Acoustat Spectra 11 loudspeaker Tom Norton Reviews

Thomas J. Norton reviewed the Spectra 11 in February 1990 (Vol.13 No.2):

Drat! There's just no way to do it. To let you, the readers, know all about one of the better loudspeaker deals around, without letting Acoustat (and their dealers) in on the secret. JA is bound to spill the beans just by sending the traditional prepublication draft to the manufacturer. Then Acoustat will send dealers copies, and before you know it, Bingo! Instant pressure to increase the price. Let's see now, if JA just misplaces Acoustat's zip code, then we print the published review in two-week disappearing ink, and send out the dealers' copies about two weeks late...

Never work. If the Spectra 11 is any clue at all, the folks at Acoustat are clever enough to figure out some way to see the review. But maybe they're not so clever. I mean, couldn't they figure out that electrostatics are supposed to be fussy? And unreliable? And hard to drive? And most important of all, expensive? Whoever heard of a full-range loudspeaker with an electrostatic mid and top and dynamic woofer (below 250Hz) for under $1000/pair? Downright indecent. Can't be any good.

Maybe if I start with the soporifics—you know, meet the manufacturer, here's how it works, this is why it works, etc., etc.—Acoustat and Co. will fall asleep and, Rip van Winkle–like, wake up after the damage is done. Good idea.

It occurred to me as I was unpacking the Spectra 11s, the most recent of the new generation of Acoustats, that this was to be the first panel loudspeaker I have reviewed for Stereophile. Plenty of six-sided boxes of all sizes and pretensions, from the Munchkinesque to Goliaths big enough to give the neighborhood burglar a hernia. There have been occasional exceptions—the ZSEs were boxless dynamics, and I did have a pair of Apogee Duetta Mk.IIs in house for several months to use in conjunction with other reviews. But the reader might be justified in asking if TJN is a "true audiophile." Aren't all "true audiophiles" heavily into (or at least lusting after) panel loudspeakers?

Sometimes it seems that way. Electrostatics, particularly the full-range variety, have been something of a touchstone among enthusiasts ever since KLH and Quad came out with their respective versions in the late '50s. While Quad certainly takes the prize for singular, uninterrupted dedication to the cause, other manufacturers have regularly and eagerly jumped into the fray, only to emerge a few years later, battered and bloodied, abandoning the idea. A good electrostatic loudspeaker or electrostatic hybrid (combining an electrostatic midrange-tweeter with a dynamic woofer, footnote 1) is easy to build on paper, difficult in practice, and hard to produce consistently. But that has never discouraged the enterprising, and new manufacturers have continually sprung up to replace those dropping out of the chase. Acoustat, while a brash young upstart by Quad's standards, has been around long enough to be something of a fixture in the electrostatic loudspeaker market.

Just in case you missed it in DO's review of the Spectra 22 (Vol.12 No.10, October 1989), Acoustat, which had been building electrostatics out of Florida since the mid '70s, is presently a division of Rockford-Fosgate operating from Tempe, Arizona. Somewhere in the interim Acoustat was bought out by Hafler, which was then absorbed by Rockford-Fosgate (the in-car, not the surround-sound people), which also continues to own Carbonneau, a large Michigan-based loudspeaker driver manufacturer. Got that? The important point in all of this is that Acoustat's Jim Strickland is still chief engineer and the company now has ample resources at their disposal.

Panacea or Fool's Gold?
An electrostatic loudspeaker, at its most basic level, is nothing more than a large capacitor. One plate—lightweight, flexible, and allowed to move—is connected to a power supply which elevates it to a high DC potential (5kV in the case of the Spectra 11s). A high-value resistance between the power supply and this plate acts to maintain a constant charge on the latter as it moves in response to its electrostatic attraction to a variable charge on the other plate—the rigidly fixed stator. The charge on the latter comes from the audio signal. The moveable plate becomes the diaphragm; its electrostatically induced vibration creates soundwaves. The audio signal is almost invariably interfaced to the stator via a transformer which steps up the output voltage of a typical amplifier to the much higher voltage level required for the system to work (footnote 2). A simple, single-ended design of this nature will work, but distortion (second harmonic in particular) will be fairly high. In practice, all modern designs of any high-end aspirations have two stators (which must now be acoustically transparent) driving opposite sides of the diaphragm in a push-pull arrangement.

