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In the Forum: Horn-Loaded Speakers
In the Thread: Macondo’s Midbass Project – the grown up time.
Post Subject: Load Distribution & Rear ChambersPosted by jessie.dazzle on: 7/18/2010
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Hmmm, the steel tension rods remind me of this:
Note the graceful arcing member that in your case, is analogues to the rafters.
Unless you make these horns super heavy, my guess is you won't need the rods. In the event you do, you might as well make the most of their contribution, so here's a suggestion:
Do not attach the upper ends of the tension rods at midpoint along the rafters, but rather to the peak of the roof, at nodes defined by intersecting rafters. This will support the bending load placed on the floor of the attic, while keeping all other members acting in pure compression and tension (instead of also being subjected to a bending load). Done like this, the ridge pole cannot be driven downward at all without first breaking open the joints of the triangles formed by each set of rafters, which can easily be reinforced with simple plywood gussets1.
For even more strength, replicate the central vertical post, installing one for each set of rafters, all the way to the rear of the space; this would allow attaching the tension rods along the vertical posts (not just at the peak of the roof).
Take a look at this bridge:
Since they are acting only in tension, you could substitute steel cables for the rods; for a given diameter, cable is a lot stronger than typical threaded rod, which means if you use cable it can be of much smaller diameter.
For your rear chambers, as they will be adjustable, it would be smart to go with a max volume that is larger than the max you estimate to be necessary.
A simple way to adjust them without going up into the attic, would be to mount bicycle sprockets to the ends of the threaded shafts, then get a bicycle shop to make up a pair of chains (without the grease) long enough to hang down through slots in the ceiling, allowing you to very easily alter the chamber volume. Once its set, the chains can be stored with little covers placed over the slots in the ceiling.
A jam nut2 is all you'd need to lock the shaft; this would have to be backed off to the end of the shaft during adjustment, then run forward and tightened down once the correct setting has been found.
There is of course also the issue of a vent for the rear chamber; very necessary, but they should be small. Barring some sort of remote actuation of the vents, before adjusting rear chamber volume, you'd have to go up and open the vents, make preliminary adjustments with vents open (this is why they should be small), then verify with vents closed. Lather, rinse, repeat!3
Since English is not the maternal language of many visitors of this site...
1: Plywood gusset:
http://www.tpub.com/content/construction/14044/img/14044_92_1.jpg
2: Jam nut:
http://en.wikipedia.org/wiki/Jam_nut
3: "Lather, rinse, repeat" (1970's North America):
http://en.wikipedia.org/wiki/Lather,_rinse,_repeat
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