Wilson Audio: Authentic Excellence: Cabinet Materials

Thinking Inside the Box

A driver iinteracts with its enclosure; this is elementary physics. Violinmakers like Stradivarius and Guarneri knew this. In their case, the driver was the vibrating violin string. Its vibrations were amplified and transformed into sound of ineluctable beauty by the resonant chamber of the violin body. The secret of the sound (understood in principle but still not duplicated) lay in the choice of wood, how it was cured and shaped and varnished—even the glue they used seems to have played a part in the musical beauty of their instruments.

The example is directly applicable to loudspeaker design, but in an obverse way: resonance—the key to a violin’s beauty—is the enemy of a loudspeaker.

The ideal enclosure for any driver would contribute nothing to its sound beyond elimination of the back wave. Every coloration introduced by the cabinet is, by definition, a distortion of the original signal.

An obvious corollary of this observation is that wood—so good in violins because of its resonance—is not so good for speaker cabinets for exactly the same reason. Nor is its close cousin, MDF (medium density fiberboard), which is the cabinet material utilized for the vast majority of loudspeakers on the market.

View a short movie on the history of cabinet materials research at Wilson Audio.

Engineer Vern Credille is a true polymath, with cross-disciplinary experience in everything from crossover design to port turbulence. One of his most important contributions, however, is in the design and execution of Fast Fourier Transform Measurements of the potential cabinet materials Wilson Audio has investigated over the years. The basic experiment looks simple enough: strike a solid steel ball against the test sample, and measure the resulting waveform. Plotted on a three-dimensional grid, the resulting "waterfall" or spectral decay graph reveals a wealth of information about resonance, damping and rigidity-the three properties that predict how well a given material serves the grain-free, uncolored reproduction of music.

The graphs reproduced below represent actual data generated over fifteen years of tests of traditional materials used in loudspeaker construction, as well as of some that have become very trendy in recent years. Finally, there are graphs of two of Wilson Audio's proprietary composite materials, developed through tests such as this, as well as through countless listening trials.

The ideal loudspeaker enclosure will be highly rigid, highly damped, and monotonic. What does that mean?

Take damping. An enclosure made of rubber would score very high in this category, but without providing a good energy launch surface for the transient cone excursions of the driver, the resulting sound would be, well, rather rubbery. So while a highly rigid cabinet supports fast transient response, it does so at the cost of ringing.

All materials, including engineered composites, will still resonate, but the two relevant questions in regard to their audible “signature” are:

Do the resonant frequencies occupy a single band (monotonicity)?

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The latter question hints at why no single substance will be ideal for both bass enclosures and midrange enclosures, and why, hence, the optimum loudspeaker cabinet will probably employ several technologies at once.

	MDF (particle board) is by far the most popular cabinet material. It’s inexpensive and easy to mill. But as the graph shows, it has three resonant peaks (poor monotonicity), The high, sharp peaks indicate poor damping, and the resonant frequency lies in the 350hz range, (right in the midbass).
This is a graph for oak, a common hardwood. Notice the multiple sharp peaks,indicating a broad resonant frequency spectrum, with poor damping, centered at 558hz.
	Baltic birch plywood shows a marked improvement in both damping (notice the smaller, blunter peaks) and resonant frequency (469hz), but the multiple peaks still indicate a material that is far from monotonic. From a practical standpoint, plywood is inexpensive (like mdf) and easy to mold and machine.
Aluminum (6061-T6 Aircraft Grade) is the real curiosity here. It’s a very expensive material, but, as the graph shows, in regards to resonant frequen(cies) and damping, it’s a real train wreck. Two widely separated peaks, both of which fall within the midrange. Note also the black line on the left, at 0hz, which indicates the material actually flexing upon impact (poor rigidity).
	Finally, we come to Wilson’s X Material. Notice the resonant peak at 34 db (an average of 10 db lower than most of the competing materials). It also has the shortest decay time (around 7 msec.). Because it’s resonant frequency is at 1064 hz, X material is used for bass enclosures, where it’s properties translate into the clean, dynamic, and impactful bass response that is the hallmark of Wilson Audio loudspeakers.

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