Beveridge manufactures or has manufactured the following equipment (click to expand models list):Electrostatic Speakers
Beveridge web site (December 2003): Welcome to the Beveridge Audio web site. We haven't been making a lot of noise, up to now, but we've been very busy on things that matter. Here are some of the results of our research:
- We have re-engineered the epoxy transducers, achieving unprecedented levels of consistency and accuracy.
- We have analyzed and updated the design of the lenses, increasing their accuracy.
- We are making incremental improvements to the rest of the system, from amplifiers to cabinets to front grilles!
In summary, we are now in a position to manufacture Beveridge speakers that sound dramatically better than the originals. For more information on these results, and on our general business philosophy, see our Frequently Asked Questions page and my essay, The Beveridge Difference.
We are aware that some of the original speakers are in need of repair or restoration, so please contact us. We can restore transducers, in most cases, to factory specifications and guarantee them in full. Amplifiers, cabinets, and lenses can also be repaired and in many cases upgraded.
Finally, if you'd like to own a brand-new Model 2, please let us know. We can also offer custom (built-in) home installations.
Stefano Zaini (May 28, 2011): http://www.thesoundofthevalve.it/Beveridge_loudspeakers.html
Stefano Zaini - The Sound Of The Valve (June 3, 2011): http://www.thesoundofthevalve.it/Beveridge_preamplifier_RM1_2.html
Beveridge web site (December 2003): My father, Harold Beveridge, was a gifted engineer with a deep love for music. He studied electrical engineering at McGill University in Canada. From 1947 to 1954, he worked at Raytheon, in Waltham, Massachusetts. He was involved with designing vacuum tubes and the early commercial development of Radar.
During those years, he always bought season tickets for himself and my mother to the Boston Philharmonic and often attended the Boston Pops. He almost never missed a concert of either. When I was about six, he would take me with him at times when my mother couldn't join him; I enjoyed many concerts with him. He enjoyed these concerts a grat deal and it became his passion to reproduce that sound at home.
By 1951, he had already decided that the inherent limitations of a dynamic speaker were so severe that, even with the tremendous gains in technology that he foresaw, they would never produce a satisfactory transient response. Several people had been experimenting with electrostatic designs and had written about their work. My father read their reports and decided to look into the potential he saw in electrostatic devices.
His first effort was a single-sided electrostatic transducer, or panel. It was made from a rectangular piece of 1/4-inch gray slate. The plate, which was 12 inches by 16 inches in size, had over one thousand 3/16 inch holes drilled through it. The side away from the Mylar membrane was painted with a conductive silver paint, spreading the charge over the entire surface.
My father would put the transducer on a table with the membrane side up, sprinkle table salt on it and photograph the patterns which would form at different frequencies, amplitudes, and membrane tensions. He would also put a piece of white paper on top of the membrane and sprinkle it with iron filings so that he could observe the electro-magnetic field patterns.
I was often invited to watch. It was fascinating and good fun for both of us. My father spent quite a bit of time explaining to me how different wave forms behaved and interacted. By then, I was probably seven or eight years old. He explained concepts such as sine waves, resonant oscillations and nulls, and intermodulation distortion. He studied every detail of this fledgling transducer, and through watching him I too learned a great deal both about sound and about experimental methodology.
By 1953, we were listening to music on this electrostatic transducer. It was placed vertically in a frame and sounded pretty good. We heard highs that regular speakers couldn't come close to producing. In fact, that was part of the problem. Many speakers of this era sounded like the public address system in the movie version of M.A.S.H. My father's "speaker", in contrast, accentuated the highs and diminished the lows. So, compared to the standards of the day, it sounded far too "bright" or "tinny".
Also, the back wave interfered, producing peaks and nulls at certain frequencies. At least a half-dozen different forms of baffles and reflectors were tried in an effort to diminish these effects and, if possible, use the back wave to augment the sound. None were very successful.
Another problem arose when the speaker was driven at too high an amplitude; the membrane would get too close to the stator and stick to it, stopping the sound. At that point, we would have to turn off the power and wait ten or twenty seconds until the polarizing voltage had dropped enough to let the membrane "peel" itself away from the stator. Then we could turn the equipment back on again, this time being careful not to drive it quite so hard.
