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Re: mdf or plywood?

This is my first post on this forum but have been building speakers and furniture for years. It amazes me that there is this growing consensus that plywood is a better building material for speakers than MDF. While it might be lighter and structuaraly stronger, I can't see getting the same accustics from it. The test for me is simple, pound on the side of a plywood box and a MDF box. MDF is ten times denser, hence the reason why it is used for speakers.

People seem to think that because plywood costs more, it is a better building material for speakers and I don't think that is true. To suggest that high end speaker companies use it because it is cheaper is just crazy. 1" MDF is about $50 a sheet and 1" ply is about $100 a sheet, that would easily make a set of speakers. I can't confirm this, but every high end speaker I have seen is MDF.

Now, using if for the reasons of easier to work with, that is a very good reason.

FYI- Almost all MDF is now made without formaldehyde making it not as dangerous as it use to be.

A picture of my Seas thor... BTW- they call for MDF in the building instructions.
 

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Re: mdf or plywood?

You are right MDF is far more dense, but cabinet grade ply being more stiff allows for a less resonant cabinet. That is why ply is superior, of course, if MDF is properly used a completely inert cabinet can result.
 

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Re: mdf or plywood?

You are right MDF is far more dense, but cabinet grade ply being more stiff allows for a less resonant cabinet. That is why ply is superior, of course, if MDF is properly used a completely inert cabinet can result.
By cabinet grade do you mean A-1? This is not what you find at your local home depot and basically it is the same wood, just better veneer on it. If you are going for superior, why not recommend 1" ply?

I will still disagree that is is superior.... Maybe I will do a test and build myself a sub box out of ply and compare them...
 

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Re: mdf or plywood?

By cabinet grade do you mean A-1? This is not what you find at your local home depot and basically it is the same wood, just better veneer on it.
13 ply cabinet grade ply can occasionally be found at at Lowes/Home Depot, but not always. At least, this was my experience being a former employee of Lowes.

If you are going for superior, why not recommend 1" ply?
Increasing thickness of a material is not an effective method of removing resonances. Rather, I recommend use of a viscoelastic constraint layer coupled with dense bracing for removal of resonances.
 

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Re: mdf or plywood?

...
People seem to think that because plywood costs more, it is a better building material for speakers and I don't think that is true. To suggest that high end speaker companies use it because it is cheaper is just crazy. 1" MDF is about $50 a sheet and 1" ply is about $100 a sheet, that would easily make a set of speakers. I can't confirm this, but every high end speaker I have seen is MDF.

Now, using if for the reasons of easier to work with, that is a very good reason.
...
I certainly wouldn't say that manufacturers use MDF only because it's cheaper. That's only one factor - though one of vastly more importance to a high production manufacturer than a home builder. I would say of equal importance are it's ease and stability of machining, and uniformity for applying a veneer.

I have seen several high end speakers that do not use MDF, at least for the primary shell of the cabinet. Although true enough, the overwhelming majority do.

I use plywood myself because of the lighter weight and the fact that it comes with a ready to stain exterior. Either plywood or MDF can make a fine cabinet. I believe the cabinet shape, as well as the internal structure, bracing, and stuffing, can all play a larger role in the final sound, than the wall material does.
 

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The Danish factory where the High End B&W 800 series are made use's A 35mm thick sheet of PLY wood pressed into shape + real wood veneer for the outer skin for the main cabinet body that hold the Larger woofer Drivers and Matrix internal bracing made from MDF....

But I Agree with Wickedrx7 that 1"-2" thick MDF is the superior product in many ways for Speaker Building,
IMO the biggest + for MDF is the finnish that can be obtained and ease to work with if you posses the Gear and wood working Skills as compared to PLY....

Cheers...
 

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I'd use MDF for sealed enclosures and plywood for ported. The possibility of "voids" in ply makes perfect sealing difficult. For ported enclosures the perfect sealing quality is not as important.
The mechanical linear strength properties of (either) material can be braced to reduce spurious vibrations to meet design minimums.
Over sized sub enclosure designs IE: LLT, IB, EBS, Reflex, folded horn, etc. would benefit from weight reduction using ply. Some of the over sized sub enclosure designs are able to use "sonotube", a cardboard like material shaped into tubes of various diameters, used in the construction industry to form concrete piers. No one that I've read yet has tested and determined the vibration or inert characteristics and quality of sonotube.
 

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Ive found that mdf seems to sound better and the walls of the box vibrate less. I have found that plywood is a lot more durable, finishes nicer if you do a stain, and is a lot safer on the lungs. Ideally I would do oak or birch plywood and just reinforce with cross braces on the inside. MDF dust is some nasty stuff (Formaldehyde).
 

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Re: mdf or plywood?

You are right MDF is far more dense, but cabinet grade ply being more stiff allows for a less resonant cabinet.
A stiffer material will raise the resonant frequency of the material while higher mass (MDF) will lower it. In either case if the requency is placed outside of the bandwidth of the system it is an effective solution. Of course bracing and other methods of damening help further.
 

