Solid wood vs MDF

Discussion in 'Archived Threads 2001-2004' started by Jim Williams, Feb 22, 2003.

  1. Jim Williams

    Jim Williams Second Unit

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    almost all of the diy cabinets that I have seen use mdf. Is there any reason, other than cost, why you cannot use solid wood for the cabinet?
     
  2. Allen Ross

    Allen Ross Supporting Actor

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    the mosture in the air will flex, expand, narual wood, causing gaps, craking and just plan pain in the butt.

    there was a thread about the benifits of MDF a while back do a search and you will find all you need to know
     
  3. AllanRW

    AllanRW Second Unit

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    http://www.ellisaudio.com/hardwood.htm
    Read this.
    The very best speaker cabinets are made from MDF. This is because MDF is far superior to particle board, has very good internal damping, and machines nicely. But MDF can be improved further by the addition of hardwood. Hardwood is brutally stiff, and improves the appearance of the cabinet. IMO, even very good veneers fall slightly short of the rich/deep hardwood appearance.
    Most folks really like the look of hardwood on a speaker box. My favorite happens to be striped walnut. There are some problems with walnut, and all hardwood for that matter. The first problem is resonance. Resonance is good in an instrument, but bad in a speaker cabinet. The back of a piano isn’t covered in MDF. It is covered in solid wood. This is because solid wood (lumber) resonates very nicely. While this is good for instruments, this is not good for a speaker cabinet. Ideally the cabinet should be acoustically dead. There is room for debate about the box being part of the speaker’s sound, not something to eliminate sound. I agree with most that solid & dead is better than live & loose. Most excellent commercial cabinets are very heavy and strong. A wimpy cabinet will produce warm, sloppy, and inaccurate bass. This is because audible bass is emanating from the sloppy cabinet walls. I prefer my bass tight, lean and accurate. This requires a strong cabinet. Therefore I err on the side of solid and dead than on the side of resonant and sloppy. This means that I need to have a strong/dead internal structure and have lumber only on the outside. The inside skeleton of my cabinets are 3/4"+ MDF and the exterior is veneer on the sides and lumber on the baffle. The lumber on the baffle is glued to the MDF.

    Over time and changing environmental moisture, lumber will expand and contract. MDF doesn’t (relatively speaking). I consulted the Madisound discussion group and a local Cabinet shop owner concerning this issue. A few offerings were conveyed. One was a sand gap. Another was a constrained layer with asphalt roofing paper. Another was to use some esoteric EAR dampening as a constrained layer material. The general consensus on gluing the oak to the MDF was that if the span across the grain was great enough the oak would split over a period of time. Byron Studer of St. Peter, MN glued solid wood to MDF with poor results. The sides cracked. He used good lumber from his brother's cabinet shop on a speaker cabinet and the sides (not the front) cracked in about one year. Another gentleman said about the same thing. I asked Dave Baker, owner of Custom Cabinets, here in Great Falls about gluing oak to MDF and he said it wouldn’t split. Hmmmm...

    I can attribute the difference of opinion to one of four variables. One, Bryon Studer said he used Silicon glue to allow for some movement. Maybe a more solid glue would have been better. Presumably this might hold the oak in place where it would form smaller cracks instead of a big crack after the Silicone allowed the oak to shift and squirm. Two, having grown up in Minnesota I know that most folks homes are very dry in the winter, and quite humid in the summer. Great Falls, MT is always very dry. This would explain why the cabinet shop owner said I would have no problems. The varying home moisture content may play a role in causing Bryon Studer’s Minnesota lumber to crack. Because of the extreme hot/cold and resultant home humid/dry conditions (relative humidity), Minnesota furniture is placed under severe stress. Third, I don’t know how the other folks applied pressure to the material (MDF and lumber) to glue it together, but this may be an issue too. Bryon Studer probably did the glue-up in the cabinet shop, so this probably isn’t the causation variable. Fourth, the tangential distance across the material was likely a contributor. None of the Gentlemen who responded said that their baffle cracked. They all said that the sides of their speaker cracked. Since the sides are typically longer than the baffle width, the wider span across the grain was the most likely factor. Hence, there is veneer across the sides of my speakers and solid wood on the baffle.

    The big advantage of hardwood is it's superior strength. MOE is a stress to strain ration. Higher number's indicate stronger materials. The Young's modulus of elasticity (MOE) of red oak is about 1.55. The MOE of MDF (medium density fiberboard) is about .55. This means that red oak is approximately 3 times stronger than MDF. When oak is glued to MDF it makes a very desirable baffle. The baffle becomes incredibly strong (oak) and dead (MDF). MDF comprises the other factor desirable in a speaker panel. The MDF is very dampened. It doesn't vibrate. I think of MDF having little "shock absorbers" built in. The rap test on the oak/MDF baffle results in, well... sore knuckles.
     
  4. Doug_M_Fraser

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    Jim:

    I have used MDF and Cherry to build speakers.

