Enclosure walls construction for 3012LF Neo

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Next subject … Porting the beasts. I routed a 1/2" deep by 3/4" wide recess, around the 12" x 7" square I cut across the back of my bass bins, right at the very bottom. This allows for bolt in port tuning. I can play around with different tuning set ups, to see what sounds the best to me. I made it large enough to accept a 6" PVC port …… But my question is this. Any reason at all to use a PVC port, instead of just making it slot loaded, since it can be either way with the modular port system?
 
I have not used BB since it is hard to find where I am at any reasonable cost.

If you want BB just for structural qualities (strength, consistency) and/or the nice end grain, you can buy this format for half the price of BB that has a wood veneer.
Birch Filmface Plywood (Black / White) | Plyco
Plyco are Vic based, but you might be able to get something like it in Sydney (e.g. Gunnersens sell 12mm Birch formply, and they have resellers everywhere)

Even without the veneer, Birch is still overpriced here. Local (eucalyptus) structural plywoods with similarly nice grain & the same strength rating cost about $30 less per sheet.

Secondly, in the past I have laminated 5mm hardboard (masonite or similar) to the back panel of mdf

I've tried hardboard (I got a couple of packs of a really dense, high gloss flooring material for $0) as a backing for 19mm timber. It seemed good, but the weight was pretty serious! I never finished the (fridge sized) project because I thought the weight might literally kill me. Poor planning, that.

This has lead to my preferred format of, glued 12 mm mdf, laminated with 5mm hardboard.

If you're making a laminate anyway, consider braceply.

Most structural plywood (including BB) that I see for sale is rated F14 to F17, so the strength:weight is roughly 4 times as good as MDF (example numbers here:
MDF vs. Spruce-pine-fir :: MakeItFrom.com)

Cheap braceply is F27, which is almost double that.
2440 x 1200mm 4mm F27 Hardwood Plybrace | Bunnings Warehouse

...this means that an 8mm layer of braceply (8^3 * 27 = 13824) would be about as stiff as a 15mm layer of MDF (15^3 * 4 = 13500).
 
But you can still hear music playing through the walls just slightly.

I once built a bass box into a structure (a corner firing quad 15" - to get the LF boost from corner loading, without the hassle of a bass horn). The 'lid' of the bass box was my bed (in a separate room to the sound system).

Original lid: pine 2x4 bracing, 18mm MDF, topped with a futon. I could clearly feel the bass rumbling through this.

Modified: added second layer of 2x4, poured concrete between them, and topped it with a layer or sheetrock. After that, I could still feel the bass, but it was damped considerably.

My take home message: brute force has diminishing returns.
 
Next subject … Porting the beasts. I routed a 1/2" deep by 3/4" wide recess, around the 12" x 7" square I cut across the back of my bass bins, right at the very bottom. This allows for bolt in port tuning. I can play around with different tuning set ups, to see what sounds the best to me. I made it large enough to accept a 6" PVC port …… But my question is this. Any reason at all to use a PVC port, instead of just making it slot loaded, since it can be either way with the modular port system?
I'd think you'd need to run some sims in winISD to see where a good starting point may be.
 
If you're making a laminate anyway, consider braceply.
I have seen the Bunnings braceply and it is not void free. The sheets seem to warp quite easily in storage. Now formply is a totally different animal. I use it for my workbench tops (see my previous photos) and it is very stiff and remains straight. It is designed to be used as concrete formwork so it can resist high forces and humidity without deforming. Unfortunately it can't be easily glued or painted. The phenolic surface coating is designed to resist adhesion.
Most structural plywood (including BB) that I see for sale is rated F14 to F17, so the strength:weight is roughly 4 times as good as MDF (example numbers here:
MDF vs. Spruce-pine-fir :: MakeItFrom.com)
Cheap braceply is F27, which is almost double that.
2440 x 1200mm 4mm F27 Hardwood Plybrace | Bunnings Warehouse
The stiffness properties have to be taken in context. The two other relevant properties are mass and damping. High stiffness just moves the resonances higher in frequency. They will still need to be damped. High mass means the panels move for longer when excited. The worst material I ever used was cast concrete. It was very stiff and massive but rang like a bell.
...this means that an 8mm layer of braceply (8^3 * 27 = 13824) would be about as stiff as a 15mm layer of MDF (15^3 * 4 = 13500).
My concern would be that the braceply tendency to warp would pull the mdf out of shape. Even when glued up, the warping forces might exert an unwanted force on the box joints.
 
I have seen the Bunnings braceply and it is not void free. The sheets seem to warp quite easily in storage.

Maybe they've changed supplier since then?

I have a stack of the reddish hardwood stamped F27 (which Bunnings resells from Gunnersens), and this is good: it cuts cleanly, doesn't warp, and it even looks nice - it is made for utility, but most sheets have large sections that are flawless.

Now formply is a totally different animal [...] Unfortunately it can't be easily glued or painted. The phenolic surface coating is designed to resist adhesion.

I hadn't noticed that was your workbench material :)

Yea, it seems to be an oddly overlooked (cheap-but-good) material. Particularly for PA boxes - you can leave it in the rain ~forever.

