Caixa Acústica Wilson Audio Gran Slamm X-1 (via Stereophile)

How can a reviewer possibly put a value on a loudspeaker as costly as the Wilson Audio Specialties X-1/Grand SLAMM? When he reviewed Wilson’s WATT 3/Puppy 2 system ($12,900-$16,000/pair, depending on finish) a few years back (footnote 1), John Atkinson said that it was “one of the more expensive loudspeakers around.” The Grand SLAMM costs almost five times as much!

On the other hand, we’ve all heard about the legendary Wilson WAMM system, which costs a staggering $130,000. At a mere $65,000, the X-1 could be regarded as something of a bargain, especially as it’s said to provide a performance envelope approaching that of the bigger, more expensive speaker. Even so, the new Wilson system costs way more than most enthusiasts can afford. (According to Stereophile‘s most recent reader survey, the average price of a complete high-quality audio system is around $11,700.)

Other big, costly speaker systems represent their designers’ attempts at achieving the state of the art. In historical order, we have the Infinity IRS in its various incarnations, the Goldmund Apologue, the MartinLogan Statement, the Apogee Grand, the Genesis I, and the B&W Nautilus. Reviewing such systems can be something of a contest between designer and critic: Can the former win over the latter by the quality of his work alone, regardless of price?

To judge by his WATT/Puppy design, David Wilson of Wilson Audio Specialties is a formidable opponent. I lost the battle several years ago when I bought a pair of WATTs/Puppies for my own use.

The X-1/Grand SLAMM

X-1 stands for the “First eXperimental” system of this type, and SLAMM for “Super Linear Adjustable Modular Monitor.” “Super Linear” derives from Wilson’s own design target: to make the system subjectively distortion-free over a wholly natural and realistic dynamic range. “Adjustable Modular” reflects the user’s control of drive-unit delay, hence system phase accuracy.

What exactly do you get for your $65,000? A pair of imposing, 6′-tall, impressively engineered loudspeaker systems, each containing seven moving-coil drive-units ranging in diameter from 15″ to 1″. The speaker is finished like a fine piano in black mirror-gloss varnish. (Alternative auto-finish colors, such as Mercedes Gold, may be ordered; a range of veneered side panels is also available.) Together, the two enclosures weigh about half a ton; when I spiked them to my floor, they partly sank into it.

Following the idea that the proportions of the X-1 should roughly follow those of the WATT/Puppy, albeit on a larger scale, the speaker looks slim.

The SLAMMs exert a definite presence in any room; it’s definitely a case of beauty being in the eye of the beholder. Responses elicited from visitors ranged from “No way!” to “Superb functional beauty,” with “Engineering as art” noted along the way. Silent, the X-1s stand like monoliths, their dark silhouettes reminiscent of the black transmitter on the lunar surface in Kubrick’s 2001. But feed them audio signals in electrical form and one’s view changes—the resulting sound quality has power and beauty.

The claimed performance envelope approaches that of the WAMM. Key X-1 features include easy, trouble-free use with a single pair of input terminals (see below); and a comparatively kind amplifier loading. This speaker has a very high dynamic range, typically capable of 120dB spl at 1m with a music-related peak program input of 350W—well within the compass of the Krell KSA-300S, for example. The high sensitivity (95dB/W, 8 ohms) is as equally important as the high dynamic range; pretty decent 102dB in-room sound levels will be possible from 20W of single-ended triode power, even if the speaker could never be driven to full stretch by such a source.


The X-1 is a pure example of the art of loudspeaker engineering—its form truly follows its function. Those requirements inherent in its design concept are realized without compromise, the angled, faceted surfaces of the functional, structural elements accented by their mirror-gloss finish.

The foundation of the design is the generous bass system, the lower of the two modules of the stack. Everything starts here: contact with the floor, the reference plane for the mid and treble enclosures, and, last but by no means least, the location and support for the bass drivers themselves. Above 30Hz, the bass range is handled by two large moving-coil drivers, a 15″ and a 12″, working in tandem. Instead of the usual four-point mounting, eight Allen-head bolts securely bind the drivers to the rigid enclosure.

Why do two differently-sized bass units share the same enclosure? They don’t. Subcompartments in the bass enclosure have specific damping and air-flow control elements graded to ensure the appropriate acoustic power sharing between the two drivers. Working as a pair, they’re equivalent to a single 18″ woofer, but the combo chosen by Wilson has far superior transient control and maintains higher quality into the low midrange—necessary in view of the overlap required for the first-order crossover (see below).

The considerable enclosure-panel area of such large speakers potentially increases the cabinet’s ability to re-radiate unwanted resonant energy. Setting reference standards for low panel resonance and coloration in such a large enclosure is a daunting task; a designer must work much harder than he or she would on a more modest system’s far smaller panel area.

