Wilson Benesch Eminence Loudspeaker

Towering Achievement

Equipment report
Wilson Benesch Eminence
Wilson Benesch Eminence Loudspeaker

The Eminence bristles with technological innovation. Starting with the enclosure, it is an evolution of the A.C.T. (Advanced Composite Technology) cabinet Wilson Benesch developed in 1994 for its first speaker (which was itself based on the A.C.T. plinth for Wilson Benesch’s turntable in 1991). This latest version builds on that foundation, maintaining the curved monocoque structure built from carbon-fiber panels around a high-compression core. The new enclosure, designed with FEA (finite element analysis), features a sandwich of carbon-fiber panels around a proprietary new core material that reportedly increases the stiffness-to-damping ratio by 30% compared with the previous A.C.T. enclosure. The combination of the extremely stiff carbon-fiber panels around a highly damped inner core both resists and damps resonances so that the cabinet is an inert platform for the drivers. The cabinet’s shape is based on the curvatures found in nature, which exhibit the maximum strength for a given thickness. Wilson Benesch says that this new A.C.T. enclosure is one of the lightest, stiffest, and best damped structures ever manufactured. 

The boat-tailed enclosure terminates in a thick piece of aluminum that runs vertically up the entire cabinet height, a component Wilson Benesch appropriately calls the “backbone.” The baffle is also machined from aluminum. Viscoelastic damping material couples the various cabinet parts to suppress resonances. In addition, the direction of the carbon-fiber weave directs energy down the fibers and into the viscoelastic material that connects the cabinet sections. The carbon fiber and inner core are mutually self-damping, as are the interfaces between the carbon-fiber composite and the aluminum baffle, backbone, and foot. 

Inside the enclosure a series of 14mm (0.55 inch) steel rods compresses the entire structure vertically. This tensile stress acts as a damping mechanism—a technique used in other fields where damping is desired. The enclosure is bolted to a massive aluminum plate that has been artfully machined into a sculpture that suggests flowing water. The aluminum plate, which Wilson Benesch calls the “foot,” is machined from a 265-pound raw aluminum billet. Seven hours on a CNC machine turns the raw slab into the elegant finished shape. The speaker is anchored to the floor with four vertical “kinematic bearings” designed by Wilson Benesch and first used in the A.C.T. tonearm in 1991. Each bearing is composed of a finely threaded stainless-steel shaft 28mm in diameter that screws through a corresponding threaded hole in each corner of the foot. The top of this shaft is a large round hand-wheel that allows you to turn the shaft and raise or lower that foot. The other end of the shaft is the business end; it terminates in a 12.5mm steel ball that rests in the middle of three other 12.5mm steel balls that are held captive in a cup that sits on the floor. The steel ball on the shaft is held precisely between the three steel balls in the cup, eliminating any motion (see inset photo). The contact points for the speaker’s 320-pound weight are less than one square millimeter, resulting in a downward force of hundreds of tons per square inch. This design, called kinematic coupling, was developed by James Clerk Maxwell in 1871 and is used in a range of industries where precise alignment between two parts that can be separated is required.

At the speaker’s other end is a carbon-fiber top piece whose shape is reminiscent of the creature in the movie Alien. This odd geometric shape is designed to reduce interference in the wave launch from the baffle, and to diffuse reflections. It was created by converting sketches into a clay model, and then laser scanning the model into a 3-D digital image that could be analyzed and manipulated in software. 

The cabinet has been made as small as possible so that its sonic contribution is minimized. The smaller the cabinet, the easier it is to damp, and any resonances that remain will produce less unwanted cabinet sound. The narrow baffle and its curved shape also reduce diffraction. Despite the cabinet’s relatively modest size, the internal volume is considerable, owing to the thinness of the carbon-fiber composite structure. The speaker has a very small visual presence when viewed from the listening position. I can’t think of another flagship-level loudspeaker with this small an enclosure or footprint.

