DIY Audio & Video

MDF (Medium Density Fiberboard) Guide / FAQ

Medium Density Fiberboard.

MDF belongs to the hardboard family of products which are made from wood fibers glued under heat and pressure. Medium Density Fiberboard typically has densities between 33 and 50 pounds per cubic feet while High Density Fiberboard (HDF) ranges between 50 and 80 pounds per cubic feet. Hardboard was first produced in 1924 by W. H. Mason, founder of Masonite Corp. The term Masonite is therefore often used to denote hardboard products, especially HDF.

Physical and dimensional tolerances for MDF are specified in ANSI A208.2-1986

MDF has many qualities that make it an ideal replacement for plywood or particle board. It is dense, flat, stiff, has no knots and is easily machined. Its fine particles provide dimensional stability without a predominant "grain" (as is the case with lumber). Unlike most plywoods, MDF contains no voids, and will deliver sharp edges with no tearout. MDF is very well damped acoustically thus making it an ideal material for speaker enclosures.

Below are some metrics for MDF and other types of wood. As you can see, MDF is very dense and heavy, but is not as stiff as other types of wood which is why bracing is suggested.

The modulus of elasticity (MOE), also called Young's modulus, is the ratio of stress to strain, where stress is the force per unit area placed on the item and strain is the deformation caused by the stress. The MOE is therefore a measure of stiffness.

Here is an image of a birch veneered MDF board on top and for contrast an image of veneered particle board below. Notice the much larger and obvious particles in the particle board.

While MDF has been in use for almost 30 years, it is only now becoming available to the general public. Finding MDF may end up being the single toughest part of using it. As its density implies, MDF is very heavy and thus potentially difficult to handle. See the safety issues below.

MDF is typically made with urea-formaldehyde resin totaling 9% by weight. While most people will not be affected by this, people sensitive to formaldehyde emissions should consider low formaldehyde or formaldehyde-free MDF, or consider methods of controlling these emissions through proper finishing. Finishes that work best at controlling formaldehyde emissions are solid add-on surfaces such as high pressure laminates, vinyl covering, and finished wood veneers. Less effective at controlling emissions are simple seal coats, oil and latex paints, danish oil, and wax. Plum Creek makes low-formaldehyde MDF, while Medite II and Medex from Medite Corp. are formaldehyde-free MDF.

Dust is another MDF hazard. The large amount of dust released when working MDF makes proper respiratory and eye protection mandatory. At a minimum use a dust mask. A respirator is preferable. Shop dust collection (or even a ShopVac) would greatly help the removal of dust from not only the air but also the working surfaces, making them easier to see. Goggles should always be worn while using tools.

No. MDF from different sources will vary in texture, density, color, etc.

MDF is manufactured in sheets up to 8ft x 25ft. Typical consumer level sheets are 2x2, 2x4, or 4x8 and 1/2 inch, 5/8 inch, 3/4 inch and 1 inch in thickness. Thicknesses can also be metric - an important consideration for the use of English system tools (such as router bits). MDF is also available with a variety of veneers and laminates pre-applied, which may affect its actual thickness.

Medium Density Overlay and High Density Overlay are plywood products with a resin impregnated paper coating. They are often used for exterior painted surfaces. These are not fiber based products.

Likewise, particle board is not fiber based; it is a solid wood composite product. Along with flakeboard and other engineered lumbers, composite products are made from wood flakes, chips, splinters, etc., formed into layers and held together by resin glues and heated under pressure. Being layered and consisting of larger chunks, particle board does not have the uniform texture of MDF.

While MDF is a hardboard, the term hardboard is often used to refer to 1/8 or 1/4 inch thick HDF, usually containing a screen pattern on one surface. As previously mentioned, this is commonly referred to as Masonite.

Plywood is made from an odd number of lumber plies, each layer having a grain direction at right angles to the previous layer. This arrangement provides a dimensionally stable product. Void-free plywood uses plies with supposedly no holes, thus the completed plywood has in theory no voids. Baltic birch plywood is often sold as void-free plywood though some users have encountered small voids in these products. Be sure to ask specifically for void-free plywood if this is what you are looking for.

You can build speakers with whatever you like. However, MDF is often the material of choice. Its stiffness and density yield good acoustical damping properties. Particle board and plywood are cheaper and can still be used if cost is an issue. Plywood (especially if void free) can be used in the main baffle as a sandwich material to better hold fasteners. Plywood is also a good material for making braces inside speaker boxes. Solid wood (lumber) suffers from movement - the swelling and shrinking of wood due to environmental changes such as humidity - and is therefore not a good material for speaker enclosures. Lumber is also not as acoustically dead as MDF.

MDF can be treated much like a fine grained hardwood. Its high glue content means that steel cutting tools will dull VERY quickly; thus the use of carbide tools is highly recommended. Always keep your tools sharp for efficiency and safety.

The following recommendations are from The National Particleboard Association publication:

• For general shop or table saw use with decent cut and good blade life, a 50 tooth, 10 inch combination blade may be used.
• For those demanding a better cut, consider a 60 tooth, 10 inch blade with alternate top bevel (ATB) teeth at 15 degrees, 10 degree positive hook, 5 degree side clearance, 10 degree outside diameter clearance, and low approach angle (blade projecting no more than 0.5 inch through top of material).
• For an even smoother cut, consider an 80 tooth, 10 inch blade with 15 degree ATB, 10 degree alternate face bevel, 15 degree positive hook, and 7 degrees side clearance. This is costlier and may result in a shorter blade life.

