Hardwood baffle over mdf, what material between?

[dlneubec]

Hi all,

I'm wondering what you all would recommend to sandwich in between if you wanted to use a 3/4" hardwood baffle over a 3/4" mdf backer board to inhibit baffle vibrations, seal the cabinet, etc.

The baffle would be floating and held in place with something like socket head cap screws and hurricane nuts glued and screwed into the mdf.

Anyone have some suggestions?

[dlneubec]

Would something like this work?

Acoustic Dampening Foam

Would it help or just add cost over just using a ring of closed cell foam around the outside and maybe around any speaker openings?

[joecarrow]

The problem with 3/4 inch hardwood is that it can expand and contract with temperature and humidity, and it doesn't do so at the same rate in all directions. I could be wrong, but I think that not many people use such large sections of real wood in baffles.

If I were you, I'd consider making the cabinet as structurally sound as possible without the natural wood, and using the wood more or less as a thick veneer- using the thinnest possible pieces of bulk wood.

I had a set of speakers I made with natural birch baffles once, but I had them for less than 6 months before I had to move. I don't know how well they've held up.

[wkhanna]

Opps double post!

[wkhanna]

All I know is that some have used bitumen as a middle layer damping material inside their cabinets.

What size is the baffle? How many drivers & what sizes?

Would using a 1" or 1-1/2" inner MDF panel help, too?

[dlneubec]

The problem with 3/4 inch hardwood is that it can expand and contract with temperature and humidity, and it doesn't do so at the same rate in all directions. I could be wrong, but I think that not many people use such large sections of real wood in baffles.

If I were you, I'd consider making the cabinet as structurally sound as possible without the natural wood, and using the wood more or less as a thick veneer- using the thinnest possible pieces of bulk wood.

I had a set of speakers I made with natural birch baffles once, but I had them for less than 6 months before I had to move. I don't know how well they've held up.

Hi Joe,

Yup, I'm familiar with the theory on hardwood expansion and contraction, though in the controlled enviroment of a home, where their is AC and whole hous humidity control, etc. I have my doubts that those old woodworking adages are accurate. I've done a few speakers so far with hardwood baffles PVC glued to 3/4" mdf and have not had any problems yet, though they are only about 18 months old.

Still, I'm talking about providing somehwat for that expansion since the hardwood baffle sits on top of the mdf backer, with the dampening material in between, and the hardwood baffle is allowed to kind of float since it would be held only by socket head screws in oversized holes and hurricane nuts. The cabinets themselves will be structurally sound via lots of bracing, except that eveything is 3/4" mdf except the baffles, where I would like to double up the thickness. The hardwood allows me to cut some substantial chamfers without either very tedious application of veneer or requiring the baffle to be painted.

[Jed]

Hardwood is such a pain. I used 1" thick hardwood corners and it ended up warping enough to create some gaps between baffle and cabinet on my 3-way project. So I'm going to just redo the baffle with veneer/ including the chamfer edges in veneer. Luckily I tested this situation out first before attaching the baffle to the box.

I'm sure there are ways around the warping of solid wood, I just don't have tools to plane everything down when this happens so I'll be sticking to veneer in the future.

Jed

[dlneubec]

All I know is that some have used bitumen as a middle layer damping material inside their cabinets.

What size is the baffle? How many drivers & what sizes?

Would using a 1" or 1-1/2" inner MDF panel help, too?

Hi Wkhanna,

Baffle sizes vary. Two will hold a single RS225 (about 13"x15"-16") and one will hold a single RS52 (about 10"x12") in a waveguide. The 3/4" mdf behind the hardwood baffle will also be braced from underneath.

[JRT]

Hi all,

I'm wondering what you all would recommend to sandwich in between if you wanted to use a 3/4" hardwood baffle over a 3/4" mdf backer board to inhibit baffle vibrations, seal the cabinet, etc.

The baffle would be floating and held in place with something like socket head cap screws and hurricane nuts glued and screwed into the mdf.

Anyone have some suggestions?

I suggest investigating this "mineral filled viscoelastic polymer" (mass loaded vinyl?) to see if it would be suitable for use between the solid wood panels and the MDF.

http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=268-035

[dlneubec]

Hardwood is such a pain. I used 1" thick hardwood corners and it ended up warping enough to create some gaps between baffle and cabinet on my 3-way project. So I'm going to just redo the baffle with veneer/ including the chamfer edges in veneer. Luckily I tested this situation out first before attaching the baffle to the box.

I'm sure there are ways around the warping of solid wood, I just don't have tools to plane everything down when this happens so I'll be sticking to veneer in the future.

Jed

Hi Jed,

My application is with much shorter, but a bit wider baffle. The baffles would be glued up at the local hardwood supply shop, and then planed and sanded flat.

[Jed]

Hi Jed,

My application is with much shorter, but a bit wider baffle. The baffles would be glued up at the local hardwood supply shop, and then planed and sanded flat.

In that case I don't think you'll need any barrier between the two. Just glue and lots of pressure. Jim Salk and many others use solid front baffles- looks like they are glued down to me.