    The Cherry bookshelfs look very good. I used lock miter joints and initially worried about wood expansion causing cracks as the lock miter joints are a very strong physical joint. After a year there are no cracks. The Cherry speakers look great and sound great.

    My Stryke 15.2 sub is made from MDF with a Cherry top and accents. Since teh sub was bigger I didn't want tro chance the cherry cracking as pannels would be bigger. I was very careful with the Cherry top and only glued one side of the Cherry to the MDF and used screws with elongated holes to secure other side of the top to the MDF (from the inside). This allows for the Cherry to move and not crack.

    Some of the benefits of MDF are...
    - it is less expensive than hardwood.
    - it is very stable in normal use, it won't warp or crack.
    - quite easy to cut.
    - relatively easy to veneer.
    - can be painted with good results (no grain to hide).

    Downsides of MDF...
    - creates lots of dust when cutting.
    - can be problematic when securing drivers.
    - easy to obtain (in most places).
    - comes in big flat sheets - with hardwoods you normally have to dress rough sawn wood, glue up panels, smooth the panels and then saw those panels into the various speaker box walls.

    A while back, Audio Express magazine had a good article that looked at using constrained layered material to build speakers (i.e. 2 layers of MDF with rubber separating the MDF). They also looked at designs that had sand filled walls. The premis was that one wanted to dampen the natural resonance of the speaker enclosure as much as possible. I have seen this argument used when comparing hardwood vs MDF.

    The conclusion (if I remember correctly) that there was very little benefit of using these exotic construction techniques to sound deaden the walls. The key point was to make sure that the speaker was constructed properly. This includes having a thick speaker baffle, sufficient (read lots of) cross bracing internally in the speaker.

    From my own experience the decision between MDF and hardwood is mainly one of esthetics. Since I am a hobby woodworker I wanted to use Cherry. I sprayed the inside with sound deadening material. The same stuff used in car audio to sound deaden door panels etc. The speakers look good and sound great !

    Doug
     
  5. Jim Williams

    Jim Williams Second Unit

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    Thank you Doug and Allan and Allen for your very detailed answers. This is exactly the kind of information that I was looking for.
     
  6. John Wes

    John Wes Stunt Coordinator

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    The problem with expanding and contracting hardwood, isn't as much a factor in areas with low humidity.
     
  7. Aaron_Smith

    Aaron_Smith Stunt Coordinator

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    In my opinion, using hardwood for a speaker enclosure would be a lot of extra money and work for very little added benefit. From the acoustic standpoint, the only properties that matter are density, stiffness, and internal damping. MDF has higher density and internal damping than hardwoods; the stiffness can be created by using thicker panels.
    The whole idea of constrained layer design (MDF/asphalt/MDF, etc.) is that vibration will not travel well across interfaces between materials with dissimilar mechanical properties; the dissimilarity between MDF and sand or asphalt is the desired effect.
    The earlier discussion about modulus of elasticity was not entirely correct. Young's modulus (E) is unrelated to a material's strength; it is a measure of elasticity. Strength tells you how much stress a material can withstand before it fails. Strength is mostly uninportant for speaker design; the parts are generally not bearing loads anywhere near their failure limits. Elasticity is important; a very stiff material with poor internal damping is very resonant. So in this case high stiffness may be a disadvantage.
    In addition to the high expense associated with hardwoods, the expansion/ contraction is a big issue. Good furniture designers do their design to allow for movement of the wood with age and humidity; unless you plan to design with these thing in mind you can probably expect trouble down the road.
    Of course, you cannot match the aesthetic beauty of nicely finished hardwood; if this the most important factor, then the discussion changes.
     
  8. Mark Barnhill

    Mark Barnhill Stunt Coordinator

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    As a professional woodworker, I would suggest using MDF already veneered with the species of wood you prefer. This gives you the best of both worlds, and saves you the aggrivations of veneering the project yourself. I've been working with wood and wood products for all my life, and would never build a speaker enclosure with solid wood. There is no way to predict the response of the cabinet and WILL cause you problems eventually, as wood is a very unstable product. Aside from being a superior product, wood veneered MDF is environmentally friendly, as less trees are cut down for veneer, and MDF itself is using waste products in its production.
     
  9. Greg Monfort

    Greg Monfort Supporting Actor

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    >The very best speaker cabinets are made from MDF.
    ====
    We have wildly different definitions of 'very best' then. [​IMG] Those made with
    concrete, granite, slate, etc., are my idea of 'very best'. That said,
    there's no such thing as too much rigidity when it comes to the efficient
    transfer of acoustic power, so if these materials are too heavy/costly/hard
    to work with for the average DIYer, using a material that has a high degree
    of stiffness to weight is the best compromise. For the DIYer, this means
    using void free 3/4" plywood (minimum), with additional bracing if the walls
    are > ~1ft^2 in area.