By 1957, stereo was coming out and my father decided to build two new speakers. He had also decided to move to a two-stator transducer, increasing both accuracy and sound pressure level (SPL). This meant, however, that he would now need four stators instead of just the one.
Drilling all those holes in four sheets of slate was definitely out of the question. In an attempt to replicate certain electrical properties of the slate, he experimented with mixing several types of carbon powder and barium titanate into Epoxy resin. After some experimentation, he came up with a working mixture and we cast four stators.
By 1958, I was thirteen and helping to make these new stators. While my father was back east on a business trip, I followed his recipe and cast four new stators. There were many factors we hadn't learned about yet, and for reasons we learned about much later, these stators were much more conductive than the ones my father had made before. He did, however, make them into transducers and found that they had some very interesting properties. The "sticking" of the membrane to the stators no longer occurred and we could drive each transducer much harder than before.
We used a pair of 1/4-inch glass plates to mold these stators. Each plate was a little over one by two feet. We put a thin coating of grease onto one side of each plate, then spread a sheet of aluminum foil onto one of the plates. The plates were placed with their greased sides (one with aluminum foil) facing each other. The plates were held 1/4 inch apart by three lightly-greased wooden sticks. This was all clamped together and stood upright with the open side up. The epoxy mix was poured into the cavity, then allowed to cure.
This produced a one by two foot, 1/4 inch slab of epoxy composite with aluminum foil on one side and a very flat surface on the other. Using the table saw, we cut a large number of parallel 1/8 inch slots, leaving about 3/16 inch of material between adjacent slots. We then poured a non-conductive epoxy rim around the entire border, finishing the stators. These stators made fairly good transducers.
To build the new speakers, my father made two cabinets. Each one measured about 36 inches wide by 24 inches deep by 30 inches tall and had a rectangular opening in the top. A transducer measuring 12 by 24 inches was placed horizontally below this opening, with a grill cloth above it.
To disperse the higher frequencies into the room, my father created a pair of acoustic "reflectors". He made a wooden bowl, about one foot tall and eighteen inches in diameter. The sides dropped, following a parabolic curve, to a 6-inch diameter base. The bowl was then cut in half vertically, forming a pair of half-bowl reflectors. The reflectors were fastened in place on the top of the cabinets, at the back side of the transducer openings, with their curved sides facing towards the room.
The reflectors protruded out and over the center of each transducer, causing some of the high frequency sound beaming up from the transducer to be reflected out into the room. This brought the ratio between the highs and lows to a more acceptable level and did, in fact, sound excellent.
The back (bottom) wave from the transducer, or at least the lower frequencies from it, came out the bottom of the cabinet, which was open. By going to larger transducers, of a design which produced better bass, and by only reflecting part of the highs into the room, my father had greatly improved the frequency balance. The "tinny" sound, or "over-brightness", was mostly gone.
Two of these speakers were built by 1959, giving us a very good sounding stereo sound system. The cabinets looked good, but took up a substantial amount of floor space. Also, the artificial flowers my mother had put in each "bowl" for decoration had to be removed after the first time that a cleaning lady watered them, drowning the transducers!
The Acoustic Lens
In 1965, my father was more excited than I have ever seen him about anything, before or since. He had conceived of the acoustic lens and the full-range cylindrical wave front. These innovations were, and continue to be, the unique hallmark of Beveridge systems. Providing comfortable levels of crystal-clear sound throughout the room, they offer a unique audio experience for the listener.
The acoustic lens solved several problems and solved them beautifully. Because the entire lens is driven by a single, continuous driver, the sound retains complete phase coherency. All frequencies of interest, including the "beamy" upper frequency ranges, are formed into a six foot tall, 180 degree, vertical, cylindrical wavefront. Everywhere the lows go, the highs go, too. He had achieved truly uniform dispersion over the entire frequency range!
In addition, my father made an important decision about cabinet design. He could find no way to allow the back wave into the room without seriously adulterating the sound in one way or another. So, from then on, he never allowed it into the room at all.
My father's lens and cabinet combination accomplishes a tremendous amount. The lens throat, combined with the "empty" space in the cabinet, creates a Helmholtz resonator in the 40 to 50 Hertz range. This works very well, extending the bass capability of the speaker.
So, we built two cabinets, each three feet wide, two feet deep, and six feet tall. Each cabinet held a continuous six-foot lens and transducer combination. We had also managed to cast twelve new stators in a new mold that my father had made. Those made six new transducers, enough for the first pair.