[dlneubec]

In that case I don't think you'll need any barrier between the two. Just glue and lots of pressure. Jim Salk and many others use solid front baffles- looks like they are glued down to me.

That's what I've done before.

In this case, it would allow the baffle to be removable, which is a desireable benefit. I.e. driver bolted to outside hardwood baffle, hardwood baffle bolted to mdf box. I make the hole in the mdf perhaps 3/4" larger in radius, so I can get larger things in and out of the cabinet and I can change drivers by just building a new baffle front, like with the PE cabinets, even jump up in size to a 10 incher if I wanted.

[Jed]

I like JRT's suggestion then.

[dlneubec]

I suggest investigating this "mineral filled viscoelastic polymer" (mass loaded vinyl?) to see if it would be suitable for use between the solid wood panels and the MDF.

http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=268-035

JRT and Jed,

I may go ahead and try this stuff and the product I linked above. This is about 2x more expensive per sq. ft. but I could do to speakers with one sheet, so not too bad.

My biggest concern with the mass loaded vinyl is that it is only .05" thick, which is about 1.3mm or less than 1/16" thick. I would take very little variation in the baffle suface, or warping of the hardwood and there could be gaps.

That's one reason I was thinking of the acoustic dampening foam I linked. I had an online chat with PE tech support and they thought it would work fine for my application. It is 1/4" thick, BTW, so it should work well as a sealer too.

Thanks for the discussion guys. I appreciate the feedback. If I try any of it, I'll report back what my impressions are.

[Amphiprion]

though in the controlled enviroment of a home, where their is AC and whole hous humidity control, etc. I have my doubts that those old woodworking adages are accurate.

I doubted it too. The resulting cabinet split at the seams. I was using an oak baffle though, and oak is bad about expansion (plus I live in Texas). But it was always in the same air conditioned room, and I always kept it at 74.

[PMazz]

Unless you know the moisture content of the wood you're using you can't necessarily predict what it will do. Properly dried wood will be ~6 to 8% moisture content. You can do a simple test. Crosscut an 1/8" slice off the end of a board, measure it and then move it into the environment it will live in for a few days. Measure it again to see how much it's changed. It helps if it's glued up to whatever width you'll be using prior to cutting.

Pete

[spasticteapot]

Hmm...I looked into some alternative materials for my cast-speaker project, and can make a few suggestions.

First, you could try expanding polyurethane foam. It's used for waterproofing canoes, and is somewhat squishy - expansion and contraction would be a non-issue.

Alternately, a mixture of concrete and micro-baloons (or crumbled up styrofoam) will do an impressive job of acoustic damping.

[JasonB]

In that case I don't think you'll need any barrier between the two. Just glue and lots of pressure. Jim Salk and many others use solid front baffles- looks like they are glued down to me.

If I remeber right, Jim Salk has a special method of attatching his solid wood front baffles that allows for some movement. He posted it once in response to a question similar to this one on one of the other forums. Might be worth sending him an email to ask.

Jason

[cfbuck]

Hardwood Baffles-(Long)

My first post here. PMazz is quite correct in IMO. Ensure that the lumber is between 6-8% or kiln-dried to begin with. Then, to minimize the movement, it is best to use quartersawn wood. If you look at the endgrain of the wood, quarter-sawn would have grain going at rightangles to the board. Most lumber is flatsawn-you see an arcs on the endgrain lines. Flatsawn wood is the least stable in terms of movement. In addition, it will warp in the opposite direction to the arcs described by the grain. So, if the wood you are using is flatsawn, place the highpoint of the arc against the speaker. This will keep the baffle outsides against the speaker with the tendency of the hardwood baffle centre to rise but the drivers should tend to hold it down in the middle.

Another thing that you can do, is glue a 3/4--1" wide wood edge, same species as the baffle, around the MDF baffle about 3/8-1/2" thick, with a roundover edge. When you have your hardwood baffle milled, rabbet its edge back so that it will fit inside the wood you have glued onto the speaker front, with a 1/8" gap around all sides. This will allow for the hardwood movement. Round over the edge of the wood baffle. Make no mistake, domestic flatsawn hardwood will move about 1/4" on a 12" board through the seasons. Therefore, make your bolt holes oversize to allow the wood baffle to move. Cheers, Fred

[Hank]

joecarrow beat me to it, but he's correct: hardwood of very much thickness is unstable due to its level of humidity-controlled drying, internal stess buildup and resulting long-term internal stess, humidity and temperature environment. 10-mil veneer is no problem, but thick hardwood is a waste of money and brings with it the physical problems. Hardwood also has a %-age of density less than MDF, and resonates. Lots of reasons to not use thick hardwood, including: how many rain forest trees does it take to make LOTS of 10-mil veneer versus THICK slabs of the wood?

[dlneubec]

Maybe you guys are right, but I'm sort of from Missouri on this one. :B

I have a center channel, two surrounds, two MTM's and my set of NaO Mini open baffle MTM's that are about about 1 year to 1-1/2 years old in my home, all using a 3/4" hardwood baffle pva glued to mdf, sealed on all sides and there is not one bit of problem evident with any of them of the things you are warning about. No movement that I can see, no splitting, warping, leaking, etc. Most of them use purple heart over mdf, but their is also some cherry and maple.

