John,
As far as I can see (I have a couple enclosures made of it), MDF has a single constant density, and no multiple layers. A layer has a clearly defined boundary. Plywood is clearly a multi-layered material.
There are certain types of chipboard that have a layered structure (thin MDF + chipboard + thin MDF)
A variable density material (If I understand correctly what you are saying) has no layers either.
Please, explain a little better why you think MDF is a layered material...
Gastón
As far as I can see (I have a couple enclosures made of it), MDF has a single constant density, and no multiple layers. A layer has a clearly defined boundary. Plywood is clearly a multi-layered material.
There are certain types of chipboard that have a layered structure (thin MDF + chipboard + thin MDF)
A variable density material (If I understand correctly what you are saying) has no layers either.
Please, explain a little better why you think MDF is a layered material...
Gastón
ghpicard said:
Hi,
MDF has a thin outer layer (the smooth stuff). Although it seems to be single homogeneous "board", it really isn't.
MDF is pressed into a sheet form, the result of this is that the outside faces of the sheet are more compressed than the centre, where the material ends up being less dense. Admittedly, the differences are small, but they do exist.
Anything made in this way, with two more rigid layers with a lower density core is a sandwich layer construct.
Another more obvious example is particle board, where you can actually see the different densities.
Plywood, though layered, is closer to a homogeneous "board" than MDF is, as it has a uniform density though out. Each layer is of the same material and is solidly glued to the next layer. This makes for a strong panel with no room for internal movement.
MDF, with it's less dense core, will allow some internal movement. This results in better damping as mechanical energy converts to heat through this movement. Similar to constrained layer damping.
AFAIK it has slight layering properties, but not many, and the boundaries are not especially sharply defined. Anything's better than nothing of course.
Re the stiffness, the average Young's Modulus values that I've seen (a good general guide) for the materials run like this:
MDF: MOE=0.53, Density=48
Chipboard: MOE=0.54, Density=29
Fir ply: MOE=1.2, Density=38
Definitely don't be quiet John.
Nobody wants you to do that, myself least of all. Your contributions are most interesting (though personally I try not to stir things up as it's not my place to do that), especially when you're presenting a carefully reasoned argument, agree with it or no.
Re the stiffness, the average Young's Modulus values that I've seen (a good general guide) for the materials run like this:
MDF: MOE=0.53, Density=48
Chipboard: MOE=0.54, Density=29
Fir ply: MOE=1.2, Density=38
Definitely don't be quiet John.
Nobody wants you to do that, myself least of all. Your contributions are most interesting (though personally I try not to stir things up as it's not my place to do that), especially when you're presenting a carefully reasoned argument, agree with it or no.Hi,
there seems to be so much confusion in posts and between different posters that is resulting from many different terms being used.
I thought about not posting, but changed my mind.
Here goes.
Plywood always has the two outer plies oriented in the same direction. These two plies dominate the strength and stiffness (and elasticity) characteristics of the board. Rotate the test sample through 90 degrees and you will find just how much those two plies contribute to the longitudinal stiffness.
On the same tack those two plies are thin layers of good quality timber with few flaws, they were chosen for the outer layers just becuase they have few flaws. This flawlessness (horrible word) gives the elastic properties of wood as the dominant characteristic and also gives much of the lack of damping inherent in plywood.
Chipboard, without any special facings attached, is very non homogeneous. As said earlier, it is visually layered with close packing and good void fillng in the outer layers resulting in a fairly smooth flat outer surface. The core is much less densely packed and the low glue content results in a high void ratio, making both gluing and screw fixing problematic. Most of the stiffness and strength comes from the outer layers, just like plywood. However the random orientation of the fibres in the wood chips results in a stiffness and strength that is virtually independant of board orientation. This same randomness results in a strength and stiffness being much less than an equivalent thickness of plywood in the longitudinal direction.
Does anyone have comparative figures for board stifnesses and strengths for the various orientations?
MDF is virtually homgeneous. Random fine fibres bonded, badly, with glue. This randomness makes it very similar in many respects to chipboard. This board is less stiff than plywood and it is less strong. MJ's stiffness test is perfectly valid and would give useful results.
All these boards when assembled into cabinets will operate within their elastic limits, until somebody has an accident with one of them. There will be no plastic deformation when music is played.
Now consider a box, forming a cube, made from each of these equal thickness board materials. The two random board types will have stiffnesses that will be substantially identical in any direction, but probably different from each other. All the panels of the box can and will deflect due to pressure changes inside. I don't know which of chipboard or MDF will deflect more.
