Investigating different materials for loudspeaker enclosures
The main feature that loudspeaker material must posses is a low amount of resonation, in order that vibrations are not transmitted through the cabinet walls (see ‘resonances’ and ‘panel resonances’). Resonation occurs due to the difference in air pressures on the inside and on the outside of the panel. A flexible material will tend to move in and out in sympathy to these pressure differences caused by the movement of the cone. This movement produces a sound (undesirable), which in turn confuses and colours the sound that comes from the cone (the one you want to hear).
Concrete, stone and slate all are highly dense and heavy, thereby reducing resonance, but are highly impractical for use in loudspeakers due to complex manufacturing processes. They may only come into use perhaps to mass-load the bottom of the speaker or to place under the speaker.
Composite materials are the modern alternative to wood. Materials such as Glass Reinforced Plastics (GRP) imitate natural woods with inset internal fibres, which act to reinforce the matrix of material they are housed in. These are like the cellulose fibres (tracheids) in natural wood. The reinforcing fibres may include Carbon Fibre or Aramid fibre (manufactured under the trade name of ‘Kelvar’). They can be combined with a thermoplastic polymer or thermosetting resin matrix. Generally the former thermoplastic polymer is preferred for loudspeaker manufacture because they have greater toughness, vibration damping properties and a shorter fabrication time because curing is not involved. The loudspeaker market is increasingly seeing the introduction of composite loudspeakers, especially as they lend themselves to Computer Aided Manufacture (CAM), enabling acoustically sound shapes to be formed that are not possible with wood.
The most appropriate material is wood. The majority of manufacturers today manufacture from wood, or rather the improved synthetic boards. Favoured materials include:
- Plyboard- board in which layers (always an odd number) are sandwiched together to overcome grain problems. Available in many thickness’ and grades, a high grade finish is needed for the exterior of the cabinet, but an inferior finish will suffice for the interior.
- Blockboard is similar to plyboard with a number of rough blocks sandwiched between to facing sheets. Edges must be separately veneered or mitred to hide interior.
- Laminboard contains a larger number of smaller blocks compared to blockboard. Edges must be separately veneered or mitred to hide interior.
- Chipboard is made by compressing resin coated wooden chips between two steel plates. Multi-layered grades are available and have a high degree of vibration damping. Densities are rated by weight and range from 400-900 kg/m3.
- Fibreboard is fabricated by reducing the wood to fibres which are felted together and pressed into sheets with little or no resin being added. The bond results from the felting and natural adhesion between the fibres. The sheets are tempered into hard boards by impregnation with hot oil or sometimes resin and then heat-cured. Fibreboard has a generally higher density ranging from 800-2000 kg/m3. It also has frictional loses between the fibres which increase it's damping factor. Medium Density Fibreboard (MDF) would be more suitable for most projects, being cheaper, more readily available and available in greater thickness’.
Constrained layer materials use wood with dampeners to create extra vibration damping. Constrained layer construction board is made of two layers of MDF or similar material with a layer of wall resonance damping material sandwiched in between. However, this board is very difficult to manufacture and not readily available.