Polyurethane glue seems to work well on it. I score the gluing surface a little, but I dunno if that's needed.

I assume PVA would work fine if you cut through the phenolic layer (e.g. in a a build that used dado joints, like your picture in post 37).
 
I'd think you'd need to run some sims in winISD to see where a good starting point may be.

I have already done that, and have several tuning points already selected for comparing. My question was, Any reason at all to use a PVC port, instead of just making it slot loaded, since it can be either way with the modular port system?

Slot or round ported should sound the same, if tuned identically. I was asking about differences in the two, as related to my particular build, with the slotted recess, for interchangeable ports of ANY kind. Thanks
 
How did you arrive at those numbers?
F27 is a nominal stress grade, 27 is not the stiffness.

I haven't paid the ~$100 for the standards document, but I've seen it referenced in a few places. Stiffness is definitely part of the rating.

Timber Plus Toolbox, Assembling wall frames, Wall frame components, F grades and MGP grades

An F grade is a measure of the bending strength of a piece of timber. 'F' stands for force in megapascals (MPa), and is the amount of force a piece of timber can withstand without bending beyond an acceptable limit. A piece graded to F11, for example, will have a safe working stress in bending of 11 MPa.​

Structural grading | WoodSolutions

For example F8 timbers have the following properties (from AS 1720.1 Table H2.1):

f 'b = 22 MPa the characteristic bending strength
f 'c = 18 MPa the characteristic compression strength (parallel to the grain)
f 't = 13 MPa the characteristic tensile strength (parallel to the grain) - hardwoods
f 's = 2.2 MPa the characteristic shear strength
E = 9100 MPa the characteristic modulus of elasticity parallel to grain​

I note that:

1) Hoop pine is a standard softwood (maxes at F8 as plain timber) but is used in marine plywood (Aus standard for marine ply is F14 minimum), so stiffness must improve substantially when a wood is turned into plywood.

There must be a limit to this, because:

2) The highest F rated ply seems to be made from local species that are themselves very hard, like Spotted Gum and Ironbark.
 
..decent softwood ply has 4x the stiffness of MDF:
MDF vs. Spruce-pine-fir :: MakeItFrom.com.

But if the MDF is 1/4 the stiffness of ply then the correct number would be 27/4 = 6.75 .
Have you used the actual value of the MDF in the second comparison, by coincidence also 4 in SI?

Best wishes
David

Perhaps we should not clog this thread, and take this elsewhere if you are interested.
 
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Perhaps we should not clog this thread, and take this elsewhere if you are interested.

Sure, but I think I'm about done :)

I found a better info source for F rating vs. stiffness - see page 6 of this:
timber pdf

It shows that strength is 1:1 with the F-number, but stiffness only goes up by about 50% when the F-number doubles. My earlier assumption (and maths) was thus a bit off, but the practical difference is small*

For stiffness calculations, thickness of panel is cubed, but the material properties are linear. The cubing means you can make a weaker material just as stiff by using marginally thicker panels.

It is that only when you compare ply vs very low stiffness materials like MDF that the difference (in modulus of elasticity) is so extreme that the weaker material will add serious weight to a project.

*so you'd need 9mm of F27 to match 15mm of MDF. I thought you'd only need 8mm.
9^3 x 18.5 = 13.5k
15^3 x 4 = 13.5k

EDIT: if the link is odd, try searching for "AS 1720.1 boeing pdf"
 
I have already done that, and have several tuning points already selected for comparing. My question was, Any reason at all to use a PVC port, instead of just making it slot loaded, since it can be either way with the modular port system?
Slot ports will tune a bit lower for a given length than a cylindrical port, so may reduce length dependent pipe resonance problems.
 
Thanks for replying! I had decided I was being to OCD with that one. I Iprefer a slot, for looks alone. I personally do not believe a difference can be heard, if the two are tuned identically. I was just fishing for anything really cool or interesting. Have you seen the build thread?
 
... My earlier assumption (and maths) was thus a bit off...

It's still off, a bit arithmetically but more so conceptually - if you actually want to do this correctly then we can move it to Construction Tips, if you are "done" then no problem, the observation that stiffness scales as the cube of thickness is correct and tends to dominate in practise.
In the event that anyone else is actually interested to understand the concepts then start a thread and I'll try to explain.

Best wishes
David
 
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I have seen the Bunnings braceply and it is not void free. The sheets seem to warp quite easily in storage.

Here are pictures of the braceply I'm using. I'm tweaking an old box, doing a re-skin with a layer of 4mm brace ply, then 5mm of bamboo.

The bamboo costs literally 10x as much as the braceply. My GF saw the project halfway through the addition of bamboo - and she preferred the look of the braceply :(

As you can see, the end grain is fine, no voids. I've had some in storage for months, and it is still fine, unwarped.

The rest of the ply in the photo is some random stuff that I bought years ago because the end grain looked nice. It had a pretty tropical veneer which was a waste - irrelevant in this project. The darker section is waxed, the paler bit is where I've cut a new 45 degree chamfer.

EDIT - triple posted cos the forum kept giving an error message. I've removed the duplicates (I think).
 

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