Because the diaphragm of an electrostatic loudspeaker is driven over its entire area, it is less subject to nonlinearities and breakup than is the cone of a typical dynamic system. Its mass is low in comparison with the mass of the air it drives, in theory resulting in good transient response. The mass of this air load, combined with the low stored energy of the relatively "lossy" diaphragm, combine to damp diaphragm resonances. And since most electrostatics are allowed to operate as open dipoles, cabinet colorations are virtually nonexistent (footnote 3).

But the electrostatic is not a magic feather; it has its own unique set of problems. The large radiating area needed to overcome dipole cancellation and provide good low-frequency extension can result in limited dispersion, especially in the high frequencies. And while the dipole radiation pattern has certain advantages (notably in its restricted radiation from the sides and top), its strong rear radiation can make placement tricky, especially in a small room. And there are those who argue, not without some justification, that the rear radiation rebounding off of the back wall results in time-smearing of the primary, frontal wave. Reliability can also be a problem with electrostatics, though more recent designs from Acoustat and others seem to have solved many of the more common failure modes—notably the shorting together, and resultant arcing, of stator and diaphragm when driven hard (footnote 4).

Furthermore, the need to clamp the edge of the diaphragm somewhat negates (but not entirely) its theoretical lack of resonant breakup modes. Not only that, forces which help to damp resonant modes also conspire to inhibit motion in the first place; the air load on the diaphragm can make it difficult to obtain the extended high-frequency response and fast rise time usually expected of an electrostatic—a considerable amount of force is required to make the diaphragm react quickly against this air load (footnote 5). And lastly, an electrostatic can present an inefficient, reactive load, conspiring to make the choice of driving amplifier difficult.

The Spectra 11 sidesteps some of the complications inherent in a full-range electrostatic by going the hybrid route. Below 250Hz the load is taken up by a dynamic woofer, in this case an 8" driver in a sealed cabinet. The woofer low-pass is 12dB/octave (electrical). High-pass on the electrostatic midrange/tweeter is at 6dB/octave electrical, though overall rolloff (adding in its natural low-frequency limiting) approximates 12dB/octave. Wall-socket mounted transformers (footnote 6), one for each loudspeaker, feed low voltages into power supplies mounted atop each woofer enclosure. The Spectra 11s arrive in three cartons—one for each woofer and the third for the electrostatic arrays. The user (or dealer) must mount the arrays atop the woofers. It's a relatively simple job, but does require opening the power-supply case. I found the fit between woofer and panel to be very tight, but a smart rap with a hammer did the trick. (Considering our similar experience with the Spectra 22s, Acoustat may want to include a hammer with each pair of loudspeakers!) The assembly instructions were clear; the job took less than an hour, total, for both loudspeakers.

The internal construction of the electrostatic array is similar to that of the Spectra 22. The panels are configured as mirror-image pairs with the outer portions driven full-range, the remainder rolled off above 2kHz. Thus the high frequencies are reproduced only by a long, narrow array, maximizing high-frequency dispersion. The network which acts as the low-pass filter on the inner segment also slightly delays the signal to it, causing it to "electrically curve." Thus SPECTRA: Symmetric Pair Electrically Curved Transducer. The Spectra 22 differs from the 11 in that it is larger, and to accommodate its greater low-end extension (for the electrostatic element, that is), it is divided into three segments instead of the 11's two. The 11's power supply is essentially the same as the 22's—tightly regulated and factory-adjusted for consistency—except for the 22's pilot light.

I encountered only one minor difficulty with my early-production sample Spectra 11s: one of the power supplies arrived damaged. Its circuit board had pulled loose from its plastic-stud moorings. Acoustat quickly furnished a replacement. Mine was apparently not the only such failure in early versions. The design will be changed in early 1990 to eliminate this potential problem—Allen-head screws and spacers will replace the plastic studs holding the circuit board in place (footnote 7).

In its general layout—electrostatic panel atop a small woofer enclosure—the Spectra 11 is strongly reminiscent of the Martin-Logan Sequel. And its size is very nearly the same. But it would hardly be practical to configure such a system in any other fashion. Cosmetically, it's quite apparent where compromises were made to keep the price down. Only one finish is available: black grille with black woofer cabinet (the sides are a metallic black laminate which Acoustat calls "black matrix"). Particle-board edges are visible at the rear of the woofer cabinet; Acoustat's ads claim that this is medium-density fiberboard, but the review pair appear to be built of particle board. A knuckle-rap test on the back and sides of the enclosure indicates some resonance and a possible lack of heavy internal bracing. The overall fit and finish are more than acceptable for the penurious audiophile, the size manageable, and certainly the resources have been concentrated where they belong—on the sound. But the look is decidedly Spartan, the WAF (Wife Acceptance Factor) dismal at best; the availability of a lighter finish would help considerably. Good-quality banana jacks are fitted (one set only, bi-wiring is not provided for), but their shanks were too thick to accept the spade lugs from Monster M-1 and AudioQuest Green Hyperlitz speaker cables. I had to first attach the lugs to Monster Exterminator banana plugs—an added interface I would prefer to avoid.