Plywood on the other hand will gain most stiffness from the longitudinal ply direction and this may result in slightly less defection than the two random board types when we are considering a cube. I bet these three boxes will sound different.
I will also bet that the MDF will sound most dead and the ply box most alive. I believe this difference in sound will be substantially down to the differences in damping between the materials and much less so to stiffness and certainly not due to strength. Some of the difference in sound will be due to the difference in weight resulting in different resonant frequencies of the panels.
Is any of that wrong?
there seems to be so much confusion in posts and between different posters that is resulting from many different terms being used.
I thought about not posting, but changed my mind.
Here goes.
Plywood always has the two outer plies oriented in the same direction. These two plies dominate the strength and stiffness (and elasticity) characteristics of the board. Rotate the test sample through 90 degrees and you will find just how much those two plies contribute to the longitudinal stiffness.
On the same tack those two plies are thin layers of good quality timber with few flaws, they were chosen for the outer layers just becuase they have few flaws. This flawlessness (horrible word) gives the elastic properties of wood as the dominant characteristic and also gives much of the lack of damping inherent in plywood.
Chipboard, without any special facings attached, is very non homogeneous. As said earlier, it is visually layered with close packing and good void fillng in the outer layers resulting in a fairly smooth flat outer surface. The core is much less densely packed and the low glue content results in a high void ratio, making both gluing and screw fixing problematic. Most of the stiffness and strength comes from the outer layers, just like plywood. However the random orientation of the fibres in the wood chips results in a stiffness and strength that is virtually independant of board orientation. This same randomness results in a strength and stiffness being much less than an equivalent thickness of plywood in the longitudinal direction.
Does anyone have comparative figures for board stifnesses and strengths for the various orientations?
MDF is virtually homgeneous. Random fine fibres bonded, badly, with glue. This randomness makes it very similar in many respects to chipboard. This board is less stiff than plywood and it is less strong. MJ's stiffness test is perfectly valid and would give useful results.
All these boards when assembled into cabinets will operate within their elastic limits, until somebody has an accident with one of them. There will be no plastic deformation when music is played.
Now consider a box, forming a cube, made from each of these equal thickness board materials. The two random board types will have stiffnesses that will be substantially identical in any direction, but probably different from each other. All the panels of the box can and will deflect due to pressure changes inside. I don't know which of chipboard or MDF will deflect more.
Plywood on the other hand will gain most stiffness from the longitudinal ply direction and this may result in slightly less defection than the two random board types when we are considering a cube. I bet these three boxes will sound different.
I will also bet that the MDF will sound most dead and the ply box most alive. I believe this difference in sound will be substantially down to the differences in damping between the materials and much less so to stiffness and certainly not due to strength. Some of the difference in sound will be due to the difference in weight resulting in different resonant frequencies of the panels.
Is any of that wrong?
Actually I have heard very good things about "a huge amount of bracing" in combination with MDF.
The following link points to a pdf in Danish, but hopefully the pictures will say enough. If not then ill try to translate parts of it for you.
http://www.steenduelund.dk/download/Duelunds_hifi4all.pdf
Its a 2-way speaker, I know. But it fits this discussion well.
AndrewT said:
The strength axis of plywood is a good point, no one has thought to bring that up yet. Although, it is not as much of an issue with the multiply stock, such as Baltic birch.
For the most part, I think your assessment is right on the money, Andrew.
I did some testing here:
http://www.diyaudio.com/forums/showthread.php?postid=1214560#post1214560
That demonstrates, for a small sized panel, not much of a difference exists. The MDF box emitted less sound during the panel test than the plywood box did (3db higher noise output).
Considering that a well built box will have effective bracing that will essentially divide the panels into smaller ones, my test results are valid.
Differences between these materials exist, but can be reduced/negated with good construction techniques.
Source rather than one. It seems difficult to dig out such information and very few appear to be comprehensive for wood products. Obviously there's room for individual variability, it being a natural product (you can apply that to sheet materials, ply, MDF etc too for that matter -how many times do you see moisture content etc stated for those?) but an average would be useful. Oh well.
Regretably no, they didn't do so in both axis IIRC, though as John points out, the strength axis is far less of an issue with multi ply stock like baltic birch, apple ply etc. Best to keep the external grain down the long axis IMO, but I doubt many people would want to be trying it the other way in any case, if only because the aesthetics would be a bit strange.