In his previous designs, Dave Wilson has shown an uncompromising approach to enclosures and their acoustic signatures. In the case of the WATT, itself a landmark in low coloration, a cast, mineral-impregnated resin material was used. Many panel materials were researched for the X-1’s bass cabinet, including everything you might have thought or heard of, and then some. Wilson especially sought to avoid the characteristic, if not necessarily unmusical, sonic signature of wood and wood-composite materials. That overlay of “panel sound” common to the construction of nearly all box loudspeakers was considered to act as a barrier to both the bass articulation and to the ability of a speaker to clearly differentiate types of bass instruments. In the Sonus Faber Guarneri, for example, curved, laminated wood surfaces are deliberately exploited to impart a more natural sound to the system. Impulse analysis led to trials with a proprietary phenolic material—a very hard, very high-density laminated polymer long used in the electrical power industry because of its strength, fine electrical properties, and dimensional stability. Figs.A and B (supplied by Wilson) show the difference in cumulative spectral-decay behavior between the ubiquitous MDF and Wilson’s high-density composite used in the X-1. This material is generously cross-braced in the X-1’s 140-liter (5ft3) bass enclosure to an almost matrix-like complexity. The result feels almost solid, like a block of stone.

Fig.A MDF panel, cumulative spectral-decay plot of accelerometer output (linear frequency scale).

Fig.B Wilson X-1/Grand SLAMM, phenolic bass enclosure panel, cumulative spectral-decay plot of accelerometer output under the same conditions as fig.1 (linear frequency scale).

Bass-reflex tuning is provided by a huge ducted port, 6″ in diameter by 17″ long, made from machined alloy and bolted into the center of the rear panel. (This port’s 8.5-liter volume of moving air is greater than that enclosed by many sealed-box miniature speakers!) Bass-reflex ports are characterized by terms such as “volume velocity,” whose magnitude describes how much acoustic output can be delivered over what frequency range for a given distortion criterion. Even when tuned to a demandingly low 24Hz, as in the X-1, where the useful output covers a range of about three-quarters of an octave (18-30Hz), this size of port is capable of producing room-shaking acoustic bass power without significant distortion.

System & Crossover: The low-frequency crossover filters are potted in resin to minimize self-induced and drive-unit vibration effects, and are isolated from acoustic pressure changes by being mounted in their own sealed section of the main enclosure. The speakers are fitted with large, low-profile, alloy cone feet which in some cases may be sufficient for optimal floor coupling. As specified, however, hardened steel spikes are threaded into the cones and adjusted to give the best lock to the floor, as well as to facilitate system leveling.

It may use seven drive-units, but the X-1 is a four-way system. The first crossover is set at a low 120Hz, but as it’s electrically first-order, 6dB/octave, there will be considerable overlap between the outputs of the bass and midrange drivers. Significant output from the woofers will be present up to 500Hz, this intended to reinforce the lower midrange and to help drive the system toward maximum conversion efficiency. Likewise, the “reach” of the high-power midrange drivers down into the upper bass improves the low-frequency “speed” and overall transient performance, helping to define the leading edges of notes. It also blends the transition between the different acoustic heights of the bass and mid sections. This increase of the effective vertical length of the overall low-frequency source tends to reduce the depth of the reflected floor-notch effects by spreading them over a wider frequency range.

Such a low-order crossover is made possible thanks to the use of two 7″-cone midrange drivers—high-power bass-midrange units that would be capable of full-range performance in a smaller system. Sealed in their own enclosures, the midrange drivers will also have a natural second-order acoustic rolloff below 70Hz or so to give an ultimate 18dB/octave rolloff slope. As a consequence, they’re not called on to deliver any musical information of significance below 40Hz.

These units cover a nominal 4½ octaves up to the second crossover at 3kHz, where both high- and low-pass transitions are second-order, 12dB/octave. This doesn’t appear to make immediate sense for a vertical midrange/treble array that has the tweeter located between the two midrange units. However, the Grand SLAMM’s facility for fore and aft displacement of these three modules—lower and upper mids and treble—may be independently varied, and their acoustic delays adjusted for near-perfect phase integration through the crossover region. Such is the glorious freedom this unfettered high-end speaker design allows!

The front tweeter operates full-range to its limit beyond 20kHz, while the third crossover point at 12kHz (damped second-order) feeds to the fourth “way” of this four-way speaker system. This is a pair of rear-facing “ambience” tweeters that are run some 10dB or so below the main level.

A front-facing tweeter of the usual 1″ type becomes increasingly directional above 10kHz due to its small size, this dimension approaching the actual wavelength of the frequencies reproduced. Additional smaller tweeters could be fitted adjacent to the main tweeter, but it’s hard to blend their acoustic output; the result often mars the main signal. Ideally, a touch of extra treble energy is required to drive the room acoustic in the upper range where the energy output of the main tweeter is beginning to fall. Bi-directional panel speakers naturally have this “open” quality: rear-directed treble energy helps “open” the excitation of the room acoustic in the uppermost part of the frequency range. Many designers of box speakers have recognized this loss and tried various solutions—eg, the Shahinian designs, the Mirage bi-directional models, the top-mounted second tweeter of the Linn Isobarik, and almost all the Snell range.

Wilson’s ambience tweeters are mounted on rear-angled facets of the uppermost X-1 enclosure module. They nicely dispense a proportion of delayed, reverberant air and sparkle without any apparent loss of focus or wavefront accuracy from the main tweeter.

