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Thinking Inside the Box
A driver interacts 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)?
Does the resonant frequency spectrum of an enclosure lie outside the range in
which the enclosed driver operates?
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.
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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).
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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.
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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.
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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).
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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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