The sound
Check out the fellow in the corner—the one with the Acoustat T-shirt. And the gentleman over by the door with the Spectra tie tack. Sound asleep, both of them? And the ladies in the next room? Good. Because the secret is about to come out...

The Spectra 11 is one of the most unfailingly "musical" loudspeakers I have had in my listening room. I have never been particularly comfortable with the adjectives "musical" and "analytic" as applied to audio equipment. But it's difficult to avoid the "musical" designation when trying to describe the Spectra 11s' sound. My first impression was something like, "Gee, that sounds nice—not spectacular or even particularly goose-bumpy, but sweet." Detailed, but not obviously so. Open and unforced through most of its range—especially in the all-important midrange. Bass just slightly warm, but reasonably deep and defined. Imaging very good, depth a bit less so, but still very competent. And they are coherent; even the blend of the dynamic woofer and electrostatic mid/top is handled well. It didn't take much listening to conclude that this new Acoustat is rather special. Not perfect, as we shall see, but a loudspeaker with "entry-level high end" written all over it. A loudspeaker for people who hate loudspeakers.

It's probably easiest to start with the Spectras' main shortcomings: a subjective lack of top-octave extension and limited dynamics. With regard to the latter, we're not talking sheer volume level here. The Acoustats are unlikely to satisfy heavy-metal freaks but will certainly play loud enough to fulfill any reasonable expectations for a moderately priced, high-resolution loudspeaker with high-end aspirations. What they lack is the sense of visceral excitement that comes from finely rendered dynamic contrasts. They simply sound rather tame. They can also become a bit pinched and congested as the volume becomes louder and the scoring more complex. The excerpt from The Right Stuff from Star Tracks II (Telarc CD-80146) has knocked off my shoes and rolled down my socks on more than one occasion—with speakers as diverse as the IRS Betas and the Epos ES-14s. It didn't sound bad through the Acoustats, it just failed to raise any goosebumps. The same can be said of "Olympic Fanfare" from Center Stage (Wilson Audio Specialties W-8824). That elusive but vital (to this listener, at least) "jump factor" was missing.

The Spectra 11s also sound rather closed-in and soft, with a noticeable shortage of upper-octave air and spaciousness. This should not be taken to mean that they sound dull or lack high-frequency detail. On the contrary, definition and resolution are particular strengths of the Spectras—as I will elaborate on shortly. Rather, the feeling of an unrestricted top end and the sensation of a fully developed soundspace surrounding the performers are just not there in sufficient measure. On Antiphone Blues (Proprius PRCD 7744), a recording with a huge, spacious acoustic, the church ambience, while still quite evident, was clearly reduced in comparison with loudspeakers having a more open top octave. And the metallic edge of Arne Domnerus's saxophone had less bite than it should—though it was clearly more pleasant than through loudspeakers which overdo this quality. (After this review was written, rumblings from Santa Fe, where a second pair awaited measurements, indicated that listening on a higher than normal axis somewhat improves the subjective high-end extension. Frankly, I had found the vertical dispersion of the Spectra 11s to be somewhat better than similar, slightly shorter "line" sources. But a subsequent check did indicate a very subtle EHF enhancement about a foot above my accustomed listening height. Not enough, however, to make me consider revising my observations.)

But it's at about this point that my review notes run out of negatives on the Spectras. Once you get beyond the somewhat pallid dynamics and limitations at the frequency extremes, especially at the top, the 11s become strikingly good loudspeakers. No, the very deepest bass is not there; the lowest pedals of the pipe organ won't ruffle your trouser legs, the impact of the bass drum is softened, and the bass range, in general, is less than ideally taut and punchy. But you won't confuse the low-frequency response of the Spectra 11s with that of any bookshelf mini-monitor. Given the proper setup, the low-end performance is capable of riveting your attention. Dorian's striking organ recording of the Mussorgsky Pictures at an Exhibition (DOR 90117) came across with a solid LF foundation—the lowest-octave loss notwithstanding. And if the low-frequency response of Rhythm Devils Play River Music (Wilson Audio Specialties W8521) didn't exactly cause the neighbors to suspect an earthquake in progress, it came closer than you might expect—especially when driven by the Mark Levinson No.23. I encountered no woofer overloading at anything approaching a reasonable listening level, something that can't be said for the minimonitors which make up much of the Spectra's competition.