The mid-treble crossover is accorded its own solid, cast-resin enclosure at the back of the module stack, well clear of acoustic, vibrational, or electromagnetic interaction with the enclosure or the loudspeaker drivers. Wilson puts great emphasis on removing the induction-sensitive crossover coils from the reach of the distorted magnetic flux field of the drivers. He reports a smearing of transient decay due to this type of poor crossover isolation.

Drive-Units: Both woofers are made by the French company Focal specifically for the Grand SLAMM. (If sold on the consumer market, these units alone would cost upward of $1000/pair.) They both feature a diecast chassis and have the same type of very large magnet and motor system. The high-power motor coils (2kW short-term) are wound on 3″-diameter Kapton formers—Kapton withstands very high working temperatures—and are ventilated to improve long-term thermal dissipation. The woofers use a distinctive composite, ventilated-magnet design. The pole pieces are chromed, while the high-intensity magnetic flux required for the target sensitivity is achieved by using an array of smaller ferrite magnet rings, their red-painted finish lending them a distinctive appearance.

The cones are made of a glass-fiber-reinforced pulp impregnated with a catalyzed resin, and are both rigid and pistonic over the required frequency range. Placed uppermost on the baffle, the 12″ driver has an additional surface treatment to damp cone resonant modes, since in terms of its natural response and position in the stack, it reaches further into the low midrange than the 15″ unit. The edge suspensions are foam half-roll surrounds of high mechanical “Q.”

These are low-loss drivers, with high electrical damping and consequently great electromagnetic control (high Qm and low Qts, a highly desirable if expensive combination). Their corrugated spiders are surprisingly stiff; these are no “soft,” long-throw units. Instead, their motor design is directed toward linear control under high-power excitation, while their fundamental resonances are matched to the requirements of bass-reflex loading. By using two differently sized bass units, sharing a common enclosure and vent, the usual single, sharp port resonance peak is broadened, extending its range and smoothing the response both in the port range and in the upper enclosure-resonance range (footnote 2).

Given the high power capacity of the bass pair, the choice of midrange driver was critical. In addition to the usual requirements for response smoothness, low coloration, and transparency, the X-1’s midrange also had to be efficient, dynamic, and remain linear under high power inputs. To meet these demands, two 7″ drivers, custom-made for Wilson by Dynaudio, are operated in parallel. Their high sensitivity is reinforced by a double-magnet system, 2.83V driving the pair to 96dB at 1m in the upper midrange. These drivers use rigid pressed-steel baskets and high-power 1.5″ alloy voice-coils with dense “Hexacoil” windings. Copper shading rings and caps minimize magnetic third-harmonic distortion. The polypropylene cones are flared BBC-style, mineral-loaded to improve both rigidity and damping, and suspended on natural rubber surrounds.

Much development has gone into the new 1″ tweeter built by Focal for Wilson Audio. It has a double magnet to raise its sensitivity to an all-time high for a direct-radiator type of 96dB. While the WATT 3 used a fiberglass material for its distinctive inverted dome (not Kevlar, as is commonly stated), the new version of this tweeter uses titanium. This metal’s great stiffness helps push the primary resonance up to 23kHz from the 16kHz of the earlier fiberglass type. Now the intrinsic response is essentially flat to 20kHz at the greatly increased sensitivity.

The new, highly stable synthetic suspension is fitted with a small half-roll termination to control sub-harmonic rocking. Wilson has also fine-tuned the viscosity of the ferrofluid cooling medium in the gap, as well as the size and treatment of the air volume behind the dome. A tapered hollow pole leads to a sealed rear chamber within the ferrite magnet rings. The 1″ ambience tweeters are single-piece titanium-dome units sourced from Audax, chosen for their good performance in the final audible treble octave.


Description: Four-way, seven-drive-unit, floorstanding, reflex-loaded loudspeaker system. Drive-units: 1″ (25mm), ferrofluid-cooled, inverted titanium-dome tweeter; two 6.5″ (170mm) plastic-cone midrange units; 12″ glass-fiber-reinforced pulp-cone upper woofer; 15″ pulp-cone lower woofer; two rear-firing 1″ metal-dome tweeters. Crossover frequencies: 120Hz, 3kHz, 12kHz (rear tweeters). Electrical crossover slopes: first-order, 6dB/octave woofer/midrange; second-order, 12dB/octave, midrange/tweeter. Frequency response: 19Hz-22kHz ±3dB. Low-frequency extension: 20Hz, -6dB. Sensitivity: 95dB/W/m (2.83V). Nominal impedance: 5 ohms. Amplifier requirements: 15-500W.

Dimensions: 72″ (1830mm) H by 16.5″ (420mm) W by 25″ (635mm) D. Weight: >500 lbs each.

Price: $64,500/pair (1994). Approximate number of dealers: 12.

Manufacturer: Wilson Audio Specialties, Inc., 2233 Mountain Vista Lane, Provo, UT 84606. Tel: (801) 377-2233. Fax: (801) 377-2282. Web:

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