[drphoto]

I been contemplating the Alpha kit for awhile and I was wondering if MDF is really the best choice for the cabinet. I read some place that the MDF has some resonace issues and plywood is really a better material. And if ply were used would the cabinet have to be so bloody thick and consequently heavy?

I suppose one issue w/ ply is that you couldn't do a round over edge, so maybe the baffle face could still be MDF.

[Andrikos]

I suppose one issue w/ ply is that you couldn't do a round over edge.

Whatcha talkin' about Willis?

[azryan]

I’ve heard Birch plywood is really good. I think that’s the type of wood that Andrikos’s plywood cabinet’s made of? No?

In the case of the Alphas the bracing goes from top to bottom/front to back glueing face to back top and base. It’s pretty much impossible to flex at any point. Imagine an I-Beam.

And MDF is so easy to work with.

You can roundover plywood but it’s much smoother easier to do it with MDF.

Side walls can be braced as much as you want. Per Danny I put in 5 wooden dowels to cross brace.

The cabinet’s VERY hard. 1.5" MDF and fully braced and filled with the fat Black Hole 5 damping sheets on the walls.

You could use Corian to make a cabinet. It’ll be a fortune though and super heavy and difficult to bond the walls to eachother. IMO a total waste to time. Way overkill knowing the end result of what MDF does.

First place to cross brace IMO is dead center of the side walls. This is where the most flex would otherwise occur. I  spread the other 4 dowels out from there but the corners are going to be where the least flex is so no need for a brace a few inches from the top or bottom of the cabinet.

You could add even more dowels or MDF strips, triangle corners, etc..

The two side walls are bonded together though as I have it. They just can’t hardly vibrate at all even when hammering the deepest of bass and the rest is even stronger/harder.

Remember the Alphas have 4 flared ports per cabinet too which relives air pressue vs. a sealed cabinet.

MDF I believe is non-resonant and why it's been so popular by nearly all speaker comapnies. -but someone can correct me here if I’m wrong.

It can flex under pressure but that’s not a ‘resonance’.

Plywood can flex too. I mean that’s usually the good thing about plywood in other applications -that it’ll flex instead of snap. Very rare use in speaker cabinets though.

I don’t think you can use thinner plywood and make a lighter cabinet that is as strong. The rock hard walls is all in the bracing IMO.

I’m not sure what you mean about the Alpha’s being so thick? The cabinet’s external dimensions would remain the same no matter what you use.

It's a kit though so you can do anything you want.

[Christof]

It is possible to purchase furniture grade plywood which has no voids located in the inner layers.  This would be recommended for this type of cabinet work.  Baltic or Finnish Birch would be ideal and I would look for the type containing many layers, perhapse 13 ply.  From what I have read there is a little hype about using plywood vs. mdf in that the different layers in plywood and their slight density differences should, in theory, be more helpful than a solid material.  

I do not think there is an advantage to MDf const. over plywood, sonically speaking.  MDF is easier to work than plywood and takes veneer better.  It is, as far as I'm concerned, a misconception to build heavy cabinets and expect them to be superior just based on the fact that they have so much mass.

Search http://www.diyaudio.com  Andrikos has some input going at this thread:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=32131&perpage=10&highlight=&pagenumber=1

christof

[azryan]

“-furniture grade plywood - - This would be recommended for this type of cabinet work.-“

Why?

“-From what I have read there is a little hype about using plywood vs. mdf in that the different layers in plywood and their slight density differences should, in theory, be more helpful than a solid material.-“

Is this your personal opinion why plywood is better?

I can’t tell if you’re even saying plywood’s better? And then you say ‘there’s a little hype’ about this? I don’t get your position. Sorry.

“-I do not think there is an advantage to MDf const. over plywood, sonically speaking.-“

Neither IMO have a 'sonic advantage' but one’s a LOT cheaper and easier to work with -but you’re saying that too but your other comments sound like you endorse plywood over MDF?

“-It is, as far as I'm concerned, a misconception to build heavy cabinets and expect them to be superior just based on the fact that they have so much mass.-“

Do you think anyone said that a heavy cabinet means better? It seems like you do ‘cuz otherwise I don’t get why you said that?

But no one said heavier’s better so I'm really not getting you?

You mention Andrikos's comments on DIY.com.

Note, his speaker is not a GR Alpha -though it kinda looks like it is.

He added a damping sheet between layers of plywood and has no bracing inside but only the woofers are in a cabinet so it's very narrow. I'd still say he should have put a brace of som kind in the center of his cabinet but I can't say for sure.

His cabinet's also sealed I think?

I think it’d be much more helpful to specifically relate your opinions to the Alpha cabinets. The point of the thread question.

[shokunin]

I'm no expert in speaker building, but I am an avid woodworker.  MDF is dimensionally stable, more so than plywood (baltic, finnish, russian).  I read somewhere than MDF has a lower resonance than other materials and thus is ideal for speaker cabinets where the resonance would be in bass region and not the critical midrange on up.  plywoods have a slightly higher resonance that could potentially affect lower midrange performance.

both are easy to work with, but MDF dust creates a royal mess when routing.       just like plywood there are various grades of MDF.  The type you find at the bix box stores (hd, Lowes) seem to be of lower quality that others that I have bought at cabinet and hardwood dealers.

