Every fixed body vibrates at its natural resonance when it is stimulated by a sonic event. The sound energy radiated by the back of the loudspeaker sets up resonant vibrations in the cabinet walls. However, vibrating cabinet walls act as sound sources, whose sound components are then added to the sound emitted by the speakers. This creates tonal distortions that are more or less apparent depending on the density of the material. Ideally, the sound is only emitted from the speaker membranes and the intended cabinet openings (bass reflex channel, horn opening, transmission line, …). In addition, the cabinet material must prevent sound from emanating directly from the inside of the box. Therefore we recommend that you use high-density materials with a high level of internal damping when building speaker cabinets, and that you include additional reinforcements for the cabinet walls. The following will compare the advantages and disadvantages of the most common materials.
Compressed into a board made from wood chips and glue, this material has a relatively smooth surface and a looser middle layer. This provides stronger damping for cabinet vibrations, but a higher sound emanation. The cut edges are very porous and hard to work with. The boards are glued using wood glue or mounting adhesive. Most hardware stores offer precisely pre-cut boards measuring 10, 12, 16, 19 and 22 mm. For smaller boxes with volumes up to about 10 liters, the wood should be at least 16 mm thick, and at least 19 mm for larger cabinets. Raw particleboard can be veneered, glued with film or fabric, or (more expensive) puttied and painted. Pre-cut boards cost between €5 and €10 per m², making them the most cost-effective material for your first test cabinet.
A wide range of wood and plastic veneers glued to both sides of a particleboard can be found in almost every hardware store, in pre-cut sizes of 16 or 19 mm. They have the same characteristics (naturally) as their base material, raw particleboard. Wood-veneered boards are glued using wood glue, and plastic veneers use a special glue. The cut edges can be made more attractive with wooden molding or edging bands, which are affixed with a hot iron. The pre-cut price is between €25 and €35 per m2.
A traditional building material is coreboard, which is made from wooden strips and thin wooden boards and is available in various wood types. It is processed the same way as the veneered particleboard. The advantage is its much lighter weight and the ease of processing the cut edges. It can be found at many hardware stores in thicknesses of 16 and 22 mm, pre-cut for €22 to €35 per m2.
Oriented strand board (OSB)
Various veneer pieces and binding agents, compressed into boards, create a patchwork-like mosaic in yellows and browns. The surface is uneven, but it can be smoothed with a belt sander and then coated in clear varnish. Oriented strand board is fairly unsettling to the eye – but if you like the look, it gives you emanation-damping as well as vibration-insulating boards at a square-meter price of €6 to €15 in 15, 18 and 22 mm thicknesses. It is glued using wood glue or mounting glue and is easy to work with. Wood stain gives OSB a special look. The many-colored surface loses some of its contrast, but is still lively. Before staining, be sure to sand off the clear varnish that always comes with the OSB as a seal.
Medium-density fiberboard (MDF)
Compressed into an even board format and made from fine wood fibers and binding agents, this board has a smoother surface and much finer-pored cut edges than raw particleboard. It is better at damping the sound emanations, but is slightly more vulnerable to natural vibrations; however, due to the heavy weight, greater impulses are required to set it in motion. Its fine structure makes MDF especially well suited for building designer cabinets because the cutouts, diagonal cuts and miters come out better than with other materials. It can be quickly and easily glued together with mounting glue. MDF can be veneered or puttied and painted; the cut edges require special treatment. Pre-cut sizes are available in almost every hardware store, in thicknesses of 10, 12, 16, 19 and 22 mm and costing between €6 to €15 per m2; (these prices do not apply to the “OBI” in Kerpen, where in 2001 I literally paid DM42.90 for one square meter of 16mm board, which today would be about €21 – extortion!). For satellite speakers with volumes up to about 3 liters, a wood thickness of 10 mm is plenty; for cabinets up to about 10 liters, 16 mm; larger cabinets should be at least 19 mm thick. Its price and the wide range of processing options make MDF the preferred material for box construction.
Made from stacked, glued birch or beech boards each about 1 mm thick, with each layer rotated 90 °, multiplex is the best material both for resonance insulation and for damping sound emanations. Wood glue is best for gluing it. The surface can be stained and coated in clear varnish, or oiled and waxed. The light and dark contrasts of the cut edges are attractive, and the box design should take advantage of them. Thicknesses of 15, 18, 21 and 24 mm are available for box construction. Prices can vary widely for the same thicknesses at different hardware stores and lumber yards. Often, you will need to buy a full board (1.5 m x 3.0 m), and the cutting will cost extra. So I pay between €25 and €40 per m2 for 18-mm birch multiplex, including the cutting. Multiplex is much harder to sand than MDF, so a very precise cut is important. Because it weighs less than MDF and is more durable, musicians’ boxes are usually made of multiplex, since they need to withstand frequent moves without being damaged. Besides, you would hate to put the musician out of commission with a slipped disk before the concert even starts!
Marble, granite and slate are the most common natural stones that can be used for building unusual cabinets. They are hard to work with, and they require the machinery and skills of a stonecutter. Slate in particular is an excellent material, since its structure makes it relatively soft but prevents wall vibrations. Unfortunately you need a crane to lift tower speakers, and even if you want to build shelf boxes they will put an enormous strain on the shelf. For instance, one customer’s 2-way shelf box weighed 54 kg instead of the 6 kg that the same model weighed when it was made from OSB. Still, the sound was worth the extra weight. For pricing, please consult your doctor or pharmacist – they’re better at naming big numbers without feeling too faint.
Acrylic or Plexiglas/ glass
People always ask me if a speaker box can be made out of transparent material. A cleanly built cabinet that doesn’t hide its inner value is something special – especially if fancy components were used for the frequency crossovers. But please note that a speaker box without insulation usually sounds terrible. On the other hand, it doesn’t make sense to fill a transparent cabinet with insulation, which would ruin the look.
Glued wood/ real wood
Almost every hardware store now carries beautiful glued pine, eucalyptus and beech boards (planed, glued wooden strips whose surfaces create an interesting patchwork pattern) at reasonable prices, for furniture-building. These boards are recommended for building speaker cabinets only with reservations. Wood “works” – in other words, it expands when the air is humid, and contracts when it is dry. But if the wood is glued in every direction (which is essential for box-building), tension builds in the walls and is only released when one piece gives in. A board will crack at its weakest point, and that’s the end of your air-tight cabinet. Still, wood is intriguing for loudspeaker designers – it does make an outstanding front-mounted baffle board.
Unfortunately we can’t talk here about every conceivable material that has been or will be invented (the chemical industry in particular is always amazing us with new, even more rigid, lightweight, omni-purpose, never environmentally damaging materials, like plastic bags and those yellow bins…). But any material used needs to be air-tight and non-bendable. Nor, if you rap on them with your knuckles, can they keep on vibrating – like metal plates.