Endcaps (absorbtion, reflection etc) (1 Viewer)

[Jedd]

After wandering through Home Depot, I'm thinking about these endcaps for my sonotube:
1/4" plywood + 3/4" MDF inside + 3/4" baltic birch outside
a few questions I have:
- is it a good idea to glue MDF and Birch together?
- do I really need this 1/4" plywood inside? What if I put t-nuts into the birch before gluing? (maybe there are other ways to mount the driver?) How sonic-wise(unwise) is it to put the plywood inside?
- I've read a couple of posts here that said that 1 Birch is as good as 2 MDF's, does it mean that 1 layer of birch flushed in will be good enough?
Those posts of Greg Monfort and Jack Gilvey have made me worry.

 

[Jack Gilvey]

Worried about what?

What diameter tube, what driver?

 

[Jedd]

About all this sonic-dumping-absorbtion-resonation stuff.

The tube is 22", 2' long, sealed, 48 ounces of polyfill. I'm going to turn it on its side and flush 15" Dayton into one endcap and PE amp into the other one.

 

[Jack Gilvey]

About all this sonic-dumping-absorbtion-resonation stuff.

Hmm. I might be worried too if I knew what that was.

3/4" MDF plugs and endcaps are fine...that means four total pieces. A laminate of BB and MDF is even better. In that case, I'd use the BB on the inside so the T-Nuts grab well. Make sure it's BB you saw, and not just regular birch ply.

 

[Ron D Core]

MDF is a "dead" wood, meaning it resonates much less than any other wood out there. Plywood is anything but dead which is why its popular with guitar cabinets. But anything highly resonant with something non resonant attatched to it will deaden it. The ply will not hurt any of the dampening properties of MDF, if anything it will add to it. Besides its high frequencies that are most affected by these types of resonances with this type of wood. What you really need to worry about is the flexing which is solved by making your baffle extra thick or crossbracing. I wouldnt bother with the ply, you might want to just triple up the MDF or 1.5" of MDF and .75" of Birch if you want the look. And some all-threads to help keep it together.

 

[Jack Gilvey]

The reason that "no-void" ply (Baltic birch, marine-grade)works well for sub duty is that it's much stiffer than MDF and resonates at a higher frequency. With decent bracing it's easy to push the resonance out of the passband of a sub. It's also much lighter than an MDF cab of equivalent stiffness.

MDF is more "damped", but resonates lower in frequency.

With the relatively small potentially resonant surface area in a sonotube-based sub compared to a box, it's probably not a big deal, especially if tied together end-to-end.

 

[Jedd]

Bottom line:
if I put t-nuts into 24"x24"-square 3/4"-birch (it has 13 layers in 3/4"), glue with a 3/4"-MDF 22"-dia-circle, using yellow glue, and flush it into 22"-tube, so MDF circle goes inside and birch square stays outside it should work fine?
Or I better add another 1/4"-ply circle inside and put t-nuts in it?

MDF is more "damped", but resonates lower in frequency.

Does it mean that glued together with a baltic birch it wouldn't resonante at all?

 

[Jack Gilvey]

Does it mean that glued together with a baltic birch it wouldn't resonante at all?

It wouldn't elimiate it completely, although I'd have to think such a lamination would attenuate resonance to the point of inaudibility.

 

[Robin Smith]

I am going to be making my first DIY project soon, and surprise surprise its a sonosub.

If I were to use two layers of 3/4" MDF (one inside, one outside), could I simply cut ~1" squares of 1/4" plywood and glue them to the inner layer of the MDF at the points where the T-nuts need to go? Instead of using a whole layer of 1/4 ply.

Is there any advantage or disadvantage to this? I'd love to here opinions.

Jack, while Jedd hasn't answered your question, I would assume he is using the baltic birch for looks and would thus want it on the outside. For that matter, is there any reason you can't use one layer of 3/4" MDF inside and an outer layer of ANY kind of attractive wood?

Thanks

Robin

 

[Patrick Sun]

I'm giving the 1x1 squares of plywood a try for the sub I'm building this week as well. I see no drawbacks to this method.

 

[Jack Gilvey]

Robin,

Like Pat, I don't see why using squares of ply instead of a whole layer wouldn't work just as well, it's only needed under the t-nuts.

As far as laminating another hardwood to MDF, I usually see recommendations against it. MDF is very dimensionally stable, as is ply (although to a lesser extent), and a hardwood might tend to expand and contract at a very different rate causing the caps to possibly split and separate. I guess it depends on the specific wood and size of the area.

 

[Jedd]

could I simply cut ~1" squares of 1/4" plywood and glue them to the inner layer of the MDF

That's a good idea! I'll probably go this way too. (could save on weight a bit and return plywood back to HD )
By the way, how do you usually finish baltic birch? I bought some wood edging for it, what else do you use?

 

[Robin Smith]

Good point Jack, you are probably right. Norm Abrahm would flog me just for suggesting it.

My plan is to go two layers of MDF on each end. One inside and one outside. I am going to route a recess for the driver into the outer layer so it sits flush. I will go with the 1" squares for the T-Nuts then since it has essentially now been given thumbs up (or at least not thumbs down) from two of the DIY masters (that's you Jack and Pat, BTW!)

Good luck Jedd!

 

[Jack Gilvey]

Jedd, although I see the acoustic properties of BB as its main attribute, it's really beautiful also, and I like the idea of a nice finish on it (can't offer specifics, haven't done it). I'd also leave the edges exposed, they're very nice looking, unique among plys.

 

[Jedd]

I'd also leave the edges exposed, they're very nice looking, unique among plys

Looks like a plywood with 13 layers to me: one light, one dark, one light, one dark..

 

[Patrick Sun]

I spent this afternoon making those 1" squares of plywood for the t-nuts I'll be using. I used a power jigsaw to make the squares, and power drill to drill in the appropriate sized holes for the t-nuts.

The trick is to lay out the holes on a usable size of plywood, drill the holes, and THEN use the jigsaw to cut the plywood into the squares. This does require being careful in the hole placement in relation to the edge of the squares so that the edges don't hit the driver near the cutout of the driver.

 

[Jack Gilvey]

Looks like a plywood with 13 layers to me: one light, one dark, one light, one dark..

All birch though, no cheap stuff or voids like regular ply.

 

[Jedd]

Pat, why not to glue squares first (and who told us they should be square)? 1"x1" is big enough to not miss it with a drill. Or drill MDF first, glue square and finish the drilling.

 

[Patrick Sun]

You have to be careful with plywood and how close the hole needs to be w/r/t the edge of the plywood. If you are impatient and just drill with a 1/8" pivot bit, and then the required width bit, you may rip through the plywood material and be left with a messy block that's glued to the MDF. It's best to use multiple successively larger drill bits to get to the right width bit in order to minimize the splintering of the plywood.

I plan on using the screws screwed into the t-nuts while the glue dries to ensure the holes line up correctly with the driver and/or plate amp. If you have a drill press you don't have to take suck precautions, but I do DIY on a budget, so I have a few extra steps in my process when building stuff.

 

[jeff lam]

I took the advice of many here and used threaded inserts instead of T-nuts for my driver mounting. They seemed like they would be much stronger and really grab the wood much better. I used the 1/4" screw-in ones from HD and some 1/4" machine screws. They are solid!