YMMV.

[PMazz]

Let's not forget that MDF also changes with humidity. The difference is wood expands/contracts across the grain (for the most part) and MDF in all directions. So if your baffle is relatively narrow (<12"), and the wood was properly dried, and you didn't subject it to radical changes in humidity, it should work just fine. Don't forget too that as a baffle with driver cutouts in it, you would most likely see problems first around the cutouts between drivers (when the wood shrinks too much) because the surface area of the glue bond at the edges is much greater.

Pete

[cfbuck]

Dan,

Like PMazz, I encourage you to use hardwood on the front baffle. I posted a rather poor text description of the technique that I use to allow for movement earlier.
I have been able to find photos of a project I just completed to better illustrate the textual description.

In the first pic you can see a batten applied around the edge of the cabinet. I used 1" wide x 3/8" thick wood batten with a 3/8" roundover.

http://www.execulink.com/~kandfbuck/BaseCab1.jpg

The outer baffle is sized 1/4" smaller all around the outside of the cabinet edge so it fits on the flat of the batten, just inside the roundover.

http://www.execulink.com/~kandfbuck/Usher03.JPG

The 3/4" thick outer baffle is cut back underneath each of the four edges, (rabbetted), so that it fits inside battens with 1/8" clearance
on all four sides. The rabbet is about 5/16"+ deep.
Thus, the back of the outside baffle sits off the MDF inner baffle slightly. The underside of the hardwood baffle rabbet rests on the outside batten.
The hardwood baffle is then glued to the MDF inner baffle with Liquid Nails which remains somewhat flexible.
In your case, you want to be able to remove the baffle so the "mineral filled viscoelastic polymer" described in an earlier post
would replace the Liquid Nails as the filler. The depth of the rabbet would be the thickness of the batten less the vinyl thickness
or a bit less to allow for some compression of the vinyl.

In my case, I take 1" rough lumber, bandsaw it in half and glue the two edges together to get a bookmatch.
This results in the hardwood being only 3/8" finish thickness. So I back it with a rectangular Baltic Birch ply which forms the rabbet instead of cutting it.
The BB ply is a bit more than 5/16" thick forming the rabbet.
If you look closely, you can see the BB ply. It is glued to the hardwood with PVA. It is scalloped back for driver air.

http://www.execulink.com/~kandfbuck/BCBaffl2.jpg

http://www.execulink.com/~kandfbuck/3WaySun2.jpg

Hope that this is a better explanation and a possible method for utilizing the vinyl damping layer between the two baffles. Cheers, Fred

Edit by moderator huge pictures changed to links

[dlneubec]

Fred,

Thanks for the detailed post and photo's! Very nice work.

I more clearly understand what you were suggesting now. I can see how the dampening material could easily fit with that concept. I'll have to see if your idea can fit in with my rather unconventional design.

[cfbuck]

Yeah, photos are so much better than words for a technical concept.

If I could add one more thing--the bolt holes through the outer baffle should be slotted. As PMazz pointed out, the hardwood will only move across the grain. As the baffle will remain centred, the wood will expand and contract a maximum of 1/8" so the slotted hole will take care of that. Just wiggle the bit at 90 degrees to the grain and ream it out parallel and the movement will be no problem, even if your outer baffle is all hardwood. Cheers, Fred

[Jonathan DA]

Dan,

You can probably accomplish what you want easily by using a flexible glue and decoupling your baffle from the cabinet. Use a fairly generous layer of Green Glue between your hardwood and MDF, and you'll probably end up with a nice constrained layer damping system that also allows the hardwood to expand and contract without impacting the soundness of the cabinet structure. If you go this route I would use T-nuts or inserts to attach the driver to the MDF portion of the baffle so the hardwood can expand and contract without affecting the driver mounting.

[dlneubec]

Dan,

You can probably accomplish what you want easily by using a flexible glue and decoupling your baffle from the cabinet. Use a fairly generous layer of Green Glue between your hardwood and MDF, and you'll probably end up with a nice constrained layer damping system that also allows the hardwood to expand and contract without impacting the soundness of the cabinet structure. If you go this route I would use T-nuts or inserts to attach the driver to the MDF portion of the baffle so the hardwood can expand and contract without affecting the driver mounting.

I'm not familiar with green glue. What is it and where can it be purchased?

[Jonathan DA]

https://www.greenglue.org

It's a viscoelastic glue that's perfect for what you want to do. In fact, I'm working on a new set of cabinets and I might try it as well just to see what happens.

[dlneubec]

Is this indeed an adhesive? I'd actually prefer any hardwood baffles to remain removeable, so I'm not sure how well that would work.

[Jonathan DA]

If you want the baffle to be removable, then green glue the hardwood to a piece of MDF and make a removable baffle where the MDF is inset so it can't be seen. Why do you want a removable baffle? That's just an invitation for resonance.