MDF vs. Plywood

[Russ Farrell]

Intotubes:

Tech Letter 245, page 7, lists displacement volume of components. I couldn't find the TL on line after a very quick search.

416-8B: .25 cu. ft.
802, 808: .025 cu. ft.
811B: .19 cu. ft. behind flange

FWIW enclosure materials are discussed on page 14 of the following:Loudspeaker Enclosures Their Design and Use (1974 version)

I asked the acoustic engineering guys (Mark Ureda & others) about why we used this stuff instead of plywood. They felt it was better acoustically, and also happened to be cheaper. They did use ply for some front panels and for boxes that were meant to be moved around. Could be wrong, but in this case cost didn't seem to be the determining factor. They did use graded density stuff for some enclosures. The particles were bigger and packed less dense in the middle than on the outsides to give better control over a wider bandwidth. It cost more than fixed density board but I don't know by how much. Then I asked about baltic birch. The reaction was sudden and severe. Cost made it a taboo topic at Altec.

[zelgall]

I'm with Gearjammer on this one. I have model 19's and a pile of altec drivers and horns. I thought I could improve on the box while maintaining the volume by building a pair with baltic birch plywood. The result was not similar in sound. I don't know why, just that with the same internal volume and the same components the sound was both bass shy and less defined by comparison. I did however enjoy using them in the backyard for the summer.
I found the same with my 620 boxes with 604-8gs. Veneered particle board for altecs may be the way to go for original sound.

[intotubes]

Thanks for all the great info. I may have to rethink my choice of Baltic birch ply and go with particle board! I wonder if "graded" board can still be found? It makes sense to my feeble mind about the wide absorbance(sp?) factor of the graded density.

[gamalot]

I wish I had kept better photos of my original 19s purchased in 1976. I was almost certain the front panel was ply and all other panels where particle board.

Just my opinion here but I seriously doubt particle Vs Baltic Birch ply is the reason for such drastic sound differences.
Port tuning probably accounts for more of the end result then the materials used when the box is changed from it original and exact design.

The best test would be to build one with PB and one with BB in the exact 19 dimensions and then see if there is a noted difference in sound.

I do think the PB cabinet would weigh a great deal more then the BB cabinet but would expect the BB to hold up much better over the years.

Gary

[bowtie427ss]

They did use graded density stuff for some enclosures. The particles were bigger and packed less dense in the middle than on the outsides to give better control over a wider bandwidth.

<font size="2" face="Verdana, Helvetica, sans-serif">This exactly describes the HD cabinet stock at my favorite lumber yard. I'm pretty sure it's a product of Canada. It becomes coarse in the middle, and as you move toward the outer surfaces of the sheet, the particles become finer and much more densely compressed to a very smooth and almost glossy appearing surface. This non uniform density is not evident from the factory edges, they are some how filled and closely resemble the large surfaces. I try to treat all of my cut edges with wood glue to hopefully "seal" it, since the core is a sponge for moisture. Another advantage is it's 49x97 inch oversized dimensions which allow you to layout a full 4x8 sheet and not have to allow for any kerf.

The bad points are it's overall strength, or lack there of. It bruises very easily on corners and edges, even lower quality plywood does not.

I recently trashed a pair of 1234 cabinets, both had MDF backs, both had their "towel bar" handles broken/pushed thru the backs, i don't believe that would have ever happened with plywood backs, and being the back of the box, i find it difficult to believe Altec used MDF for anything other than economy in that instance. The remainder of those boxes were built from plywood.

MDF is heavy, but if you can find genuine marine grade 13 ply baltic birch, it is just as heavy, any weight difference between the 2 is negligable. However, lower grade fir and pine plywood is far lighter, as much as 30 to 40 percent. My nearest source for genuine 13 ply void free (yes it is void free, if you hit a void they'll take the sheet back and replace it) is 130 miles away, and at last quote was $118.00 per 4x8 sheet. HD cabinet stock is 26 bucks a sheet and 10 miles away.

The other point i'd like to make is that genuine 13 ply is the only sheet product i'm aware of that is a HARDWOOD, all other plywood, MDF, HD cabinet stock, and particle board are made of conifer/softwood.

For the particular project i have upcoming, i'll be building four 20 cu ft boxes. Obviously there will be large panels involved and material cost is definitely one of the considerations. My "inner battle" revolves around the idea that if there is an audible difference between materials, then boxes of this size would surely demonstrate it. I'd hate to think that i allowed economy to be the cause of failure or my displeasure of the finished boxes.

[gamalot]

Another consideration when useing the particle board is waste.
Unless you plan to veneer the entire box after building it with birch ply you will have to pay strict attention to grain patterns and cut the sheets to keep grains going correctly. This might cause considerable waste in unsed cut offs where the grain is not going the right way.
With the PB just cut away for maximum panel yields and know you will be veneering all sides.

I am not advocating useing one material over another but at $118/ sheet it sure will hurt when you have 40% of it unusable.

Gary

[zelgall]

I've never been able to get 4x8 sheets of baltic birch ply. It seems to come in 4x5 or 4x4 sheets most of the time. When I built my 19 knock offs I kept the same volume but made the sides straight instead of having the recessed part between the upper and lower sections as well asadding bracing to the cabinet. I don't feel that this would make a difference in the overall sound of the box. The important part is the volume, not the shape. The bottom line is that the original Altec 19s sound better than the baltic ply boxes that I built, even with the same components and crossovers.

[intotubes]

zelgall, did you use factory XO's or did you build your own? I'm a little surpirised you heard sucha large diff between your boxes and factory if volume, components and porting were the same.

[Old Guy]

The logic escapes me- you made changes- but the reason they sound worse is they are made out of plywood?

Bottom line is the cabinets you made don't sound as good as the originals. That's all that can be proven.
Again- whoever wants to use MDF- go right ahead. I'll continue to use well braced plywood.
Different strokes. The world would certainly be boring if we were all the same.

[Altec19]

The only speaker that sounds like a Model 19 is a Model 19.

<font size="2" face="Verdana, Helvetica, sans-serif">Amen.