[Charles Calkins]

Solid core and veneer core plywood reflect sound waves. MDF is a dead material thus it does not reflect sound waves.

[azryan]

MDF is a dead material thus it does not reflect sound waves.

Wha? So it's a scam to have polyfil, fiberglass, open cell foam, black hole five, etc inside a cabinet 'cuz the internal sound waves from the drivers hit the MDF which does not reflect sound therefore it can't come back to hit the drivers?

I don't think so.

Fairly dead/ fairly non resonant isn't the same as 'doesn't reflect sound'.
Otherwise a transmission line wouldn't work either.

[Tyson]

Pretty much any solid material will resonate if energy is applied.  The question is at what frequency does the resonance occur.  MDF does indeed resonate at a lower frequency than no-void ply.  Which means that most speakers work better with MDF, but subs are often better with no-void ply.

[Agrippa]

Of course MDF reflects soundwaves - any solid material does.  The reason MDF is so popular, is that 1) being "constructed" from fibres and glue, it's soft and therefore resonance will only occur in the lower frequencies (as stated earlier by shokunin); 2) it's probably the easiest material of any to work with and 3) it's cheap - all of which are of prime importance for a speaker maufacturer.

In terms of resonance, there are certainly better materials available, such as HDF (High Density Fibreboard) which has a greater density, while not shifting the resonance-point much upwards, and various more or less exotic types of wood. Plywood, being "unintentionally" acoustically dampened due to its construction from several thin boards and glue, is certainly as good as MDF, but more expensive and far harder to work with.  Also, its exact properties will vary according to what kind of wood it is made from.

All in all though, what's the point in NOT using MDF?  Its cheap, it's easily available, its easy to work with and it's good enough for just about any speaker manufacturer on the planet, be they low-end or high-end.

Agrippa

[shokunin]

I think more importantly would be how the cabinets are constructed and how it's internally braced to help reduce resonances in key areas.    MDF will reflect sound waves, as others have pointed out.   HDF is another option, although it's harder to find and even heavier than MDF.

MDF is used by many because as Agrippa points out that it's cheap and easily availabile.   MDF is also more easily milled on a CNC machines with less tearout than plywood.  

For DIY that's is less of a concern, but is another reason why MDF is used.

[DSK]

I agree that MDF is cheap, readily available, minimally resonant, and easy to work with, thus a great choice for speaker enclosures.

Although routing MDF is easy (eg. a nice round over on each edge),  the routed edge will have a fibrous texture rather than the sealed finish on the faces. In my experience it is very difficult, regardless of sanding, to get this edge sufficiently smooth enough (even when using MDF primer) to get a seamless painted finish across faces and edges. However, I am no expert on painting and perhaps there is something I'm missing? Should we use some sort of filler or sealant on routed MDF edges before the primer?

The 'inner' MDF box and 'outer' ply box seems to be a great way of getting wall thickness/rigidity and providing something better than MDF for mounting screws to grab into, as well as providing an outer surface that can be stained/varnished etc better than MDF. I would have thought that a decent quality ply (eg. void free marine ply or hoop ply) would have provided a more 'closed'/smoother routed edge that would be far better for finishing. However, posters above have suggested that it is difficult to route ply ....why? .... does it tend to fracture or burn? Does the routed ply edge need any special treatment before staining/varnishing etc?

[jjb]

Simple test: Take a 12x12 square of MDF and a 12x12 square of high grade plywood, raise each square 12" above a table with each corner being held by a brick with felt between the brick and test square. Use a sound meter 24" from test square and drop a weight (anything will do) measure the difference. I am certain you will from this point forward build only with MDF. (No comments, or opinions until you try the test, as they would just be unfound opinions)    MDF/

[shokunin]

Although routing MDF is easy (eg. a nice round over on each edge),  the routed edge will have a fibrous texture rather than the sealed finish on the faces. In my experience it is very difficult, regardless of sanding, to get this edge sufficiently smooth enough (even when using MDF primer) to get a seamless painted finish across faces and edges.

Painting MDF requires a bit prep to get an excellent finish, as you have noticed the routed edges are fibrous and in order to get a smooth corner you will want to fill in the edge with joint compound or use a good high-solids primer.

If you use joint compound it is easily sanded afterwards and can then take a layer of oil-based of shellac-based primer.  I never get good results using a water-based primer on MDF as the primer coat.  The water based stuff tends to absorb into the MDF and raises some of the fibers making it difficult to get a nice smooth finish.  Use an oil-based or shellac based primer and your choice of oil (if you used oil primer) or water based paint.  For smooth finishes, I'll usually spray a colored lacquer finish to get the best gloss look.

[DSK]

Painting MDF requires a bit prep to get an excellent finish, as you have noticed the routed edges are fibrous and in order to get a smooth corner you will want to fill in the edge with joint compound or use a good high-solids primer....

