A/C Fir ply for 2-ply sub cabinet? (1 Viewer)

[Vince Bray]

The stryke has entered the building. Holy cow, if you ever needed inspiration for a project, this thing is it! No one has even slightly exaggerated the build of this thing!!!
I am planning a modified version of John's cabinet that has grill cover for all drivers. It will still be 1.5" thick but I seek a lighter product than mdf. I would like to be able to heft the cabinet easily with no drivers installed.
Home depot has a fir plywood that is 23/32" thick, with few voids except at the exact half-length point, where the voids line up all the way down the stack. This seems like a decent lightweight wood for a cabinet that has double walls. The majority of the voids could be easily avoided because of their location. Anyone tried this wood? Also the outside could be finished directly, although not exotic, it would look good stained black or painted. I don't think the 20lbs removed will matter at all.. the drivers are said to weigh over 120lbs alone ( i'm using the pr-18's with the he15) Another option is to use oak ply with the fir as the inner layer, but the oak has more irregular voids...
I'm trying to avoid the expense of marine ply, as it would be 150+usd versus 60usd for the fir.
Comments anyone?

 

[Julian Data]

I wouldn't use any ply from HD. They aren't void free.
If you want a lighter and stronger solution look for Apple Ply, Baltic Birch or marine ply.
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[Mike Temple]

Removing the weight of the ply doesn't sound like a good idea. I might be wrong, but the wieght of this thing is going to be one of the reasons ( density ) that you wouldn't get cabinet resonances. I have a single Shiva with MDF heavily braced and where the largest open spaces are I can feel the cabinet resonating during loud passages.
I don't know if the resonances will be heard in your case, but if you are going to use ply, find the highest ply count ( usually HD and other depot stores have fewer plies per sheet as compared to other stores, this is why they can offer it cheaper, it cost less. )
Maybe not a feasable idea, but how about removable drivers which seem to be the bulk of the weight.
Good luck, and remember, do what YOU want, not what others think is right for them.
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[Dennis Kindig]

Check out Phil Abbate's site on speaker-building. His research indicated that the lowest panel resonance he experienced was with two layers of MDF glued directly together, as opposed to layers of MDF separated by homosote or separated by linoleum. His test methods and results are outlined here:
Link Removed
I built my Shiva sub using two 3/4" layers of MDF, first building one complete box, using eight 1 1/2" dowels across all sides secured with screws through the panels directly into the ends of the dowels. I then coated each panel completely with wood glue and attached the next layer one panel at a time. Using a good carpenter's wood glue, the MDF will actually break before the joint does.
This cabinet is very heavy and I have not detected any resonance, at least by hand. I'm sure I would using Phil's test method, but I trust his results in terms of his constrained-layer tests.
MDF is generally considered the most inert material for speaker panels since it has no voids, no grain and has a high resin-glue content. If you feel that you have to use plywood I would recommend using what I have seen called ParaPly. This is a cabinet-grade plywood that has almost twice the layers that a standard-grade plywood contains. I think that a standard 3/4" plywood has 5 layers while the ParaPly has 8 to 10 (again, a higher glue content). It also generally comes with a very smooth Birch veneer on both sides that will help when it comes to finishing the material.
Dennis
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[Vince Bray]

The HD used to have paraply, but I can't find it anymore. I agree that it would be worth a try.. I think it's the same species as luan ply, the 1/4" stuff, but as you said, it's more plys and very consistent. It is guatambu or a related species of semi-hardwood. Wish I could find it around here anymore...

 

[Vince Bray]

Thanks for everyone's input!
A quick search for paraply resolved that...
Paraply is banned in the US because of some chemical in it
that is at least a skin irritant.
I think I will use a marine ply and mdf combination.
I dont' want to end up with a half sheet of marine ply
wasted (left over) when it's $78 for a 4x8 sheet.
I'm going to double up the box with mdf on the panels with drivers on them, and the top, thus the additional weight will be minimized, as the cut-outs are pretty big. I need 12 total panels ~2x2, so I can get 8 from the marine ply, and 4 from mdf for a cost of $10.
This also might produce a more non-resonant cabinet...