Large amounts of bracing are fine -you can't have too stiff a box; problem is, things can get extremely heavy, especially with large cabinets, so I'd always go for something that's naturally stiffer & requires less bracing.
Regretably no, they didn't do so in both axis IIRC, though as John points out, the strength axis is far less of an issue with multi ply stock like baltic birch, apple ply etc. Best to keep the external grain down the long axis IMO, but I doubt many people would want to be trying it the other way in any case, if only because the aesthetics would be a bit strange.
Large amounts of bracing are fine -you can't have too stiff a box; problem is, things can get extremely heavy, especially with large cabinets, so I'd always go for something that's naturally stiffer & requires less bracing.
layering, diffuse or distributed
The glue that binds the layers of plywood together is stronger than the wood it connects. I suggest the glue lines act as transmission lines for energy movement. This leads me to believe that energy travels further in plywood before being converted to heat or radiated out of the material.
Mdf is layered...soak a piece in water to find out...but the boundaries are not so well defined as in plywood. I suggest the energy transfer in mdf is more diffuse than in plywood. Can I also say it is SLOWER? Generating more heat per unit distance travelled?
I think both materials require bracing above a certain panel size (throw in frequency range also). Can I suggest that the different natures of the two materials will require different bracing schemes, and that suitable enclosures may be made from both of them?
The glue that binds the layers of plywood together is stronger than the wood it connects. I suggest the glue lines act as transmission lines for energy movement. This leads me to believe that energy travels further in plywood before being converted to heat or radiated out of the material.
Mdf is layered...soak a piece in water to find out...but the boundaries are not so well defined as in plywood. I suggest the energy transfer in mdf is more diffuse than in plywood. Can I also say it is SLOWER? Generating more heat per unit distance travelled?
I think both materials require bracing above a certain panel size (throw in frequency range also). Can I suggest that the different natures of the two materials will require different bracing schemes, and that suitable enclosures may be made from both of them?
Re: layering, diffuse or distributed
Perfectly logical Ed -they will. The MDF will need somewhat more, for a given panel thickness, especially with large areas.
The glue does form a layer, but the idea of it being a tl for energy movement doesn't really take into account the presense of the multiple boundaries and the losses encountered as vibration attempts to travel through it. Interesting thought though, as the speed of sound does vary with the material.
If Peter Comeau (ex Heybrook & Mission) will forgive my quoting his blog on the subject:
"The first I knew of [MDF] was when my cabinet maker for my Heybrook speakers rang me up and suggested we try it in our speakers. [they were using particleboard] Wary of any change to my designs I requested a sample. The results, when we made a pair to listen to, surprised me.
In a word they sounded awful. Gone was the full, lucid bass. Gone was the open, informative midrange. Gone was the sweet, sparkling treble. What could have happened?
I didn't find out until much later ...I started a project with Plymouth University to create the 'perfect' speaker cabinet material. One of the first investigations I did with the University was to study why MDF 'sounded' bad. We quickly discovered that the resonant structure of an MDF panel has only a few, high Q (sharp and strong), resonances at midrange frequencies. In fact this resonant character is so strong that the output of the cabinet can swamp the output of the drivers at these frequencies. Imagine taking the overall energy from the driver, transmitted both as vibration through the cabinet and from the trapped sound in the enclosure, and re-emitting it over a narrow range of midrange frequencies. Don’t forget that the ear is most sensitive in the midrange. Then you will start to understand why MDF cabinets have a strong acoustic character.
It is possible to overcome this. By using extensive cross-shelf bracing you can cut down the level of resonance. But it is difficult to shut those midrange resonances up completely."
This is for somewhat larger panel areas than John used in his own most valuable tests, which explains the differences in the findings. Note that Peter isn't a massive fan of ply himself -he prefers a chipboard / particleboard & MDF combination.
Ed LaFontaine said:
Perfectly logical Ed -they will. The MDF will need somewhat more, for a given panel thickness, especially with large areas.
The glue does form a layer, but the idea of it being a tl for energy movement doesn't really take into account the presense of the multiple boundaries and the losses encountered as vibration attempts to travel through it. Interesting thought though, as the speed of sound does vary with the material.
If Peter Comeau (ex Heybrook & Mission) will forgive my quoting his blog on the subject:
"The first I knew of [MDF] was when my cabinet maker for my Heybrook speakers rang me up and suggested we try it in our speakers. [they were using particleboard] Wary of any change to my designs I requested a sample. The results, when we made a pair to listen to, surprised me.
