I have listened to tests of various cabinet materials, and I always thought MDF sounds best in those tests, as in neutral. I assumed it is partly because of it's porous nature.
I'm aware of MDF's porosity because my CNC router has a vacuum table, which works by pulling air through an MDF spoil board. To prepare the spoil board I face off both sides of the MDF, because the surface of MDF is denser than the middle. Once faced off, it is quite porous.
I guess someone would have to do some experiments to prove this, but I suspect an MDF cabinet with a sealed interior would have more resonance than unsealed? Because sealing the MDF prevents energy from dissipating into the material.
I'm aware of MDF's porosity because my CNC router has a vacuum table, which works by pulling air through an MDF spoil board. To prepare the spoil board I face off both sides of the MDF, because the surface of MDF is denser than the middle. Once faced off, it is quite porous.
I guess someone would have to do some experiments to prove this, but I suspect an MDF cabinet with a sealed interior would have more resonance than unsealed? Because sealing the MDF prevents energy from dissipating into the material.
Think about paint, bitumen sheet reinforcement, foam, and what they do to porosity.
The MDF pores would be blocked.
The MDF pores would be blocked.
FYI, MDF doesn't have pores...I guess that's obvious, but I wasn't implying that air has to move from the inside of the box to the outside of the box. What I believes happens is that the pressure wave hits the surface of the MDF and gets diffused as it passes through the matrix of fibers and air spaces within the MDF.
It didn't seem relevant to specify that there was an outside coating, since every speaker gets some type of coating that effectively seals it. (But most coatings are actually porous on a microscopic level, though that isn't strictly relevant to this conversation.)
It is just the way MDF is. But that said, saying "MDF" is like saying plastic--there are multiple types of MDF. There is the cheap hardware store variety that most people think of, there are moisture resistant types, and there are types designed for machining. The density of the material, the type and sizes of the fibers, and the type of binders used can all vary. I expect they will all be somewhat porous, but how much will vary by the specific product.
My knowledge of the different types comes from currently not being able to buy the normal cheap MDF I am accustomed to using. Turns out there are major shortages of the stuff right now, and the "MDF" currently being sold at the hardware store is completely different from what I expect, and just doesn't work for me.
But this experience isn't speaker related. I'm machining molds on a CNC router. Or I would be, if I had MDF!
Molds for what item?
Consider wood filed composite, high density fiber, cement binded particle board and rice husk board as alternates.
Consider wood filed composite, high density fiber, cement binded particle board and rice husk board as alternates.
From a pure physics perspective, a rough porous surface like MDF is what you want as the "lossy" surface produces a low Q-factor which greatly reduces the amount of stored energy. As the waves bounces back and forth between the walls of the cabinet, the only way to lose energy is to 1) loss through the air 2) loss at the cabinet surface. There is no other way to reduce stored energy.
MDF is definitely porous, especially if you machine one or both of the faces off. You can make a spoilboard for a CNC router from MDF, skim a mm off the top and then pull enough suction through the board to hold parts down on it! That's non-trivial airflow.
But for speakers it's fine.
Personally, I coat the finished boxes inside and out with sanding sealer to prevent absorption of moisture and swelling. No discernible or measurable change in performance.
But for speakers it's fine.
Personally, I coat the finished boxes inside and out with sanding sealer to prevent absorption of moisture and swelling. No discernible or measurable change in performance.
I'm most likely to graduate to urethane tooling board if I can't come up with the right MDF soon. It is the right material to use, but it's quite a bit more expensive.
I invented a hard shell knee cap/ knee pad that I'm making out of carbon fiber. And I'm working on making production tooling.
I think it's worth adding that facing off only one side of a piece of MDF will cause it to warp. I think this is due to the wood fibers on the surface being compressed. When both surfaces are intact, the pressures are balanced. But remove one side, and it curls toward the surface that was removed. This warping is also non-trivial! (Ask me how I know....)
Wow- Really!? I mean I believe you because I've never 'defaced' MDF but that's still gotta be a lot of suction!
In any case, 100% agree with your sentiments- there will be no discernable or measurable differences in performance for 'sealed' vs unsealed MDF boxes.
That's not to say sealing is not preferred and ideal to keep any wood from swelling/deforming over time.
Its only about a 10mm MDF sheet that is vacuumed down. Vacuum pump is large, spinning +400mm disc that will hold down a spoiler sheet of MDF and then vacuum your part down to stop it moving when routing. Spoiler sheet is so you don't damage the main steel bed when cutting out your part.
MDF is fine to use in speakers as long as thick enough in the walls and proper bracing. Like mentioned above l normally seal the inside and outside of the enclosure with a wood sealer to stop the mdf moving and cracking your paint along joins. I would still do this even if l was going to veneer them.