MDF vs. HDF

For decades, Medium Density Fibreboard has been the enclosure material of choice for the huge majority of loudspeakers. MDF is made from compressed softwood fibres held together with resin and wax. MDF is cheap, very machineable and has reasonable acoustic properties.

Unfortunately, as one attempts to extract maximum performance from a design, MDF also reveals some shortcomings. It is not stiff enough and does not have sufficient mass to resist ringing. The main body resonance of enclosures built with MDF is typically between 280 Hz and 400 Hz. This is at and near the "power range" of the musical spectrum, where most of the fundamental musical energy is contained, and most of the amplifier power is expended. Knowing how to brace and damp an MDF enclosure is the key to getting decent performance out of this material. But "decent" performance is probably the most you will ever get, no matter how thick you make the enclosure walls.

MDF also absorbs atmospheric moisture. Over time, the acoustic and cosmetic qualities of the cabinets will change. The woodscrews holding the drivers in place will loosen, and refuse to tighten back up. If you want to build a superlative loudspeaker, and have it retain its performance over time, a better cabinet material must be found.

At Lenehan Audio, the performance of our speakers improved out of sight when we discovered HD3 - an epoxy resin/hardwood composite material. HD3 has 1.6 times the mass of MDF - as is immediately apparent when you pick one of our speakers up. It is very rigid, highly self-damped and has very low levels of energy storage.

In practice, it also has several disadvantages for the manufacturer. It is difficult and time-consuming to machine and, due to its hardness, very tough on cutting and router bits. It is also expensive; seven times the cost of high quality MDF in Australia. But when you hear the sonic benefits, that seems like money well spent! Without it, we could not achieve what we do with the ML1 loudspeaker.

Comparing two pairs of loudspeakers, absolutely identical in every respect except that one pair had cabinets made from 18 mm MDF and the other 18 mm HD3, was a revelation. The MDF sounded glazed, slow and blurred - with a midrange "humming" that significantly reduced resolution and rhythmic precision. The HD3, by comparison, was jet fighter fast, and utterly free of unwanted resonance. Music had a pristine purity to it, with superb timing and resolution. The natural decay of a piano note, for instance, was eerily real.

Finally, the cabinets are constructed using an exceptional 2 part epoxy adhesive - the same one used to build ocean going maxi yachts. If it can handle the immense torsional stresses to which they are routinely subjected, the London Symphony Orchestra playing Mahler should be a piece of cake!

HD3 Surpassed - the new ML1

With HD3 no longer available, its nearest commercial equivalent is HDF and this is also a far superior material to MDF for constructing a loudspeaker cabinet. While HDF does, indeed, preserve many of the advantages of HD3, it is not quite as stiff and heavy, nor is it as optimally self-damped. An identical ML1 cabinet constructed from HDF was clearly inferior in performance … and that we could not tolerate.

The solution we reached is, to say the least, radical! The HDF cabinet is clad internally with 1/8" steel plate, using a special (and very labour-intensive) technique that uses the metal's inherent 'springiness' to render the cabinet walls incredibly inert. The finished weight of each cabinet is increased by 1 kg (2.2 lb) as a result; the energy storage is reduced below that of the HD3 cabinet, resulting in an even lower level of colouration.

Enclosure Bracing.

Even a cabinet material as optimal as steel-clad HDF needs to be correctly braced to give of its best - and this is far from simple. You see, conventional bracing techniques are successful in reducing the amplitude of particular resonant modes; but you may recall the first law of thermodynamics ("energy can neither be created nor destroyed, but only transformed"). In this case, the resonant frequency is often shifted higher, where it can sometimes cause even more problems.

Over a period of seven years, utilising the results from accelerometer testing of enclosure walls, we have succeeded in developing a bracing technique we call "differential bracing cancellation". Using a combination of materials, including hardwood dowel and ply laminate, it allows the resonance in one cabinet wall to work towards actively cancelling the equivalent resonances in the opposing and adjacent walls. This, combined with the structure's inherent rigidity, renders the enclosure acoustically 'dead'. And a dead enclosure puts more life into your music!

Enclosure Damping.

Two distinct types of enclosure damping are typically required. Due to our cabinet's inherently high level of self-damping, the cabinet walls do not require further attention. However, damping of the internal air mass is critical in controlling the air flow from the bass driver - affecting bass extension and the speed of the midrange. A proprietary 'Stiky Foam' material is employed, in two different densities, to reduce internal standing waves and control reflection back to the bass/mid driver diaphragm.