In a word they sounded awful. Gone was the full, lucid bass. Gone was the open, informative midrange. Gone was the sweet, sparkling treble. What could have happened?
I didn't find out until much later ...I started a project with Plymouth University to create the 'perfect' speaker cabinet material. One of the first investigations I did with the University was to study why MDF 'sounded' bad. We quickly discovered that the resonant structure of an MDF panel has only a few, high Q (sharp and strong), resonances at midrange frequencies. In fact this resonant character is so strong that the output of the cabinet can swamp the output of the drivers at these frequencies. Imagine taking the overall energy from the driver, transmitted both as vibration through the cabinet and from the trapped sound in the enclosure, and re-emitting it over a narrow range of midrange frequencies. Don’t forget that the ear is most sensitive in the midrange. Then you will start to understand why MDF cabinets have a strong acoustic character.
It is possible to overcome this. By using extensive cross-shelf bracing you can cut down the level of resonance. But it is difficult to shut those midrange resonances up completely."
This is for somewhat larger panel areas than John used in his own most valuable tests, which explains the differences in the findings. Note that Peter isn't a massive fan of ply himself -he prefers a chipboard / particleboard & MDF combination.
Nearly forgot to mention -if anyone's interested in the BBC's research findings & you haven't run across them, you'll find Harwood & Mathew's paper here: http://downloads.bbc.co.uk/rd/pubs/reports/1977-03.pdf
Scottmoose said:Source[s Obviously there's room for individual variability, it being a natural product (you can apply that to sheet materials, ply, MDF etc too for that matter -how many times do you see moisture content etc stated for those?) but an average would be useful. Oh well.
.
Hi!
Moisture should be the same as wood. In the furniture industry one has to have economy in mind. Lower than 5% moisture and the lacquer/ paint consumption increases. Especially when waterborn paint is used, also the fiber rising will be worse.
Two of our customers churns out 5 million sqm, of MDF based furnitures per year, so have quite good wiew of consistency of MDF.
I think most of my aversion against MDF comes from all the problems I get from customers using lousy MDF.
It exists good MDF board on the market BUT my insights in manufactoring of boards tells me otherwise. The different wood used in production of MDF varies due to accessibility + bark content should also be minimized. Last mention is not true according to me.
I'm not judging the whole industry but the ones I've visited.
Peace brothers!
Good points all Peter. I wasn't focusing on a single aspect, merely mentioning the moisture content as an example -as I say, how many times in sheet materials do you see detailed specs like that (or others) cited? Almost never. Yet they suffer from the same quality variability as natural products.
As it happens, if I couldn't have ply, for whatever reason, I would favour particle/chipboard over MDF any day of the week. Peter IIRC found that a particleboard cabinet with an MDF baffle (because it was easier to mount the driver on) worked for him, for his commercial floorstanders.
Just for fun, here's a thought for the day from our departed friend Terry Cain: "They’ll never make an airplane from spars of MDF."
As it happens, if I couldn't have ply, for whatever reason, I would favour particle/chipboard over MDF any day of the week. Peter IIRC found that a particleboard cabinet with an MDF baffle (because it was easier to mount the driver on) worked for him, for his commercial floorstanders.
Just for fun, here's a thought for the day from our departed friend Terry Cain: "They’ll never make an airplane from spars of MDF."
peterbrorsson said:
But carbon fibre & aluminum do have potential for boxes... with all the aluminum oxide on the moon, i imagine one day manufacturing facilities at L4 or L5 that will "grow" very complex organo-metal cabinet structures using nanobots in a bath of "nutrients and raw materials" in micro-gravity.
dave
Metal boxes? Now you're talking. Serious efficiency. 12 - 14ga CRS should do according to GM, and he should know. Incidentally, is it just me who's thinking that some of our high-tech materials (resins etc) seem to be trying to aquire the properties of a certain natural product called wood? PR and marketing gone mad.
Nope, I'm not a fan of metal for regular dynamic drivers. They can be good but are often more trouble than they're worth IMO as their breakup modes aren't pretty. Carbon has some real promise; I haven't heard a stellar pair yet, but the material itself should be good & nicely damped. When it comes to compression units, a whole different set of considerations comes into play of course.
Nope, I'm not a fan of metal for regular dynamic drivers. They can be good but are often more trouble than they're worth IMO as their breakup modes aren't pretty. Carbon has some real promise; I haven't heard a stellar pair yet, but the material itself should be good & nicely damped. When it comes to compression units, a whole different set of considerations comes into play of course.
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