We later called this pair our model 1's. Only one pair was ever made. Eight years later, when we decided to go into production with this concept, we honored this first pair by calling our first production speakers our model 2's. The pair of 1's was sold to a friend in Santa Barbara who had a huge house overlooking the ocean. The pictures taken of our first model 2's (the white lacquered ones shown in our first brochure) were taken at his house.
What my father had created was truly novel: a high performance, phase coherent, full range, full height, full width, cylindrical wave front, line source speaker with incredible transient response. A series of patents attests to the novelty of his designs.
The new design had the added benefit that it was a true line source, as opposed to a point source. Consequently, sound pressure level drops off as one over the distance from the speaker to the listener. In contrast, a conventional cone speaker is a point source and the sound pressure level drops off as 1 over the distance squared.
Because these new speakers were true line sources with a 180-degree dispersion pattern, their placement in our home was novel. Our living room at that time was about twenty feet wide by fifty feet long. There was a large circular fireplace on the centerline and about twenty feet from one end. Each speaker was placed with its back to a long wall, facing the fireplace and the other speaker. The highs, mid-range, and lows were dispersed uniformly throughout the entire room. There were no more "bright" and "dull" spots.
The manner in which these new speaker introduced sound into the room was astonishing. As one moved about the room, the relative change in the volume from each speaker was minor. We found that we could set the volume for an acceptable listening level, go right up to either speaker, put one ear right up against it, and still hear the other speaker with the other ear.
In 1972, Akio Morita, the founder of Sony, spent several days at our house in Santa Barbara. He was very impressed with the sound made by my father's speakers. He wanted to purchase all of the rights to build them and made my father an offer to do so. Unfortunately, this offer precluded any further involvement by my father and was unacceptable to him because of that. My father had some other ideas and wanted to remain involved with development.
I suggested to my father that I could help and that we could build speakers ourselves. He was retired by then and really didn't want to start a manufacturing company, but my offer of a year's free labor was an significant enticement and we got started.
During that year, Don McFarland, a close friend and talented designer, helped us reshape the cabinet. Because the cabinets were so large, we couldn't employ the usual brute-force, high density particle-board style of cabinet design: the result would be impossibly heavy.
So, we tested a thin-walled cabinet at every possible frequency, making notes of all the resonant locations. These places were strategically reinforced, adding little to the overall weight. The result was a remarkably light cabinet which was remarkably sound-dead. It was, however, not completely sound dead; some listeners believe that the speakers sound better because of this fact.
My father and I spent an intense five weeks brain-storming and developing the tooling to manufacture the lenses. Several modifications were made in the stator molding and coating and transducer assembly processes. A new amplifier was also designed, using conventional tubes instead of the radio transmitter tubes used in the model 1's. The result of this effort was the Model 2.
The systems performed beautifully, as far as acoustic accuracy. The major thrust in those days, however, was for the highest possible sound pressure, with real fidelity taking a back seat. Consequently, some speaker designs were beginning to reach some pretty amazing sound pressure levels. One very highly respected reviewer even refused to audition ours until we made them louder! Hence the birth of the model 2SW.
The addition of a subwoofer allowed us to redesign the lens and open up the throat of the lens: a more constricted throat reinforces the bass response. With the line source pumping out a few more decibels, and the subwoofer pumping out more, we actually did get higher total sound pressure level. The model 2SW then enjoyed some very favorable reviews. It was even called, by one of the most critical reviewers, "a prime candidate for the world's best speaker." One reviewer kept the speakers, using them as the standard against which he measured all other speakers for nearly ten years.
They were, however, expensive to build. We had been selling the Model 2's for $2,000, wholesale to dealers, for about two years before we found out that they were actually costing us about $2,500 a pair to build! So, we simply doubled our price and kept on going.
For the Model 2SW's, we were paying a little over $500 per pair just to have the cabinets built. The Model 2's used three two-foot transducers, and also three two-foot-tall lenses per speaker. Both systems were very labor intensive to produce.
The Model 3 was my father's attempt at cutting the cost of production. The two-foot diameter cylindrical cabinets were intended to be much less expensive to build. It was also hoped that, by building three foot long transducers and lenses, we could seriously reduce the production costs. Eliminating a built-in amplifier also reduced costs and increased our customer base.
