MDF vs. Plywood

[gamalot]

Great info here!
I use the hand circular saw for ruff cuts with the 2X4 supports as well. Fortunately my table saw can handle 35" to the right side so I can clean up most cuts.
I bought the lock miter bit to construct 2" X 2" posts I use for legs in making mission oak furniture. I use quartersawn white oak and have all faces with the fleck exposed. It does a great job and is a very intricate set up proceedure getting both male and female to mate propperly. I am just not sure yet how this might work in plywood so some experimenting must be done. With solids I can usually sand out inaccuracies in the fit but with veneered ply this won't be an option.
Butt joints might end up being the way to go but then I would miss out on all this fun in the shop trying to figure out different techniques.
I have also seen a router bit that does a butt joint with a 45 at the end which could also be worth trying. My reason is simple, trying to get all end grains to match.

Gary

[MikeInNH]

Take a small piece of plywood and put one end to your ear and tap the other end with your finger. Repeat this with a piece of MDF or particle board. With the MDP or particle board the sound should be significantly less. In building a speaker you don't want to ADD sound. Plywood will do that....even if you properly brace it.

The other problem with plywood is that plywood has voids between it's layers. These voids can also add color to the sound.

The question is...can you hear this added sound from the plywood?? I don't really know.

But if I'm going to spend the time and money to build a nice speaker I'd want to make it sound the best I possibly could. I'd hate to build it out of plywood and then find out I have a problem.

As for adding veneer....You're going to have to add veneer anyways using plywood..at least on the endge. Not going to look nice if you don't.

Building the box....I like to use a standard butt joint with a biscuit joiner. The biscuit joiner makes for a strong joint, but it really helps in aligning everything up easily. Then I brace the corners with 2x2's. And screw into the 2x2's from the outside. Then before I apply my veneer I fill in all the screw holes with a epoxy filler, then sand smooth and apply the veneer. I use to use the veneer that you apply contact cement to. Now I use the kind you Iron on. It's about 1/32" thick. It looks GREAT and easy to work with.

[Old Guy]

Different opinions I guess. I've got to disagree that plywood properly done necessarily adds more coloration. Resonance is a function of stiffness and mass mostly. MDF is heavier. So it is easier to get it less resonant.

But good quality plywood has few or no voids.

I just don't care for MDF, and like the dust less.

Different strokes for different folks.

[MikeInNH]

But good quality plywood has few or no voids.

<font size="2" face="Verdana, Helvetica, sans-serif">Never ever found plywood that doesn't have any voids. Even the best plywood made will have voids in 10% of it's whole volume....As stated from the plywood manufacturers. They try to avoid (pardon the pun) them, but 10% is about the best they can do.

I know for a fact that plywood will add coloration....How much and at what volume level I don't know. If it's a insignificant amount that can't be heard then fine.

The ONLY thing I ever like to use MDF for is speakers. Outside of that I find the product useless.

[GM]

Greets!

Right on! This is really a simple choice once you understand the physics of the situation. You ideally want the speaker cab's Fs to either be below or above its passband, so as we go down in frequency, the amount of mass required to keep its Fs below the passband increases, rapidly reaching concrete bunker construction requirements. Factor in that the lower its Fs, the more energy it stores, the wider its decay BW, ergo the higher its amplitude decay rate. From this we see that it potentially can 'color' it more than plywood, so it becomes the 'no-brainer' choice to push the cab's Fs either above the passband or at least as high as practical since it's easier to attenuate the higher Q, but fewer, faster decaying, lower amplitude resonances.

MDF specs is 'all over the map' depending on who's mixing it, but the last time I checked several years ago around here, its MOE (modulus of elasticity) was ~527 k psi/19mm, while Baltic Birch 13 ply/19mm was ~1.8 m psi. Stiffness goes up at the cube of thickness, so it takes ~1.125" thick MDF to match the BB ply's stiffness, so it obviously increases much quicker with bracing than MDF, ergo the oft used 'bracing' argument to justify using it isn't valid per se since it's a 'chasing your tail' situation and all you're really doing is mostly adding mass while not affecting its Fs much; so as a general rule-of-thumb, void free plywood or a similar/greater stiffness material is the performance choice for any cab with a BW that extends below ~250 - 400 Hz depending on the local 'mix'.

WRT the 'knuckle' test, knock on a concrete slab and compare that to a MDF cab someone deems 'dead'. The only thing you should hear is your knuckles cracking. So far, I've yet to find one that's 'dead' enough for a typical speaker app. My braced 3/4" marine grade 11 ply cabs have a metallic-like 'ring' to them when struck with a hammer, but tests at the time of construction showed no output above a typical room's noise floor.

'Dead' enough for me, but as always YMMV.

GM

[gamalot]

Just a question in line with the above.
Particle board, the type available at my local big box Depot. Any specs on this material.
I own a number of vintage speakers and all use at least some particle board in them.

Funny part about this discussion is that if I can build a box from available materials to sound as good as the speakers we know and love then this will be perfectly satisfactory for my ears.

I have often wondered if lining the interior of a cabinet with DuraRock would make for good sound. I know it would make for some mighty heavy cabinets for sure.

Gary

[quindecima]

When I did my speakers and also the current ones (pic soon Gary)) I knew a 45 angle was going to be tough to do in a garage shop. I rabbited all joined edges, clamped, glued and screwed them togeather much easier and very strong. I used a precision straight edge, worm drive skill saw and good ply blade, router with a carbide bit. Cover with veneer and never see the edge

[GM]

Greets!

I don't have any, but I'm sure your local supplier can get it from the distributor. Anyway, PB has come in myriad mixes over the decades as binders and processes have evolved and I seriously doubt they sell an exact cabinet grade equivalent today, though if you have a broken scrap, the manufacturer can analyze it and recommend the closest product.

Some DIYers use Durarock to damp MDF cabs and I've built outdoor speakers with it, but prefer to make cabs that don't need such a drastic 'bandaid'.

GM

[gamalot]

I plan to build 19 cabinets this winter.
Two seperate boxes placed one on top of the other with a seperator in between. Each box is open to the other!
Is there a specific purpose for this opening between boxes?

Gary

[speakerdave]

"I plan to build 19 cabinets this winter.
Two seperate boxes placed one on top of the other with a seperator in between. Each box is open to the other!
Is there a specific purpose for this opening between boxes?
Gary"

It adds quite a bit to the internal volume, enabling a few Hz of lower bass.

David