Acoustat Spectra 3 loudspeaker

Founded in the mid-1970s, Acoustat was the first manufacturer of full-range electrostatics literally forced to address what had long been a major weakness of such speakers: high-voltage breakdown, or "arcing." The original design was built and used in JP (Jeep) Harned's home, where the living-room french windows opened out onto a stream in the back yard. That, plus Florida's legendary humidity, conspired to produce summer days when moisture would trickle down every vertical surface in the house, including the speaker elements. With 8000 volts or so running around in the speakers, and the close element/grid spacing necessary for adequate efficiency, heroic measures were needed to produce a system that would operate on its home ground at all without going up in a spectacular display of pyrotechnics. The result was the basic design of what subsequently became the first full-range electrostatic ever sold with a lifetime warranty: the conductive wire stators are sheathed in insulating plastic.

It was that reliability, as well as the consistently high quality of Acoustat's speakers, which caused such consternation in the high-end fraternity when the company went bankrupt two years ago, and such relief when it was rescued from oblivion by David Hafler, who purchased Acoustat in 1985 as an independent subsidiary of the Hafler Company.

The theoretical advantages of a crossoverless loudspeaker using an extremely large, lightweight diaphragm driven uniformly over its entire surface have been obvious from the start, but so, unfortunately, have the practical disadvantages. Some of the latter, such as dielectric breakdown (which resulted in holes punched in the diaphragm, each one of which then became a constant breakdown point), had to wait for improvements in materials technology before they could be solved. Others, such as the need for a matching transformer with the formidable impedance ratio of 40,000:1—ie, a voltage step-up ratio of 200:1—required rethinking and refining old design concepts.

One such refinement was the inspired idea of using not one but two step-up transformers to feed the same set of radiator strips. Each transformer could then be optimized for a specific part of the audio range; while it did necessitate using a simple high-pass crossover network (for the smaller high-frequency step-up), a complementary recombining network could be used to cancel out the phase anomalies which normally result when a crossover is fed directly to individual driver units.

Then there was the treble-beaming problem.

Because of the lateral displacement in space, signals radiated from the sides of a flat panel are in-phase with those coming from the center of the diaphragm only along the axis of the panel. To either side of that axis, interference causes signal cancellation or reinforcement, depending on the angle from the central axis. The effect worsens with increasing frequency, with the result that all flat-panel radiators tend to produce an on-axis "hot spot"—a beam that becomes progressively narrower with increasing frequency. And the wider the panel, the lower into the audio range the beaming occurs.

The method employed by Acoustat for its first model was to construct the electrostatic speaker from several tall, narrow vertical strips arranged as segments of a shallow arc (footnote 1). But this merely exchanges one severe wide-range hot spot for several less severe ones setting in at a higher frequency, and tends to produce severe vertical-venetian-blind effects. Later Acoustats were tall and thin, to maximize lateral dispersion, in order to restrict interference effects to the high treble.

The Spectra 3
The latest Acoustat speaker, the Spectra 3, uses a different, better, approach. Each unit consists of three identical, tall, relatively narrow electrostatic strips, each of which is split into two electrical sectors. There is no crossover as such, but one sector is operated full-range, the two next to it are rolled off above 1.7kHz, and the final three operate in the lower few hundred Hertz. Doesn't this low-pass filtering cause phase shift and, as a consequence, interference? Of course, but now these side-effects can be used to the system's advantage.

Through careful balancing of strip widths and rolloff frequencies, the Spectra 3's phase "anomalies" are used to shape its polar pattern into a single, broad lobe aimed at an off-axis angle of about 20°. Each speaker unit is a mirror image of the other, and when oriented with their widest-range strip toward the inside, the result is two broad energy lobes intersecting at a point in front of and midway between the speakers. Does that sound familiar? (If not, read the reviews of the dbx Soundfield 1-A and Ohm 5 loudspeaker in Vol.10 No.4.)

Essentially, what this radiation offset does is compensate for the timing and amplitude changes which occur when one moves out of the "sweet spot" on the center line of the speakers: it prevents the lateral image shift and collapse of the stereo stage which would otherwise occur. In other words, this is the first full-range electrostatic which should allow the listener to hear a stable stereo image right across the listening area!

The specs at the head of this review mention an "optional" subwoofer. It isn't exactly optional: it comes with the speaker whether you opt for it or not. (It adds no more than "a few dollars" to the cost, according to Acoustat.) The woofer is optional only in the sense that you don't need to use it if you don't choose to. It operates below 100Hz, and is mounted, face-down, at the bottom of the speaker's base unit (which strikes me as being one of the least attractive-looking pieces of audio furniture I have seen in years). The bottom panel of the base is slanted upward toward the rear, so that the speaker operates into a loading arrangement which combines elements of the classic slot load and a linear-flare horn.

The module containing the electrostatic power supply, audio step-up transformers, and woofer crossover fits into an opening in the bottom panel next to the woofer. On it are two sets of 5-way binding posts (with standard ¾" spacing for dual banana plugs) and three bat-handle switches. One switch selects between direct (full-range) connection of the electrostatic panels, or connection through the woofer crossover network. The second switches the woofer from direct-in to the internal crossover, while the third turns the woofer on or off. (In its Off position, the woofer is connected to its own pair of binding posts, for biamping or biwired operation.)

The Spectra 3 system is shipped from the factory in four pieces—two bases and two panels—which require assembly prior to use. Although this is simple and straightforward enough, it is nonetheless a royal pain in the butt, because attaching each panel requires applying appreciable force between it and the base, to compress some air-seal gaskets, before attaching the assembly screws. It was a chore for two of us working together; it would be almost impossible for one person to accomplish alone. And once assembled, the speakers were quite awkward to move, again requiring two persons (or one plus a hand cart). I strongly recommend that you have your dealer do the assembly, in your home if possible. If not possible, you may then have to hire a van to get the things home, and will need at least one other person to help you get the speakers from the van into your listening room. Once set up, it is easy enough to move them a couple of inches at a time to tweak their placement.

It is necessary to remember that the panels are mirror images of each other. The bases are identical, however, and the only way the L and R panels can be distinguished is by a tiny hole drilled in the metal strip that tops off each panel, at a distance of 7" from the edge of the panel. Viewed from the front (the side with the Spectra 3 logo on it), the right-side panel has its hole at the right, while the left one has it at the left. Once the speaker is assembled, you have to be a lot taller than my 5'6" to see the hole (although you can feel it with a fingertip), and that, plus the awkwardness of moving the assembled speaker, make it advisable to identify and install the right-side panel on the rightmost base to begin with.

Footnote 1: The English Quad ESL-63 uses another approach. The diaphragm is electrically divided into seven (I think) concentric rings, with the outer ones fed through delay lines of increasing length. This causes a signal to be radiated first from the diaphragm's center, then by each adjacent ring in succession, simulating the temporal ideal of a point source situated several feet behind the diaphragm, thus minimizing off-axis interference.
Company no longer in existence (2017)

dalethorn's picture

I never got to hear the model 3, but one day in 1977 or 78 I drove to Columbus OH to visit a shop that just got the Acoustat X. I remember that speaker very clearly, as it was the first full-range 'stat I'd heard that had satisfactory bass for pipe organ music. The demo music at that time was a version of the Bach T&F BWV 565, and it sounded quite good. I've been happy with many speakers that roll off the bass below 50 hz, since recordings of tracker organs with their low-pressure pipes don't generally have the weight in the pedal notes that the "modern" organs have.