So, If the internal bracing needs to be ply, why not the speaker baffles or the other panels? It is not as though the structure is like a steel framed building where the walls just stop the wind blowing through and people falling out. The whole box acts as the structure.

Hitting a material with a hammer simply tells you how it reacts to being hit with a hammer. The point I was trying to make is the fact one material is not quite as strong as another doesn’t by default make it unsuitable. If the loads it can sustain are greater than the loads imposed on it, then it will be fine.

Johan gives us a figure for Young’s modulus, but doesn’t give any values (4 what???) It is only about 4.2 to the nearest star but I wouldn’t like to walk. No explanation regarding the implication of Young’s modulus is given either, except that bigger is better. On that basis we should perhaps be using aluminium which has a value of 69 or even better stinging nettle fibres with a value of 87.

The units for the values given are GPa or gigapascals or if you prefer KN/mm². The imperial value for MDF is 580,000 lbft/in^2. So is 580,000 beefy enough?

Young’s modulus for those that didn’t do physics tells us how elastic a material is. It is the ratio of stress to strain.  Rubber can have values as low as 0.01 GPa, yet if you hit it with a hammer it probably won’t be damaged but the resultant bounce of the hammer might damage your head. Young’s modulus tells us how much a material will stretch before it breaks. Johan is correct in that less stretchy is better, but like in all engineering problems, the question is how unstretchy is sufficient?

If we need to know what load will break the material we could try the tensile strength. According to the MDF manual, it has a tensile strength of 0.7 N/mm²  or 0.7MPa. So how does this compare with other materials? Concrete has a value of 3MPa, Glass fibres have a value of 3500, carbon fibres 6300.

0.7 doesn’t seem a lot which would tend to confirm that it is not a good material to use. So given its common use in loudspeaker cabinets why don’t we see more falling to bits?

First tensile strength just tells us the strength when the material is in tension. If the load is compressive then the value is different. I couldn’t find a value for MDF but concrete which has a tensile strength of 3MPa can have a compressive strength of up to 40MPa.

Also the cabinet is a compound structure. A box or tube is able to withstand bending loads far better than a simple piece of material with an equivalent cross section.

Going back to simple, and using the figures above, we can consider an experiment. The brace in the cabinet looks about 260mm in length and is I presume 18mm thick. That gives us a cross sectional area of 4680mm². If we suspend this and hang a load off the bottom, it should be able to support 4680 x 0.7 = 3276N. For those not familiar with Newtons it is the equivalent to 334 Kg force. So the brace should be able to support the full weight of the cabinet.

Given that GPa is a unit of pressure and we are discussing loudspeakers we can look at the actual pressures of the sound. 0dB our reference level is 2 x 10^-5Pa while 130dB is 63Pa. Note that these are Pa and the values for materials above were MPa or  Pa x 10⁶.

As I stated previously just coming on and stating that ply bracing is better than mdf is likely to lead down the slope to where at some point in the future, because it has been repeated so many times, that you can’t use mdf for internal bracing. Look how many people think that you need 4 bass horns for them to work properly without understanding that it is actually the combined mouth area that matters not the number of horns.

note that values are published averages. Any individual example of a material might differ from these values.

Tony.A.S.S. wrote: My only comments were that Ply would be more solid, am I wrong, but hey what do I know?

I don’t know what you know, as I said before, the ability to successfully build and sell loudspeakers doesn’t require much knowledge about the theory behind it. Maybe you know a lot maybe you don’t. I am just replying to what you wrote.

The problem is, that by posting

“One thing I would say about the bracing though, MDF had very little Integral Strength, so although acoustically you can get away with the material for the main build, Plywood bracing would be more solid.”

it gives the impression (at least to me) that using MDF for the bracing is inadequate. If that is the case my question is why is it inadequate. Either explain theoretically or point to lots of examples of MDF braced cabinets that have structurally failed.

mini-mad wrote: wow!... you lot really will argue about anything wont you? LOL ..give it up and admire the OP's effort and wood working skillz!

As Tony W said this is more a technical discussion, or if you prefer argument then it is a connected series of statements intended to establish a proposition.

Perhaps you are not bothered about how things work. I am; I am curious sometimes rhetorically. The actual answer, although it might be important, is less interesting than the how or why. The latter are more important if you want to do something original.

If you design or build something there should be a reason behind every decision. They don’t always have to be technical. The OP used MDF because as he said, that is what he had. Large manufacturers sometimes tend to work round what packs well in trucks.

Take for instance the use of Birch ply in the construction of cabinets.  It seems to be an accepted assumption that it is the best material. But ask yourself why? Most, if honest, should answer because everybody else uses it, or that is what brand X uses so it must be the best. How many actually consider what properties make it a good choice and explore whether those properties can be found in other materials.

Not applicable in this thread, but if someone is unwilling to explain why they believe what they say, or write, why should they expect me to believe them? I know sometimes it is difficult to put things in words, but most of what is discussed on this forum is not new so just a link or reference to a paper or book would be useful. Trying to work everything out from first principles every time would be pointless.

The other problem of simply stating things is that they get repeated and repeated, taken out of context and repeated and so on. After a while, statements like the one above turn into the common knowledge that you can’t use MDF for bracing cabinets because it has no strength.

colint wrote: TONY.A.S.S. wrote: My only comments were that Ply would be more solid, am I wrong, but hey what do I know?   Sod all by the look's of thing's, I mean it's not like you've built many cabs is it

The number of cabinets built is irrelevant to the discussion. If I build a million cabinets with ply bracing and none fail, I can argue that from empirical evidence that ply bracing is suitably strong. It would be wrong to infer from the experience that MDF is not strong enough.