Membrane and Brawn

In the best of all possible worlds, audio systems would reproduce perfectly music's visceral bass, richly sonorous midrange, sparkling transients, and smooth high frequencies and harmonics of the high strings.

This is a very tall order. At present, the CD has matured into the prime musical source, providing a reliable format with low-frequency response right down to those 8-Hz organ notes and high-frequency response flat to 20 kHz-all with a great dynamic range. Needless to say, no real-world consumer loud speaker meets these demands due to the compromises necessary in size, complexity, appearance, and cost.

There have been countless loudspeaker designs since the beginning of the hi-fi era. As a rule of thumb, the biggest, most expensive loudspeakers usually afforded the fewest audio compromises, but many attempts have been made to provide a "10-gallon" performance from a "pint pot." Even with today's most sophisticated computer-aided design, advanced technology, and the availability of new and often exotic materials, the circumvention of the laws of physics is just not possible.

The achievement of the best high-frequency response has fostered exotic tweeters using such things as electrostatic and piezoelectric elements, ionic drive (as in the Ionovac), plasma modulation, and even modulated flames! Frequently used were inexpensive ribbon tweeters by JVC, Panasonic, and Pioneer. But obtaining clean, extended high-frequency response was a great deal easier than trying to improve and extend bass response.

With bass frequencies, one usually had to contend with large, heavy, and complex structures such as front- or back-loaded horns, folded horns (as in the Klipschorn), acoustic labyrinths, larger bass reflexes, and infinite baffles with multiple woofers. Then there were the "cheaters"--slot-loaded woofers, Helmholtz resonators (the "air coupler" and RJ), and acoustic suspension. Acoustic suspension can pro vide deep bass, but it can also be quite inefficient.

Although the predominant design of today uses dynamic cone and dome drivers (with the better speakers using high-tech cone materials, very sophisticated crossovers, and advanced methods to attenuate cabinet resonance), quite a few audiophiles are enamored of loudspeakers which roughly can be considered diaphragm or membrane-type designs. These include electrostatic, planar magnetic, and magnetic ribbon speakers. They are admired for their inherent transparency of reproduction, their exception ally fast and accurate transient response, and their overall smoothness of response (particularly in the high frequencies). Unfortunately, as a class, they are not very efficient, they have fairly limited dynamic range, and their low-frequency response is usually not very extended and is of limited weight, power, and projection. This is especially true when membrane speakers are used in a so-called full-range con figuration.

Manufacturers who concede that full frequency response is difficult to get from their membrane-type loudspeakers often resort to the hybrid approach.

They cross over the membrane driver at a low frequency that will be compatible with a typical dynamic woofer placed in either a chamber within the main speaker or a separate, dedicated woofer enclosure. But there are problems that are difficult to solve. For example, the dynamic characteristics of the woofer are at odds with those of the membrane driver. Also, the bass en closure may have inadequate volume, or the type of bass loading may not provide enough bass extension at low distortion.

In more than 40 years, I've used just about every kind of speaker ever marketed. I've had the big back-loaded horns, the folded horns, even a monster exponential horn. I've had the 600- pound, sand-filled-panel, 16-cubic-foot infinite baffle. I've had all the smaller electrostatic speakers. I've used the huge Acoustat Model Eight electrostatic with enough panel area to achieve a fairly flat response at 35 Hz. I have auditioned "full-range" ribbon speakers as well as many ribbon tweeters. And I have enjoyed membrane speakers in the sonic ranges in which they excel.

All of the foregoing is a preamble to the main thrust of this column. Many people have asked me if there is a straightforward or hybrid membrane-type speaker with wide frequency range for $2,000 to $3,000. I found this a rather daunting requirement in respect to performance and price.

Frankly, I was prepared to state that I did not know of any speaker of this type that I could comfortably recommend. However, a friend of mine, whose ear and musical perceptions I have long respected, said he had been very impressed by a $2,595 pair of ribbon hybrid speakers he had heard at the Summer CES. He described the performance and configuration of this speaker, the Reference RT-7 manufactured by Clements Audio of Toronto, Ontario. The name rang a bell, for at a CES quite a few years ago, I heard a large, heavy, full-range ribbon speaker that was made by Clements, then located in Texas. The loudspeaker was around $6,000, was very clean and had all the good attributes of a ribbon driver, but it just rolled off too high to provide satisfactory bass response.