Unfortunately, it turned out that the costs did not actually go down. The increased transducer length meant that everything in the tooling had to be that much bigger. With the increased difficulty in handling, it actually took about twice as much labor to build a three-foot transducer as a two-foot one. The same was true of the lenses.
In addition, the round tubes we used to build the cabinets took up a tremendous of space to store and were, in many ways, more time-consuming to build than we had thought they would be. I personally believe that we didn't save a dime on any of those round cabinets.
It was in 1980, during the development of the Model 3's, that I left the company. I was never again involved in any of the companies that followed. What I know from there on, I have gotten second hand.
After I left, my father designed an all-new type of transducer. The stators were made from circuit-board material. They measured one foot by three feet and performed very well. They required less labor to make, but they didn't hold up very well over time. They would burn up the Mylar much more readily because they lacked some of the electrical characteristics of the cast epoxy-composite type of stators. These circuit board type transducers were used in Models 5's and 6's, which also had cylindrical cabinets, but smaller diameter, about eighteen inches, and were sold without their own integral power amplifiers.
The Model 5 was five feet tall, with a single three-foot transducer and lens above. It only had about a 130-degree dispersion angle, and had a subwoofer in the base. Model 6 was a return to the full-height, six-foot line source, although not with a full width 180-degree dispersion, but again more like 130 degrees, and with a separate subwoofer cabinet about eighteen inches tall underneath it.
The Model 2 was the last speaker my father engineered solely to satisfy his own personal criteria. That is:
It was a full range electrostatic system: no crossovers.
It was full height: at least 3/4 of the distance from floor to ceiling.
It was full Width: 180 degree dispersion, starting essentially at the wall.
For its simplicity of concept and clarity of sound, it is the personal favorite of some, including my younger brother Ross. The step from the Model 2 to 2SW exchanged the simplicity of a single source for increased volume and bass presence. While the Model 2 is perhaps the finest for solo guitar music, the Model 2SW is better for demanding pieces such as pipe organ music. The models following the 2 and 2SW, while excellent, represented compromises that met with varying degrees of acceptance in the world.
Arthur Vered (July 2000): BEVERIDGE - THE LEGACY MODELS by Arthur Vered
There seem to be some vagueness with regard the various models produced by Beveridge during the 70s and 80s. After consulting all the literature recently sent to me by Rick Beveridge, I can now give a full picture of the legacy range.
Production of Beveridge loudspeakers is divided into three main periods:
1973-80 - Harold Beveridge, Inc. (HBI)
1980-82 - the new HBI, a relaunched second company
1982-85 - California Audio Labs (CAL Audio) the last company set up by Harold Beveridge and Michael Alexander.
HB started manufacturing the initial, commercial full range electrostatic Model II around 1972. I think that altogether approx. 50-60 pairs were manufactured and marketed before the Model IIsw came in. The Model II's relative early demise was mostly caused by its low efficiency - at least one reviewer is known to have refused to audition it just because it didn't produce enough SPL. HB introduced the Model IIsw in response to this demand for higher SPL and efficiency. In order to achieve that, he restricted the ES panel to working from 70Hz upward and added two additional external commodes that housed dynamic subwoofers which covered the lower range. Thus the IIsw became a hybrid system but an additional 10db headroom was gained. In 1979 Beveridge developed the Model III in which the subwoofer (2 drivers per speakers) were housed inside the main cabinet. In 1982 Model V made its appearance, a smaller shorter version of Model VI, which appeared a year later still. Model VI was again reminiscent in dimensions of Model III but differed in other, substantial areas. It was also the last model ever developed by Harold Beveridge. The designer passed away in 1997 at the age of 83.
Before the full production of the Model II there was a Model I. This was a prototype, a concept proof and technology demonstrator as it were, which included all the later Model II specifications - and more. The cabinet was 6' tall, as would be all future models bar Model V (5'), but was 3' wide, as opposed to 2' of Models II and IIsw. The extra internal volume allowed the full range electrostatic Model I to reproduce the lower ranger much more successfully. Space and aesthetic consideration eventually limited the production models to 2' width. The Model I was never marketed (it was acquired by a family friend but was later destroyed).
