

It isn't the seams where things usually come apart, but the middle of the panel. This can be ameriolated quite a bit by butcher blocking the wood. (note: i am far from an expert)

dave

[vitalstates]

Hi Chris

It is possible to worry too much(not that I'm guaranteeing your build)....

I used to build long case clocks(from trees that I'd chosen and had planked and kiln dried) from mid eighties to late nineties and there is still one I have in the family which hasn't moved at all(nearly 25 years).......In my experience the timber doesn't move at all if you seal inside and out(shellac, french polish or spirit based wood sealer will do).

although this is limited experience( I built about 8 of these in all as it was just a weekend hobby) I have been told by many cabinet makers that the secret of longevity is thoroughly sealing.

The other menace is varying temp and humidity in the environment they live. Controlling that will increase stability.

Having said that, all my loudspeakers are BB ply which I would advise for your next build. I don't veneer mine but spend more time choosing boards with a good, figured face.

Ed

[cpmiller]

Dave, I see. I thought it would be the joints that would fail. If a board cracks I'll get some experience figuring out how to repair that. The mahogany in my brother's guitar amp cabinets has held up well so far.

Ed, for sure we are planning to give an oil finish to the outside. I had not considered sealing the inside surfaces. That's a simple enough thing to do to try and improve the stabilty of thw wood. Thanks for that information.

Dave, you're familiar with the GM MLTL design based on the Jordan JX92. Can you suggest where I might buy some acoustic felt and speaker gasket material. I didn't use either on our prototypes - couldn't find it.

Thanks all,
Chrsi

[planet10]

Originally Posted by cpmiller

Dave, you're familiar with the GM MLTL design based on the Jordan JX92. Can you suggest where I might buy some acoustic felt and speaker gasket material. I didn't use either on our prototypes - couldn't find it.

Not that familiar. We built a set of the triangular ones for a local. I sold my JX92 because i didn't like them.

Gasket material -- get some neoprene draft exclusion tape. I get it at Home Depot.

I've started using the 1/2" recycled cotton felt from bonded logic Natural Cotton Fiber Insulation, Natural Cellulose Insulation

Bob at CSS sells it in more accessible quantities.

dave

[MJL21193]

Originally Posted by cpmiller

Dave, I see. I thought it would be the joints that would fail. If a board cracks I'll get some experience figuring out how to repair that. The mahogany in my brother's guitar amp cabinets has held up well so far.

Ed, for sure we are planning to give an oil finish to the outside. I had not considered sealing the inside surfaces. That's a simple enough thing to do to try and improve the stabilty of thw wood. Thanks for that information.

Cross grain joints are a problem with solid wood, where the wood on one side of the joint (the end grain) will expand and contract the most as opposed to the wood on the other side of the joint (the edge grain) will expand and contract very, very little.
You can ensure the wood is as dry as possible by leaving it in a heated, dry space for some time (weeks). Then, once the build is finished, apply sealer (not oil) to the inside and outside. Something that is virtually impervious to moisture is polyurethane - 2-3 coats will go a long way to minimizing seasonal movement.

[cpmiller]

Glued together the last cabinet tonight. Will be sealing the interior surfaces based on feedback. Will be ironic for sure if the crude ply test mules sound better than their mahogany competitiors. In any case, we will be sealing the interior surfaces. Thanks all for comments and suggestion. They sure are looking good though. Picked up the granite bases tonight too.

[Greg B]

You should be fine. Mahogany (including african) is about the most stable of all hardwoods, and besides there are no wide cross grain spans with that design. Shellac makes the best sealer, but mix your own or get Zinsser 'Sealcoat'. Old canned shellac sometimes won't dry.

People generally won't recommend solid wood because, apart from stability issues, a speaker cabinet is not supposed to be resonant...

I prefer mahogany with the traditional garnet/ruby shellac french polished finish, but it's hard to do.