The drivers, and the way they are deployed, are equally innovative. All the drivers are designed and built in-house. The most salient aspect of the driver configuration is the isobaric woofer array and its backward-facing drivers. The two 7" woofers you see at the bottom of the baffle are mated to identical drivers inside the cabinet that you don’t see, with their cones facing each other. Because the two drivers in the isobaric pair are driven by the same input signal, they move in unison. As the two facing drivers move together, the woofer you hear (the one you see on the baffle) doesn’t have to contend with the springiness of the air inside the enclosure, as occurs with all other loading techniques. Indeed, “isobaric” means “equal pressure,” and isobaric loading is technically known as a “constant-pressure chamber” configuration (the “constant pressure” referring to the air pressure in the space between the two woofer diaphragms in the isobaric array). A woofer loaded in this way has a very low resonant frequency, allowing the system to extend very deep in the bass yet still maintain very fast transient response. The isobaric array’s ability to start and stop very quickly not only results in better transient performance in the bass, but also allows the low-frequency section of the speaker to better integrate with the transient speed of the midrange and tweeter. In other words, isobaric loading prevents the common affliction of a big slow woofer lagging behind a smaller, lighter, and faster midrange driver, creating an audible discontinuity. Wilson Benesch says that the isobaric array in the Eminence has a better step response (how quickly the cone accelerates in response to a steep input signal) than the midrange driver, which is unheard of. A guiding principle at Wilson Benesch throughout its 25 years of building loudspeakers is that transient performance is of paramount importance to fidelity. The company has always deployed small woofers with very powerful magnets that can start and stop very quickly—like a 500-horsepower engine in a lightweight sports car. 

Besides superior transient response and better integration with the midrange, another reason Wilson Benesch chose such diminutive woofers for its flagship is that the mounting holes in the enclosure can be smaller. Smaller holes don’t compromise the cabinet’s rigidity as much as larger openings. In addition, a smaller hole results in less energy from the driver’s backwave emerging through the driver and out into the room where it colors the sound.

Unusually, particularly for a large reference-class loudspeaker, the 7" midrange driver is run full-range, with no crossover components between your amplifier’s output terminals and the midrange driver’s voice coil. Removing inductors and capacitors from the signal path, particularly on the midrange driver, confers enormous advantages in clarity and immediacy, but places quite a burden on that driver, particularly in a large full-range system like the Eminence. The 7" driver’s excursion limits will define the entire speaker’s maximum output level and ability to reproduce low-bass transients. The design goal of running the midrange driver full-range likely dictated the use of a 7" cone, which is rather large for a midrange.

The tweeter is entirely new for the Eminence, but based on a classic Wilson Benesch approach that cleverly addresses the dilemma of the relative merits of soft domes and hard domes. Soft-dome tweeters don’t ring like hard domes, but their break-up mode occurs within the audible spectrum, typically at 18kHz. At that frequency, the dome’s motion ceases to be pistonic; parts of the dome are moving forward while other parts are moving backward. Wilson Benesch addresses this problem by mounting a strip of carbon fiber across the dome surface that strengthens the dome and shifts the first break-up mode to 30kHz. In addition, the carbon fiber helps to damp resonances in the dome. (The tweeter’s voice coil also features a carbon-fiber strip to increase its rigidity.) Wilson Benesch calls this hybrid dome structure the Semisphere.

The Semisphere diaphragm has been coupled with a new technology that Wilson Benesch calls the Fibonacci Element. This is a tweeter faceplate formed in an intricate lattice structure (see inset photo on p. 134). The Fibonacci Element reportedly precisely controls the constructive and destructive interaction of the direct sound from the tweeter and reflections from the tweeter faceplate. The result is said to be extremely flat frequency response. According to Wilson Benesch, the typical flat tweeter faceplate introduces many small frequency-response irregularities in the treble. The Fibonacci Element is made through “additive manufacturing,” also known as 3-D printing. The tweeter is decoupled from the baffle by a damped substructure that you can see through the lattice.

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