The Model II - and the IIsw - comprise three 2' x 1' electrostatic panels, each with its associated front mounted lens which, stacked vertically together in the cabinet, form the Beveridge line source. The transducers are of a unique design. They are made of an epoxy mould, mixed with a special conductive material, cured at a carefully selected temperature. No other manufacturer was ever able to successfully duplicate the process. Because of their unique design and robustness, the Beveridge transducers were guaranteed for an unlimited period of time. Mylar to stator clearance in Models II and IIsw is c. 60mil. These are capacitative transducers, as opposed to constant charge, again, a feat from which other manufacturers shy away, choosing instead to manipulate voltage to achieve the desired SPL. In the Beveridge design arching is virtually impossible.
The last element that makes the Bevs so special is the acoustic lens. It basically guides and transforms the planar wave into a cylindrical waveform which disperse horizontally into the room in a full 180 degrees. In other words, wherever the low frequencies go, the highs go too. No other electrostatic loudspeaker achieves such a wide dispersion. Placement of the speakers thus become uncritical (although HB advised to place them on opposite walls, about 1/3 down the long side of rectangular rooms, facing each other!) and all "sweet spots" are eliminated.
This is not the place to go into detailed technical descriptions, but I should mention that since the Model IIsw only worked down to 70Hz, the lens were modified to accommodate that. Thus, the IIsw lens has a less constricted throat (the narrowest point where the lens converge) and a wider mouth opening in the front - 6" in Model II, 6.75" in Model IIsw. This alone allowed more sound to escape the lens, increasing SPL.
All Models II and IIsw were directly driven by integral, high quality OTL tube power amplifiers, located in the base pedestals of the towers and capable of delivering a peak 1500VA. The electronics were, again, a unique design, with two tubes working in series, and supplying the high voltage directly to the 'stats. The power amps received rave reviews in their own right. HB also produce a pre-amp, the RM-1 and later an electronic Xover RM-3 (designed by Roger Modjeski).
Many hundreds of Model IIsw were sold in their two main versions. As mentioned, the Model IIsw-1 had its xover set at 70Hz and came with an extra Control Unit. With it the customer had a wide range of control over the balance between subwoofer (driven by their separate power amps. of 60W/channel and firing downward into the floor) and the stats, the stereo separation and spread (continuously from full mono to full stereo), etc. The subsequent Model IIsw-2 (from c.1977) dispensed with the Control Unit and balance between woofer/stats was moved down onto the amp-housing pedestal. The xover frequency was raised to 100Hz, gaining some extra db in headroom. Visually, all cabinets looked identical to Model II, save the somewhat wider front vertical slot, and came in a variety of wood finishes from Oak, Cherry, white or piano-black lacquer and even fully covered in custom mirrors, as Julie Andrews chose to have them! While in name-dropping mode, other famous "golden ears" (and voices) users of Bev speakers included Frank Sinatra, Leonard Bernstein and a plethora of other speakers manufacturers who used them as reference for comparing their own products. On the side, it is interesting to mention the fact that the late Akio Morita, head of Sony, accompanied by a battery of his own engineers, had visited Beveridge in 1972, and had tested and auditioned the Model II extensively. Bowled over by what he heard, Morita made HB a huge offer to buy out the entire line, patents and all, for Sony's exclusive use. HB turned him down!
In 1979 the long awaited Model III made its appearance and it was a marked departure from previous models. Visually, the new 6' tall, 2' wide cabinets were tubular and resembled circular pillars. The subwoofers were placed at the bottom and the top of the pillar, behind the ES panels and phusically isolated from them, firing downward and upward. The transducers were again modified, since the xover point was further raised to 200Hz. Thus, the efficiency of Model III was on par with the then current crop of speaker on the market. The III however, had dropped the integral amplifiers and included the required step-up trans. and voltage divider board. The customer could now drive the speakers with the amp. of his choice, as would be the case with all subsequent models. The mylar to stator clearance was reduced to c.30mil as long excursions were no longer required for low frequency. High voltage was also dropped correspondingly from 3200V in the IIs to 1700V in the III. Each pillar now housed two 3'X 1' transducers with associated lens, the throat and mouth of which were modified and enlarged to about 8.5". Dispersion was somewhat reduced from 180 to c.150 degrees, still streets away from anything achieved or available elsewhere.