Will the edges then be as solid/hard as the faces or still a bit soft and prone to damage from knocks?

Also, for butt joint and face to face gluing of MDF and ply to each other, is there any advantage to Gorilla Glue, ployurethane glues etc over normal white PVA wood glue?

[shokunin]

DSK,

All the mdf edges will be prone to 'damage" but  you shouldn't worry too much about it.  don't use the poly glues on MDF, use PVA glues like Titebond I, II, or III.  The poly's like Gorilla Snot, Excel, etc need the presence of moisture to cure, which the MDF has little to none.  The poly's are good when you need it to be waterproof or when gluing exotics like cocobolo, ebony, & teak.  Even then, i've had mixed results with Gorilla glue, I prefer epoxy (WEST, G1/G2) when I need a watertight lasting bond.

Besides the foam created by the poly glues would be a pain to clean up inside a speaker cabinet.

[DSK]

Shokunin,
Thanks for the advice, very handy.

OK, so MDF can be finished better than I have done in the past. However, my concern is that it doesn't offer a very secure base for mounting drivers or amp (ie. on subwoofers). Wood screws or threaded inserts (for machine screws) would both be better off going into ply. So, for this reason I think I would go with the MDF inner box and ply outer box.

Also, wouldn't routed rounded over edges be more solid on ply than on MDF and not require joint putty etc to try to smooth the edges? And ply edges would not be so vulnerable to damage from knocks etc?

[azryan]

-However, my concern is that it doesn't offer a very secure base for mounting drivers or amp (ie. on subwoofers). Wood screws or threaded inserts (for machine screws) would both be better off going into ply. So, for this reason I think I would go with the MDF inner box and ply outer box.-

That's true that it's 'better' than MDF for holding in screws but in the case of subwoofers you'll find most people rec. you use T-nuts or similar Hurricane nuts and bolts into the cabinet. When you do this there's no diff. between MDF and anything and no way the bolts can 'pull out' 'cuz they're gripping the steel T-Nuts not the MDF (or plywood etc..)

FAR better too if you need to get inside the cabient ever again too 'cuz you can take the bolts out as many times as you want with no damage to anything.

I recently had to do this to my subs 'cuz I had to re-wire 'em from 4 Ohms into 16 Ohms when I added 2 more subs to my room for a 4 Ohms total at the amp (crazy I know but I already had all the stuff and still very cheap).

For speaker cabinets- say the Alphas to get back to the main topic... just painting the cutouts before you mount the drivers is usually enough to keep the drivers in place. Not that I'm saying screws aren't needed, just that I think your concern isn't needed.

I'm sure many can tell you about unscrewing a driver to take it out and finding it bonded to the cabinet and it being a real bitc- uh... real tough hehe to get out.

Of course on the planars there's no pressure of any kind on the screws and the ones GR provides (along with the woofer screws) work perfectly  and look very nice.

-Also, wouldn't routed rounded over edges be more solid on ply than on MDF and not require joint putty etc to try to smooth the edges? And ply edges would not be so vulnerable to damage from knocks etc?

Plywood would be possible a little strong to guard against dings, but I think this isn't an issue for most people.

Most people don't run into their speakers too often nor bang 'em in to any walls.

Either of those would damage the surface paint or veneer you used to make the speaker look nice so plywood's not a solution to an MDF problem here.

Plywood's not going to have a perfectly smooth roundover edge either. You're going to be exposing cross-grain layers in the roundover. Could get little splintered bits, etc... No big deal, but no reason to think it'll be dead on smooth vs. MDF rounded edge IMO.

If you are using the plywood as your veneer finish you'll want to put in sepp. solid wood roundover corners to make this look good 'cuz for sure an exposed plywood roundover won't look good.

If you intend to paint both plywood or MDF will need a little prep work to assure smooth painted corners.

And actually plywood having a grain to it's whole surface will IMO be worse for painting than MDF which has a perfectly slick/smooth surface, but both can work.

IMO, just no advantage to plywood.

[shokunin]

As azryan mentioned, plywood is even harder to get a nice smooth edge, due to the alternating layers of end grain and edge grain in the plys.  It's very tough to get a smooth edge on plywood because of that.  You're better off using MDF.  T-nuts should be used to grip into the MDF versus using wood screws to hold the drivers.

[DSK]

in the case of subwoofers you'll find most people rec. you use T-nuts or similar Hurricane nuts and bolts into the cabinet. When you do this there's no diff. between MDF and anything and no way the bolts can 'pull out' 'cuz they're gripping the steel T-Nuts not the MDF (or plywood etc..)...

This is why I mentioned 'threaded inserts' as they sit flush with the surface (better than T-nuts), have an outer thread for gripping into the cabinet, and take machine srews (bolts) so that the driver can be removed an infinite number of times.

I read elsewhere that T-nuts don't grab into MDF very well and I have come across a number of comments from people that used ply for the 'outer' box for this very reason. Are you guys suggesting that this is unnecessary? ...or are you suggesting to drill right through the baffle and mount the T-nuts on the inside of the baffle?