When to use center beam joist inside speaker cab?

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When to use center beam joist inside speaker cab?

Post by ESR » Tue Mar 20, 2018 3:05 pm

I've got two oversized 2x12's that I built from 3/4" baltic birch ply. I basically tried to copy the dimensions of the old Marshall oversized 2x12's, so these are 24" tall, 30" wide, and 12" deep. I originally did these cabs as semi open back, where I had a 5" panel going across the top and the bottom of the backside, leaving an opening of about 12". These two cabs are for a JTM50 style amp I built. I like open back cabs, and these sound good. But I've decided to try the closed back route.

I'm going to use 3/4" baltic birch again for the solid back panel on each cab. Gonna close it up as snug as I can. (Might even try doing the internal caulking that I've read about to get as good of an air-tight cab as possible--unless there's reason I shouldn't?) But my primary question is--do I need to use one of those internal wooden beam/joist/brace things that connects to the middle of the speaker baffle (inside of course) and stretches back to the inside of the rear panel?

From what I've read so far, that joist is there to prevent the back panel from "flapping" when you play the amp, and to keep a certain amount of stiffness and "solidness" in the cabinet overall from baffle to baffle. Is this to minimize rattle and/or resonance?

Given the dimensions of my cabinet(s), and the fact that they're 2x12's (not 4x12's), DO I NEED THESE JOISTS IN MY CABS?

Thanks in advance!

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Re: When to use center beam joist inside speaker cab?

Post by awangotango » Tue Apr 10, 2018 7:57 pm

i would put that post in, measure the distance and add 2-4mm extra which will slighly bow the back panel out and ensure nice amount of pressure on the post (and hence transfered to front baffle. I dont see the vaulue of caulk. just screw it on. it not only stiffens front baffle and back panel but makes the entire cab more rigid and act 'as one'.

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