Intrigued, I arranged for the Clements people to send me a pair of the RT-7 speakers. It seems that Clements had further refined and updated their ribbon driver technology, and sold the design to the Canadian company. Under the direction of president Jim Richards, the decision was made to properly integrate the ribbon driver into a hybrid system.

When the speakers arrived, I was surprised by their heavy weight, 90 pounds each. With dimensions of 45 1/2 inches high, 10 1/2 inches wide, and 19 inches deep, they were also far larger than I had envisioned. Yet the RT-7s are visually quite striking, superbly finished in beautifully grained light oak.

(They are also available in several other finishes.) An acoustically transparent, brown grille cloth covers the top ribbon section and the lower woofer section, but, with their large expanse of wood, the systems are much more at tractive if the grilles are removed.

The reason for their size is soon apparent, inasmuch as the bass loading is via a patented Compression Line system. This is a cross between a transmission line and a Hypex horn load. A proprietary 8-inch woofer, made by Clements, uses a cone of 20-mil polypropylene, which is very stiff but low in mass. The surround of the cone is custom-made in England and is one of the thinnest in the world. The surround roll is inverted to eliminate any diffraction from it. The driver spider is also custom-made and provides one of the stiffest suspensions used in any driver. The woofer uses a very rigid but light Kapton former, and the 1 1/2-inch voice-coil is driven by a 20-ounce ceramic magnet. Particular emphasis is placed on the stiffness of the suspension, the low-mass surround, and the Compression Line loading to provide the speed of response necessary to keep pace with the high speed of the ribbon. Speed mismatch and woofer lag are common problems with hybrid speakers.

The woofer is mounted at the top of the bass enclosure. Sound radiated from the rear of the woofer cone traverses the first part of the Compression Line, which is diagonally mounted of wood grooved and mitered into the en closure. The sound waves continue down the line, and then the line narrows considerably, creating a compression in this zone. The sound is directed into a fiberglass-filled trap tuned to 120 Hz to remove unwanted resonances and distortion. Simultaneously, the sound is sent through the termination of the line, and bass frequencies below 80 Hz are exhausted through a vent on the bottom of the woofer enclosure's front panel. The vent provides up to 10 dB more bass output than the direct radiation of the woofer, yet these outputs are equal at 1 meter because of the smaller radiating area of the vent. Thus, a flat transition to the woofer is achieved, which radiates directly from 80 Hz up to its crossover point of 1,575 Hz. The woofer drivers are individually matched within 1 dB for efficiency, 1 Hz for resonance, and 0.1 ohm for impedance.

Frequency response is matched to a reference driver. The speakers are even packed in left and right boxes of different colors to ensure matched pairs. The main cabinet structure and the Compression Line all use grooved and mitered construction of 3/4-inch, multiple-density fiberboard.

The Clements ribbon midrange/tweeter is an unusual design. The 7-inch aluminum ribbon is bonded to Kapton with a special adhesive. The ribbon is quite durable, handling three to four times more power than a typical dome tweeter. In fact, if its temperature reaches 300° F, the adhesive will deactivate, and the ribbon will delaminate from the Kapton before it can burn out.

Each side of the ribbon has a grain-oriented ceramic magnet 1 1/2 inches deep developing a 10,000-gauss magnetic field at the midpoint of the ribbon.

Custom-fabricated 3/16- and 3/8-inch steel plates hold the ribbon and focus the magnetic field in the gap. The rib bon coil is typically about one-twentieth the weight of a dome tweeter's voice-coil. This ribbon has virtually a 100% driven area. The ribbon is mounted directly on the magnet structure, so there is no frame that can diffract the sound. This mounting has the added benefit of providing the ribbon with a huge heat-sink and a chimney.

The ribbon has almost bipolar radiation from the negative and positive sides in the magnetic gap, and its natural low-frequency roll-off occurs at a rate of 18 dB per octave. No coils are used in the crossover, and the ribbon is used with a 6-uF Wonder Cap with 0.01-µF Wonder Cap bypass. Solen Fast Cap capacitors and Wonder Cap bypass capacitors are used for the woofer, along with custom-made 50-watt ceramic resistors to ensure constant resistance.

Double Tiffany binding posts permit bi-wiring or biamping. A three-position, 2.5-dB treble-boost switch is provided for setting the high-frequency response to personal preference.