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Re: mdf or plywood?

A stiffer material will raise the resonant frequency of the material while higher mass (MDF) will lower it.
Do you have any reference material on that? I have been trying to understand what variables affect how a material resonates.
 

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Re: mdf or plywood?

Do you have any reference material on that? I have been trying to understand what variables affect how a material resonates.
Any resonance can be thought of as a spring mass sytem. A heavier mass lowers resonant frequency while a stiffer spring raises it. Frequencies which occur near resonance are amplified and are out of phase thus distorting your sound. Bracing is a way of coupling two surfaces together in an attempt to counteract vibration.
 

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OK, a quick search answered my question and its sort of. The resonant frequency stays the same, but I am guessing the amount of energy needed to start it increases.

You answered while I was posting. Thanks. Am I right about the increase in required energy?
 

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Hmm. So, if you combine a lower mass material with a higher stiffness material with a damping layer between, you introduce a damping factor??
 

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OK, a quick search answered my question and its sort of. The resonant frequency stays the same, but I am guessing the amount of energy needed to start it increases.

You answered while I was posting. Thanks. Am I right about the increase in required energy?
Sort of, and it has to do with your second post about dampening factor. Dampening factor is basically the resistance to the mechanical motion from other forces. Think of it as drag. In the case of a speaker cabinet the primary source of dampening would be the internal resistance of the material. In an ideal spring mass sytem with zero dampening, amplitude would gradually work it's way to infinity at a rate proportional to energy input. Of course this can never happen in real life due to the mechanical limitations of the 'spring' and dampening.

For the debate of MDF vs Ply, dampening needs to be considered as the amplitude of frequencies near resonance will be dependent on this factor. Regardless, moving the resonant mode outside the bandwidth of the system will be the most effective method of reducing cabinet vibration.
 

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It is also important to note that even if a cabinet has no modal resonances within the passband it is possible for it to have significant, audible, acoustic output (resonances).
 

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It is also important to note that even if a cabinet has no modal resonances within the passband it is possible for it to have significant, audible, acoustic output (resonances).
Right. Outside of natural resonance, vibration drops off but does not disapear completely and the natural mode can be excited by harmonics, though at much lower levels. The amplitude of these vibrations is controlled by the dampening factor of the material but can more directly reduced through the use of bracing.

A cross brace running the full length of 2 opposing surfaces serves two purposes. For one, it cuts the cross section in half, and thus it's fundamental mode of resonance is doubled. For another, it couples two opposing surfaces which are each trying to move in opposite directions, thus cancelling any vibration less the elasticity in the brace (for all intensive purposes this can be assumed to be zero).

The major source of cabinet resonance in most speakers usually occurs around a few frequencies and is a result of cabinet construction that allows a natural resonance to be excited. Removing cabinet resonance has less to do with whether it is constructed from MDF or Ply than is does from proper sizing, bracing and shaping.
 

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Dampening factor is basically the resistance to the mechanical motion from other forces.
Duh. OK, makes sense. I didn't quite think it through the other night. I suspect you can increase resistance to vibration by coupling two materials of different resonant properties and stiffness together.

A cross brace running the full length of 2 opposing surfaces serves two purposes. For one, it cuts the cross section in half, and thus it's fundamental mode of resonance is doubled. For another, it couples two opposing surfaces which are each trying to move in opposite directions, thus cancelling any vibration less the elasticity in the brace (for all intensive purposes this can be assumed to be zero).
That leads to questions on the effective use of the material you have chosen. I know its ouside of the thread topic, but these two things are very closely related and should both be considered.

I have often wondered if, when given the choice of 1x1/2" brace vs 2x1/4 braces, if we would not be better served by multiple thinner braces.

That leads to another question. If you add mass via MDF to reduce a modal resonance by half, and then add a brace cutting the sheet in half thus doubling the modal resonance, are you not back to where you started (but a lot heavier)? I suspect not given that a greater mass will require greater energy to excite, but I wonder if you are limiting the effectiveness of your design because you would still have a modal resonance that will be excited by frequencies being produced by the driver.
 

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I have often wondered if, when given the choice of 1x1/2" brace vs 2x1/4 braces, if we would not be better served by multiple thinner braces.

That leads to another question. If you add mass via MDF to reduce a modal resonance by half, and then add a brace cutting the sheet in half thus doubling the modal resonance, are you not back to where you started (but a lot heavier)?

Good questions... Now that you've got me thinking about this some more I don't think adding a brace would halve the resonant frequency. Even though it makes sense from traditional wave propogation it doesn't fit nicely into the root(k/m) equation. Changing these distances would have to change the spring constant. Adding mass would alter the frequency according to that equation so doubling the mass wouldn't exactly halve resonance.

The more I've been thinking about this the more I think that a single point brace could be nearly as effective as one running the full length of the surface like what is traditionally done, giving back valuable internal volume and reducing weight and building complexity. In your statement, it would certainly be better to spread the braces out vs one thicker one.
 
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