Availability varies geographically so there is no simple answer to this question. Hobbyists have found MDF from a wide variety of sources including, but not limited to :

• large warehouse style supply dealers (Home Depot, Lowes, etc)
• small local lumber yards
• cabinet shops who buy in large quantities and are willing to part with some
• surplus building supply dealers

As MDF becomes more popular you will see it more and more in your local hardware stores. Sometimes, they will only have smaller 2x4 pieces or 1x4 pieces designed for use as shelving, so be sure to look around or ask.

Beware of clueless store clerks trying to pass plywood, particle board or MDO as MDF ! Note that many lumber yards can special order MDF but may not realize this, so it never hurts to ask. Ask them to check their price book for availability.

Because MDF can be milled to just about any profile, there are many possible joints. However, not all make sense in the context of speaker building.

• butt - this simplest of joints may not be ideal for furniture but works very well for building speakers, especially when combined with biscuits (for alignment) and screws (for holding strength while the glue dries).
• miter - works well when using pre-finished MDF (veneered or laminated) thus leaving no exposed unfinished surfaces.
• lock-miter, dovetail and other routered joints - works just like lumber. These joints have limited use in most speaker enclosures.
• rabbets, dadoes, grooves and other saw cut joints - same as with hardwood. Note that these can also be cut with a router. These joints may be useful, depending on the design of the speaker.
• spline, biscuit, dowel - as with lumber, the glue joint is stronger than the MDF. Dowel holes should be 0.002 to 0.003 inch larger than the average dowel diameter, and 1/32 to 1/16 inch deeper than the actual depth used. Plain or spiral grooved dowels are preferred over fluted or multigrooved dowels. Biscuits are very handy for alignment of parts in addition to the additional gluing surface provided.

MDF can also be edge glued to make larger surfaces, although this is not likely to happen except with exceptionally large speakers. Panels can be scarfed, doweled, tongue & grooved and finger jointed.

Some typical fasteners and their uses follow:

• staples - Do not staple within 3/4 inch of any corner. Coated staples hold better than smooth staples. Use a finer wire staple if splitting is a problem. Drive at right angle to the surface to avoid bending.
• nails - The same rules apply to nails as they apply to staples. Use ring-shank nails to avoid fiber raising around the nail head; do not use smooth nails.
• screws - Drill pilot holes between 85 % and 90 % of the root diameter of the screw used and at least as deep as the screw. Untapered sheet metal screws with constant size shank are good, as are some untapered wood screws. Pilot hole sizes and minimum edge distances for common screw sizes are :
• #6 screw - 3/32 inch pilot - 1/2 inch edge distance
• #8 screw - 7/64 inch pilot - 5/8 inch edge distance
• #10 screw - 1/8 inch pilot - 1 inch edge distance

Do not rely solely on the above fasteners for building speaker enclosures. This is especially true for butt joints. Combine glue with screws for a simple and strong joint.

Good glues to use are gap-filling glues such as polyvinyl acetate (PVA) typically known as yellow glue, modified PVA glues like Titebond II or white glues. Epoxy, urea and hot melt glues may also be used.

For that finished look, there are many options requiring different levels of woodworking skills.

• The box may be painted. Be sure to seal and prime the surface before painting to ensure even absorption on all surfaces. A high gloss piano finish can be made with combinations of spray enamel, spray lacquer or other topcoats. A little experimentation at this juncture can be very rewarding.
• The box may be laminated. Options include melamine, Formica, or even contact paper. Be sure the surface is clear of dust before applying any laminate.
• The box may be veneered. Carried out properly, veneering can yield a very professional looking speaker. Refer to the references below for veneering info. A veneered surface can be finished with lacquer, varnish, oil or wax depending on individual taste. Stains and dyes may be used to modify the color as desired.

Note that raw MDF is very porous. Use a generous amount of glue to ensure a proper bond.

Yes, some other tips include:

• Apply the finish to the enclosure after assembly. Then cut the speaker holes. This gives the best appearance for the least work.
• Use shelf braces to stiffen the box and to further support edge joints. Also use 3/4 x 3/4 inch lumber along the inside of edge joints for extra strength and for stiffening.
• Use a table saw for dadoes and grooves when possible. This usually gives better, straighter results.
• On large panels with no bracing or shelves attached, reinforce the panel with one or more 3/4 x 3/4 inch ribs.
• If using particle board, fill exposed edges with spackle or wood filler. Then sand all surfaces thoroughly to get a smooth surface. Prime before painting.
• Two coats of yellow glue applied 10 minutes apart may be used to seal MDF edges.
• Wood or autobody sealer may also be used to seal MDF edges.
• To avoid stripping threads when mounting drivers to MDF baffles use threaded inserts such as T-nuts.
• Excess glue may be removed before it dries with a damp rag. For pre-veneered MDF, care should be taken to avoid smearing glue into the wood grain. An alternative method is to scrape off the glue in its semi-hard state.