But no audiophile buys a dynamic/electrostatic hybrid for the bass response. He or she is usually delighted if the bass extension, detailing, and integration are well-handled and do not detract from the expected clarity of the upper range. The Spectra 11's low end more than meets those criteria. The real strength of the new Acoustat lies in its ability to render superb resolution, from the lower midrange to the mid-treble, while it hardly seems to be trying. It is detailed without being pushy, three-dimensional without making you immediately aware of its ability to render depth, subtle yet clear. If that sounds like music, then that's what the Spectras (with the exceptions already noted) sound like. Midrange is convincing. Human voice, in particular, has that palpable presence combined with natural, unobtrusive sibilance. Instrumental timbres seem right.

While the Spectras may not be the last word in openness in the top octave, their overall HF response is superb. There is no sense of hardness or over-etching; the combination of subtlety and detailing is as good as you're likely to find. I nearly lost track of the number of Opus 3 recordings I sampled, reconfirming my appreciation of the variety and gradations of HF detail captured on this label. Just how many different kinds of guitars have they recorded? Through the Spectras, I had no trouble telling them apart. And you want to hear the subtleties of shading that a good percussionist can create? You'll hear it here. About the only criticism I can offer is that the 11s almost never became snarly or bright, even when it seemed to be called for by the music or recording. And recordings which tend to have, on other high-resolution loudspeakers, an electronic edge, had that edge softened. A result, I feel, of the upper-end and dynamic limitations of the Acoustats. But if a loudspeaker must deviate from the ideal—and every loudspeaker does—far better that it should err in this direction.

If precision of imaging has been the Achilles heel of many panel loudspeakers, the Spectra 11 is an exception. One of my favorite soundstaging tests is the last cut on side two of Dafos (Reference Recordings RR-12)—I like to call it "The Natives are Restless" cut. The first few minutes, however, are very subdued, with quiet voices, gently struck percussion, and bird calls of unknown origin positioned throughout the soundstage. The Spectras enabled me to precisely locate the individual sounds—both laterally and front-to-back. And the image held up to reasonable head movements. The soundstage stayed where it belonged. Furthermore, while earlier Acoustats, especially the lower-priced versions, had sometimes given me the uncomfortable sensation of listening through a giant set of headphones, the Spectra 11s did not. Not that they were incapable of surprising effects. On the aforementioned River Music album, the image on cut 1 extended from slightly below and beyond the outside edges of the left and right loudspeakers at their outer limits to, in the center, about a foot in front of, slightly above, and a shade to the right of my nose!

I wasn't initially convinced that the Spectras were as effective in presenting depth as they were in rendering lateral position. When the cues were extremely subtle, and intertwined with the same cues which provide air and space, the depth was less fully realized. But on recordings with strong depth cues, the third dimension was very definitely there. If you don't hear striking depth from the Acoustats on the quiet bands ("Sub Level 3" and "Atmosphere Station" of the Aliens soundtrack CD, Varese Sarabande VCD47263), there's something wrong in your system—and it isn't the Spectra 11s. I also heard excellent resolution of fine details on these selections, details of which I was previously unaware. And while I'm on the subject of resolution, the "chime" on the Hildegard of Bingen cut from the Hi-Fi News & Record Review Test Disc 1 was plainly audible through the Spectras. (Thanks to JA for pointing this test out in his review of the Rogers LS7t in the December 1989 issue.)

The Acoustats, unlike many electrostatics, did not appear to be particularly fussy with respect to amplification. I was, unfortunately, unable to try them with a good tube amplifier, but they worked well with all three solid-state amps used during the evaluation. The 50Wpc Forté 1a is a bit below Acoustat's minimum recommended power of 75W, but it did an excellent job with smaller-scale music. The Classé, however, despite its only slightly higher 70W rating, had a considerably more robust low end through the Acoustats. Its more up-front HF also better suited the Spectras. And while I don't expect to see too many readers mating these loudspeakers to amplifiers in the Levinson's price range, a good amp of 100Wpc or more would not be wasted on them. The Spectras are moderately power-hungry; my informal measurements showed them to be 4dB less sensitive than the B&W 801s (footnote 8).