    This raises the cab's Fs higher up in the BW where it can't be so easily
    excited by the LF or midbass driver(s). Normally, all that's needed to quell
    any remaining resonances due to standing waves is a bit of fiberglass
    insulation or special damping pads such as Dynamat.
    ====
    > This is because MDF is far superior to particle board, has very good
    internal damping, and machines nicely. But MDF can be improved further by
    the addition of hardwood. Hardwood is brutally stiff,
    ====
    Hmm, according to my hardwood specs compiled from various sources, only a
    few are nearly as stiff as void free plywood of the same thickness.
    ====
    > and improves the appearance of the cabinet. IMO, even very good veneers
    fall slightly short of the rich/deep hardwood appearance.
    ====
    Agreed.
    ====
    >Over time and changing environmental moisture, lumber will expand and
    contract.
    ====
    True, but whether it's enough to be a problem WRT speaker building, it
    depends on the wood. Quoting Terry Cain of Cain & Cain Speakers, whose
    primary business is fine furniture fabrication:

    "The most stable of nearly all woods is Doug Fir. Assuming dryness. Wideleaf
    woods the winner is White oak especially when quartered. Mahoganies are
    technically a softwood and are very stable. Domestic hardwoods are quite
    active given moisture changes. Quartered white oak being again the most
    stable. Poplar is very stable once dried, nearly that of mahog. Still,
    almost no hardwood is as stable as the pines, cypress and firs *once
    dried*." TC

    So assuming the wood has been kiln dried, at least these appear to be viable
    material options. Anyway, I've used White Oak on speakers with no cracking,
    and some have been credenza sized, though with a thin ply backing and plenty
    of bracing. I figured that if all my grandparent's 1800's doweled together
    WO furniture could stay together a lifetime, it ought to work well as a
    speaker cab material.

    My fave 'hardwood' though is old, weathered, ~petrified lumber from barns,
    etc.. Once planed flat, and with knots, etc. filled with clear epoxy, make
    excellent, and beautiful cabs.
    ====
    >The big advantage of hardwood is it's superior strength. MOE is a stress to
    strain ration. Higher number's indicate stronger materials. The Young's
    modulus of elasticity (MOE) of red oak is about 1.55. The MOE of MDF (medium
    density fiberboard) is about .55. This means that red oak is approximately 3
    times stronger than MDF.
    ====
    Hmm, a material's elasticity (stiffness) isn't the same as its beam strength.

    FWIW, MDF's MOE can vary quite a bit from batch to batch, plant to plant,
    and manuf. to manuf., and the average comes out to ~.527 according to
    Ga-Pacific when I queried them about it. Anyway, since stiffness is cubic,
    the difference between .55 and 1.55 is only ~1.43x, so for MDF to be ~as
    stiff as 1" red oak, it'll need to be ~1-7/16" thick, a considerable weight
    increase.
    ====
    > When oak is glued to MDF it makes a very desirable baffle. The baffle
    becomes incredibly strong (oak) and dead (MDF). MDF comprises the other
    factor desirable in a speaker panel. The MDF is very dampened. It doesn't
    vibrate. I think of MDF having little "shock absorbers" built in. The rap
    test on the oak/MDF baffle results in, well... sore knuckles.
    ====
    3/4" MDF by itself though has a fairly low Fs, even after factoring in
    corners, bracing, etc., and not nearly as well damped as most folks think,
    just build a sealed box with a piezo attached to find how how much 'sound'
    the box makes. It must have a second layer of 'x' thickness/stiffness
    laminated to it as you've done to get its stiffness up to plywood or
    hardwood stiffness.

    If the knuckle rap test is the criteria for well damped, go rap a concrete pad
    or brick wall for a good frame of reference. All you'll hear is the knuckles
    cracking. [​IMG]

    Being lazy and having a bad back, I prefer to get my stiffness as light as
    practical with as little time/effort as possible, so opt for the
    aforementioned 3/4" void free plywood, with bracing as required. Can't make
    the nice looking, but mostly acoustically worthless, rounded edges with the
    router, but then that's what quarter round stock (or DIY) is for. [​IMG]

    GM
     
  10. Hank Frankenberg

    Hank Frankenberg Cinematographer

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    Greg, if you're using void-free plywood and then you veneer it, you should be able to do the roundover edges. I think that Northcreek's recommended cabinet construction is interesting: sandwich outer MDF and inner void-free plywood using their glue which bonds but doesn't completely harden. I'd like to hear from anyone who has used that method.
     
  11. Greg Monfort

    Greg Monfort Supporting Actor

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    Right, there's more than one way to 'skin that cat'. [​IMG]

    In experimenting with constrained layer construction, I've built cabs that were 3/4" marine ply and covered with non-hardening glue, then skinned with 1/4" marine grade ply and they worked exceptionally well, so NC's version should be a winner too. Still, I've found that just the plywood with bracing, then depending on the design, either just some fiberglass, or if more damping is required, cheap self sticking floor tiles, is audibly just as good.

    Compression horns, high taper ratio TLs, etc., OTOH would benefit much more with NC's or similar construction. I use double 3/4" BB for throats and laminated 1/4" BB ply for the walls to get sufficient strength/damping.

    GM
     

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