18mm Baltic Birch vs 15mm Baltic Birch for speakers - stiffness

[Ultralight]

My understanding is that a flat sheet of material gets stiffer by the cube, and that for a speaker, the stiffer the sides the better.

I have a 15mm BB kit coming.

However, doing a simply calculation, 18mm would be about a whopping 75% stiffer.

Questions: If I wanted a stiffer cabinet but the kit can come only in 15mm for a floor standing speaker, would it makes sense to purchase another sheet of 1/8 or 1/4" thick Baltic Birch and glue it on the outside to make the body thicker and stronger? 15mm sounds awfully thin to me but then I'm used to seeing mdf type cabinet.

What do you guys think?

This will be my first build and using a flat pack.

Thanks,
UL

[planet10]

15mm BB is on the same order of stiffness as 1" MDF, and much less massive. So to bring the resonant structure of the MDF in line with the BB you'd have to add on the order of 2x as much bracing to the MDF. The MDF would still have much greater energy storage & retransmission.

For the original frugel-horn 12mm was specifies. 15mm for FH3 is just fine. If you really want to take it up a level, i'd suggest adding a layer of plastic laminate. Very stiff for the mass, a dissimilar material, and with an addiitonal glue layer. it becomes the finish. Just doing the sides with natural everything else could have a very nice aesthetic.

dave

[Ultralight]

Dave, this post has NOTHING to do with baltic birch or FH3 or Alpair 7 or Helper Woofer or anyone of the many other subjects I've queried on different boards from here to AudioCircle.

Here's my question: HOW in the world do you keep up with so many board, forums and threads. It doesn't seem to matter whatever or wherever I post, you are on it in short order. Is there actually half a dozen people masquerading as 'Dave' or Planet 10? LOL - I'm not kidding but I'm literally shaking with laughter right now at how you seem to be everywhere all at once.

As far as I know, only one Being is omnipresent but you give me that feeling sometimes. Thanks for giving me good laugh in the best sense. It just strikes me as impossible.

Thanks again. I'll look into the plastic laminate. No idea what that is but it WOULD look very very sharp. i can see it in my mind's eye - black plastic on the side.... or perhaps dark grey with a grey Alpair driver would look sharp!

Regardless, thanks Dave.

UL

[planet10]

Plastic laminate -- arborite, formica, more -- literally thousands of colours & patterns.

An example: Formica Corporation : Laminate

dave

[Saturnus]

Originally Posted by Ultralight

My understanding is that a flat sheet of material gets stiffer by the cube, and that for a speaker, the stiffer the sides the better.

To my recollection that's only if the number of plys is the same in both which is usually not the case. Still if I recall correctly, each 3mm jump in plywood thickness with the assumption that 2 extra ply is used will double the stiffness. For example 18mm 13ply is twice as stiff as 15mm 11ply which again is twice as stiff as 12mm 9ply.

[Ultralight]

Other than cost and weight, is there any drawback with trying to go stiffer? I'll probably stay with the original 15mm. But would like to know.

Thanks,
UL

[Loren42]

Or you can do what I am doing and use two layers of 18 mm BB with Green Glue between them.

[Loren42]

Originally Posted by Loren42

Or you can do what I am doing and use two layers of 18 mm BB with Green Glue between them.

Then you could change your name to UltraStiff...

[boywonder]

Originally Posted by Saturnus

To my recollection that's only if the number of plys is the same in both which is usually not the case. Still if I recall correctly, each 3mm jump in plywood thickness with the assumption that 2 extra ply is used will double the stiffness. For example 18mm 13ply is twice as stiff as 15mm 11ply which again is twice as stiff as 12mm 9ply.

For a given material, the stiffness increases with the cube (3rd power) of the thickness.........for an easy example, take a long 2x4 and place it between two saw horses laying flat, and compare it's stiffness when laying flat to when on edge.....then a 2x6, 2x8, etc.

Your "rules of thumb" are quite close; 18mm cubed is 5832, and 15mm cubed is 3375, and 12mm cubed is 1728. So 5832/3375=1.72 and 3375/1728=1.95 (not quite 2X for either of these, but 1.95 is close). Taking 5832/1728=3.37, so not quite 4X from thinnest to thickest.

[chrisb]

UL - there are a number of reasons why we settled on the combination of 15 & 18mm BB for the FH flat-paks, including the quite signification factor of shipping cost. The approx 20% additional weight isn't the biggest issue, but the combined dimensions of securely wrapped parts would exceed Canada Post's length / girth limits per package.

For those folks building their own from FH plan-set, I can't think of any downside to using 18mm material for the entire enclosure, except for yield. Be aware that with the last couple of revisions to dimensions, there's lots to spare on a 4x8ft sheet, but I was not able to get all parts from a single 5x5 sheet, and the optimization works out quite differently on a batch run of 15 pairs compared to a single.