How does the Spectra 11 compare with its likely competition? Neither the Snell Q nor the Epos ES 14 has as extended a low end. The Snells have a distinctly more spacious sound; the Eposes are clearly more dynamic and punchy. Neither is quite as natural and subtle as the Acoustat. The Snells, with stands, are competitive in price. The Eposes, with stands, will run you considerably more. (I view the escalating price of the Epos with some alarm. If I didn't think so highly of this loudspeaker, I wouldn't care.) The Vandersteen 2Ci is strong competition, though notably more expensive with the required stands; nor have I heard them in my own listening room, although I've heard them elsewhere on many occasions (footnote 9).

The weak points of the Spectra 11s just might make them a less than spectacular experience in a dealer's showroom. But they do so many things so well, and at such a fair price, that they quickly won me over. They belong on your list of speakers that must be auditioned—even if you contemplate something far more expensive. They're a solid Class C choice, and will even give some Class B loudspeakers a difficult time of it. I suspect that more than one high-end dealer will be tempted to keep the Spectra 11s out of earshot of their more upscale models with higher profit margins.

Larry Archibald, in the October 1989 "As We See It," made a case for "buying cheap" when it comes to loudspeakers, and mentioned a few prime candidates. You can add the Spectra 11 to that list. Somewhere near the top.

With any luck at all, Acoustat didn't hear a word of this. I did hear one of them (the guy with the Rockford lapel pin, I believe) mumbling something between snores about a Spectra 11+ being readied for a post-Winter CES launch—with more up-scale cosmetics, a slightly more refined woofer with a thicker, more rigid cabinet, a high-frequency level control, and bi-amp/bi-wire capability. And a price several hundred dollars higher. But it'll be an additional model, not a replacement for the 11. Now if we can just wake up the folks from the Acoustat factory without disturbing the marketing folks, there just might be enough Spectra 11s to go around.—Thomas J. Norton

Footnote 1: That's the most common form of hybrid. Anybody else remember a unique Koss hybrid, the Model 2 of the late '70s, which combined an electrostatic woofer/midrange with a Philips soft-dome tweeter?!

Footnote 2: It's possible, of course, to use a high-voltage, transformerless amplifier to drive the stator of an electrostatic loudspeaker directly. The very first Acoustat, the ca 1976 Acoustat X, did indeed have such an integral amplifier, a transistor-tube hybrid. But commercial considerations generally demand a more universal design. The integrated amplifier-loudspeaker (electrostatic or not) has never really caught on among audiophiles in general, although the powered loudspeaker does have a certain following in some countries, notably Germany.

Footnote 3: In reality, the need to fit the elements into some sort of framework means that there may be residual colorations from vibrations of this structure. These can sometimes cause problems, especially in a full-range electrostatic—or in any other type of panel loudspeaker.

Footnote 4: I'm not aware of any consistent failure problems from today's "big four" electrostatic manufacturers—Acoustat, MartinLogan, Sound-Lab, and Quad. Certainly none to match those from earlier generations of electrostatics from other sources—which shall remain blissfully nameless (they're no longer in the electrostatic game). I strongly discourage the purchase of any used electrostatic loudspeaker from a manufacturer no longer in the business of building such systems, for that reason.

Footnote 5: Do electrostatics operating under lower pressures at high altitudes (such as in Santa Fe) have a quicker rise time, but less damping of the delayed resonances, than those used nearer sea level? And can this affect the sound, making a given electrostatic loudspeaker faster, and perhaps subjectively brighter and/or more dynamic, than at lower elevations? An interesting hypothesis. In any event, my listening is done near sea level.

Footnote 6: Similar to the dozen or so already living in your kitchen drawers and used to recharge all your rechargeable screwdrivers and water picks.

Footnote 7: If you happen to own an early-production sample which arrived in good condition, I recommend some reinforcement of the circuit board prior to any shipment or major move. A piece of foam or crumpled newspaper wedged between the board and the top of the woofer cabinet should do the trick. But remove it prior to use. And heed Acoustat's warnings about disconnecting the line cord and the audio-feed line prior to opening the transformer case, and discharging the panels. We're talking dangerous, even lethal voltages inside the power supply!

Footnote 8: If your room/listening tastes/preferred listening level let you get away with a bit less power, however, don't let any of this dissuade you from trying the Forté 1a. It's every bit as good as DO and GL say it is.

Footnote 9: A pair of 'Steens is on the way from Santa Fe as I write this; I'll relay my impressions vs the Spectras as soon as possible—might even make the same issue as this review, but I can't promise.

Acoustat, Rockford-Fosgate Corp.
Brand no longer in existence (2014)