The Model III proved extremely successful despite some drawbacks. As the cabinets now housed the subwoofers, they tended to ring and resonate a bit. No amount of internal bracing could totally cure the problem and many users solved it by taking out the subwoofers and mount them in external satellite enclosures. Since the III could be driven in a variety of ways (bi-amped, internally or externally xovered) some chose to lower the stats frequency to 100Hz, as per Model Iisw-2. Roger Modjeski told me that he recalls that the IIIs had virtually no output at or below 100Hz. From my measurements, there's certainly some output at 70Hz, the frequency at which the non by-passable, internal protective oil cap cuts off the panels at 6db/octave.
The III was the last model produced by HBI company in its 2nd incarnation. The Model V which followed was already produced by CAL Audio. The V (so named because it was 5' tall) resembled a smaller Model III but was narrower - 18" dia. It had one 12" woofer firing downwards xovered at 250Hz. The main difference however lay in the transducers. Goner were the sturdy epoxy panels, in came the circuit board, perforated type. They were far easier and quite cheaper to producer. The V sported a 3'X 1' panel per speaker. The panel design was preserved for the next, and what proved to be the last model, the VI.
The Model VI was a return to a full 6' high line source, incorporating two of the new 3'X 1' circuit board transducers per speaker (mylar-stator clearance c.30mil). It came in three varieties: Model VI, VIb and VI "short". The short had no subwoofers. The VIb had two additional round commodes housing the 12" subwoofers, reminiscent of Models IIsw. The main tubular tower could be thus placed either along, or on top, of the subwoofer and was xovered at 200Hz, as per Model III. In fact a retrofit kit which included the new transducers, new step-up transformers and the subwoofers was offered by CAL Audio to Model III owners, to be carried out at factory. I don't know how many users took up the offer, but hopefully not too many since in due course (5-6 years) the circuit board panels (the "pegboards", as they were nicknamed) proved to be less durable than the previous epoxy ones - they gradually arched and in due course burned out.
As can be seen, the Beveridge models slowly shifted from full range electrostatics to hybrids, although the stats panels still covered a considerable range of the spectrum. SPL was never at the top of HB's priorities but market demands forced him to move in that direction. It 's a shame. Model II was, and remains, the purest incarnation of Harold Beveridge vision and remains the finest, albeit not the loudest, loudspeaker system ever produced, by anyone. The rest of the models did enjoy most of Model II's qualities but the purity of the idea was somewhat marred by the introduction of dynamic cones, the very drivers that HB tried to avoid from to begin with. Still, a new Model III owner who had recently acquired his pair at an auction held by the Dahlquist company was told by its CEO he won't regret his purchase. Dahlquist used the IIIs for reference...
I personally own two pairs of the Model III and hopefully a new Model II in the near future. One of the IIIs is a uniquely silver-finished pair of some lineage. These were the personal speakers of Harold Beveridge, custom built and presented as a gift to his wife shortly before she died. They are hand-made (as all models were) and include a few extra features not found in the stock production. They sound out of this world (and so does the other, oak-finished pair). The panels work and look almost like new, as if they'd left factory today. Quite amazing. After her death, the silver pair became the property of the youngest Beveridge. Some four years later he sold them to a family friend who kept and played them until some two years ago. Rick Beveridge bought the pair back for me. They now dwell in their final home. After a recent "jaw dropping" audition session by a most respected audio magazine editor-in-chief, his $8000 offer to buy the "silvers" was turned down. He still rings periodically.
Today Rick Beveridge, who helped his father set up the original HBI company, and had built many of the transducers in the IIs and IIsw, has relaunched the production of the speakers. Apart from repairing and servicing current customers, he builds new Model IIs to even higher specifications than the originals. Casting the epoxy panels is much improved while the lens had undergone a computer-aided redesign, carried out by his younger brother, Ross (the pre-owner of the my "silvers"), who is a computer scientist and lecturer at Colorado University. The beauty of the new lens must be seen to be believed.
My modest contribution to the new effort consists of a recently built, computerised audio testing platform for the new speakers (in which all audio parameters can be measured and analyzed by software) as well as designing and setting up the Beveridge Audio website which can be found at www.beveridge-audio.com along with a lot more info and photos. Enjoy.
Refurbishing & modifications
Stefano Zaini - The Sound Of The Valve (April 20, 2012): mylar repair/assistance: http://www.thesoundofthevalve.it/Beveridge_repair_maintenance.html
Beveridge Electrostatic Loudspeakers