I started a thread https://www.diyaudio.com/forums/multi-way/378092-tannoy-i8-makes-coax-convert-3.html about my new-to-me Tannoy i8's which feature an unbraced 15mm MDF cabinet.
The frequency response shows elevated output just below the crossover point that made me wonder if it could be related to cabinet resonances. Placing my ear to the cabinet allows me to hear midrange frequencies fairly clearly. So I thought I'd try to retrofit some bracing to see if it had a positive impact before I start messing with the stock crossover.
The solution I've come up with is comprised of two shelf braces tied together with a vertical spine. The vertical spine pieces still need final trimming to clear the magnet/basket assembly and the crossover.
The shelf braces are currently 100mm from the top/bottom and 130mm apart.
Some questions I still have that I hope can be answered:
1. I plan to drill holes in the braces. Any rules of thumb? e.g. how big can they be or what percentage of material can be removed before significantly reducing stiffness?
2. Is there are a target volume reduction to stay under? At the moment with the braces as they sit (beyond worst case since the woofer and crossover won't fit), they reduce volume by 9%. After trimming and drilling I'm sure I can halve that.
3. I had a nice shelf template that fit pretty snugly in the cabinet but I damaged it when trying to cut out the first brace. So the shelf brace has about 1mm of clearance. Will an expanding polyurethane adhesive (gorilla glue, Loctite PL construction adhesive) be effective in taking care of this?
The frequency response shows elevated output just below the crossover point that made me wonder if it could be related to cabinet resonances. Placing my ear to the cabinet allows me to hear midrange frequencies fairly clearly. So I thought I'd try to retrofit some bracing to see if it had a positive impact before I start messing with the stock crossover.
The solution I've come up with is comprised of two shelf braces tied together with a vertical spine. The vertical spine pieces still need final trimming to clear the magnet/basket assembly and the crossover.
The shelf braces are currently 100mm from the top/bottom and 130mm apart.
Some questions I still have that I hope can be answered:
1. I plan to drill holes in the braces. Any rules of thumb? e.g. how big can they be or what percentage of material can be removed before significantly reducing stiffness?
2. Is there are a target volume reduction to stay under? At the moment with the braces as they sit (beyond worst case since the woofer and crossover won't fit), they reduce volume by 9%. After trimming and drilling I'm sure I can halve that.
3. I had a nice shelf template that fit pretty snugly in the cabinet but I damaged it when trying to cut out the first brace. So the shelf brace has about 1mm of clearance. Will an expanding polyurethane adhesive (gorilla glue, Loctite PL construction adhesive) be effective in taking care of this?
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Gorilla won’t fill that gap I’m afraid, I would either make a shim or re-cut the brace.
I wouldn’t recommend gorilla glue for most woodworking, is only good for laminating sheets of thin wood together in a form, I have found. Regular urea, yellow wood glue is much stronger.
Something I try to do when making or modifying speakers is to round the edges of the speaker openings towards the inside of the cabinets.
Holes in your bracing will probably be easiest, and maintain strength if they were 1” holes, staggered, leaving no less than 1” from the edges. If that makes sense.
I wouldn’t recommend gorilla glue for most woodworking, is only good for laminating sheets of thin wood together in a form, I have found. Regular urea, yellow wood glue is much stronger.
Something I try to do when making or modifying speakers is to round the edges of the speaker openings towards the inside of the cabinets.
Holes in your bracing will probably be easiest, and maintain strength if they were 1” holes, staggered, leaving no less than 1” from the edges. If that makes sense.
The shelf arms that extend to the front baffle are only 2.25" wide so 1" diameter holes won't preserve 1" on either side of the hole.
As for the gaps, if the brace is much tighter, I can't get it in place. Perhaps epoxy could do the trick?
As for the gaps, if the brace is much tighter, I can't get it in place. Perhaps epoxy could do the trick?
I would probably forget about the holes in that case.
The epoxy would work, would be messy and hard to get into the gap consistently. Maybe a shim that will slide in after you’ve got the large part in place.
The epoxy would work, would be messy and hard to get into the gap consistently. Maybe a shim that will slide in after you’ve got the large part in place.
I've always thought bolts from the outside would be good for ensuring a good coupling? I know some advocate threaded rods across a cabinet.
Make sure they are a tight (inteference) fit, I don't fancy relying on glue for contact with the box as it can become brittle over time. Screw and glue is much preferred, but having it jammed hard against the walls is a great basis (no chance of rattles). And if you aren't relying on glue for the contact you can experiment before committing to the design too.
Another way might be some simple stick braces across the case, 25mm square hardwood or 25mm steel tube perhaps - a brace is more efficient straight than as a C-shape, and its easier to get solid contact in one place than along the edge of shelf. Sticks across an enclosure don't risk creating tuned resonant sub-chambers like web bracing might.