All SP Technology Loudspeakers Millennial Reference Series™ products employ aluminum-cone woofers for true "piston-like" performance. Having said that, let's examine those woofers a little.

First, an aluminum cone is certainly not "exotic." They have been in production to varying degrees for decades now. The reason they are not often used in 2-way designs is that they tend to behave as they were intended… they operate as near "perfect pistons" up to their "mass cut-off frequency." That's the point where the effective mass due to acceleration of the cone is so great that they simply cannot move back and forth or change directions fast enough to reproduce higher frequencies. Above that point, their output drops off very quickly - too quickly in fact to be crossed over to most tweeters.

The very quality that permits them to produce extremely low distortion levels (very stiff, rigid cones) limits their use to mostly the lower frequency realm. An 8-inch unit is only good up to a little over 1,000 Hz. They really do make great woofers but… they simply do not "break-up" and flex at the center as plastic, paper, etc. cones do. Do you remember the "whizzer" cone in the center of cheap car stereo speakers? Those types are intentionally designed to flex at the center in order to go high enough to cross over to an "average" tweeter.  Fortunately, due to the addition of our waveguide we don't have an "average" tweeter to contend with though. The extended bandwidth provided by our waveguide technology permits us to take advantage of the "perfect piston" behavior of a metal cone by crossing over at such a low frequency.

A very serious side effect of center-flexing woofers is that they invariably flex in other parts of the cone as well. These unwanted "modes" of vibration are what is commonly referred to as "break-up modes" and are the source of a wide range of distortion products. Unfortunately, amplifier damping has a very limited potential in controlling these distortion products. This is because they tend to occur in a plane orthogonal to voice-coil travel and therefore produce little back EMF for the amplifier to damp.

Such break-up modes can be very complex in nature and often occur at even low drive levels and moderate volume. This is because they are not strictly a function of driving power, for if they were, then frequency response would vary with volume. Designers of this type cone desperately try to avoid such effects. Yet, often they inevitably are affected by drive power. This combined with thermal compression (further described below) effects are the reasons many speaker systems appear to change "character" when the volume changes. None of these effects are considered a "good thing" in an accurate design -- in our opinion.

The woofer units we select are chosen specifically for the fact that their massive magnet structure, magnesium frame, power handling and incredibly low distortion have no equal in their size class.  One caveat to note -- just like any other speaker, you can over drive them. They are not "burn out proof" and because distortion throughout the entire spectrum is so low, it is tempting to continue pushing them a little more at a time. They give no indication vis-à-vis traditional distortion that they are reaching their electrical/thermal limits – so be careful and use good discretion when operating our systems!

Upon inspection of these woofers, one will also note the "phase plug" at their center. This was not chosen as a means to promote a "high tech" look. Form simply follows function. In lieu of such there would have been instead the traditional "dust cap" and its associated drawbacks.

Phase-plug venting combined with rear magnet venting moderates voice-coil temperature extremes. This not only increases power handling, but also reduces the likelihood of a thermally dependent impedance increase in the woofer. As voice-coil impedance rises, current delivered from the amplifier is reduced. Because drive current should be proportional to drive voltage to avoid distortion, any dynamically changing driver impedance represents a form of distortion. In this case, both static and dynamic sound pressure level compression takes place. Dynamic range is reduced in the short term and average power output tends to decrease over time as the average voice-coil temperature gradually increases. No dynamic driver is completely free from this effect but proper venting greatly reduces the tendency.

Dust caps are notorious as sources of spurious break-up modes and require no further analysis. They also tend to introduce dynamic compression effects due to an "acoustic diode" type action. This phenomenon arises if the designer is not careful in the design of the air passages that lead from the coil gap in the magnet assembly to the backside of the dust cap. Air is forced into and out of the cavity behind the dust cap as the cone travels in and out. If the amount of viscous air "drag" is not the same in both directions (more likely than not) then positive or negative air pressure can build up behind the dust cap. This can cause dynamically changing voice-coil offset (not good -- a source of even order harmonic distortion) and overall signal compression. For all their trouble, dust caps really do not have much to offer.

The phase-plug and magnet venting in our drivers clearly represent a superior choice as far as performance is concerned. The only drawback is that one needs to keep curious fingers from poking objects in the voice-coil gap and be sure to keep metal particles far away. Magnetic materials such as iron filings and slivers can be easily "sucked in" by the powerful magnetic field. Events as these are not expected to occur with any frequency among owners of a product of this caliber, so the protection offered by a dust cap cannot be justified.

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