The entire ribbon structure is mounted on 3-inch-thick multiple-density fiberboard, and this plus the crossover are in a separate enclosure at the top of the speaker. The positive and negative halves of the ribbon are separated by a fin, or "lens," made of a very inert material. The lens keeps the two sides of the ribbon from summing with each other and is also used to provide accurate dispersion. The ribbons are mirror images, with the positive side firing to ward the middle of the soundstage. In a normal room, the negative side of the ribbon will reflect off side walls and bounce back in phase. If the sound source is in phase, a horizontal dispersion pattern of equal phase and amplitude is presented at up to 70° off axis.

To avoid interface problems, the ribbon is driven directly, without transformers. The systems are tested with a Hewlett-Packard 3561 FFT analyzer, and the test graphs are included with each speaker.

Obviously, the Clements RT-7 loud speaker employs some formidable technology in the design of its ribbon midrange/tweeter driver and its integration with a synergistic Compression Line bass system. The straightforward performance specs are quite impressive. Frequency response is rated at ±3 dB from 26 Hz to 40 kHz. As a matter of fact, the ribbon is down only 0.5 dB at 150 kHz, and the University of Maryland's biology lab uses a Clements ribbon to "communicate" with bats at about 100 kHz! Sensitivity of the RT-7 is 88 dB SPL at 1 meter with 1 watt of full-frequency pink noise. Distortion is claimed to be no more than 0.8% THD from 30 Hz to 10 kHz. Impedance is rated at 6.5 ohms, and power handling is 200 watts with a "continuous music source." But raw specs don't begin to define the merits of these remarkable speakers.

I auditioned the RT-7s in my listening room, with my usual complement of RPG diffusors and Abffusors. The speakers were about 8 feet apart on axis, and I listened to them from about 10 to 12 feet away. I drove the RT-7 with McIntosh 2002 amplifiers, FM Acoustics 611 and 811 amplifiers, and the FM Acoustics 244 preamplifier. A Sony CDP-X77ES CD player and a Wadia 2000 digital processor were the source units. It might seem a bit incongruous to partner a $2,600 speaker pair with such gilt-edged electronics, but any anomalies one hears is unlikely to originate in this kind of equipment.

I played various large-scale sym phonic works, and the speakers revealed one of their most outstanding characteristics: They simply disappeared! There was so sense whatever of music emanating directly from either speaker. On particularly well-recorded London, Delos, Telarc, and Chandos discs, the depth perspectives were not only extended, but there was specific front-to-back localization of various instruments. Add to this the striking smoothness of response, especially of first violins, with lovely extended harmonics whose freedom from grain or edginess would certainly quiet the disparaging comments of the digiphobes.

Along with the midrange and high-frequency smoothness was the virtually seamless transition to mid-bass and bass frequencies. Contrabasses were clean and darkly resonant, cellos rich and mellow. Most importantly, the bass did not seem a separate element, sonically apart in speed of response or musical perspective from the upper frequencies. The disparity in the speed of propagation between the membrane and dynamic drivers in most hybrids is obvious and musically distracting.

The RT-7s reproduced the huge bass drums in the largo movement of the Shostakovich Symphony No. 13 ( London 417261-2) and on Stravinsky's Rite of Spring ( London 417325-2) with accuracy of timbre and consider able impact, if not the weight I hear from my reference Duntech Sovereign speakers. (There is also a small matter of some $13,000 difference in price!) Much the same could be said for the excellent organ reproduction from the RT-7. Transient response was in a class by itself. One hears how incredibly clean Tom Jung's dmp recordings really are-the attack and timbre of acoustic and electric guitars, the plangency of bells, the shimmer of cymbals, the explosive energy of snare drums and rim shots, and the subterranean rumblings of synthesizers.

The reproduction of voice is also revelatory. The richness, expressiveness, and all the various nuances which are so important in vocal projection-for example, the resonant baritone of Fischer-Dieskau in the Mahler lieder (Sony Classical SK-44935)--are wonderfully delineated by these speakers.

The RT-7 does a good job of handling the extreme dynamics of John Eargle's recording of Bartok's Miraculous Mandarin (Delos DE-3083), although not with the massive weight and output of the Sovereigns.

It would be wonderful if sometime in the future a truly full-range membrane-type speaker, with high efficiency and full dynamic response, became avail able. In the meantime, the hybrid membrane/dynamic driver design is a valid approach, especially if it is as well executed as the Clements RT-7. This is not a "flashy" speaker for those who like exaggerated and highly colored sound. But for those who want an ac curate speaker which faithfully pre serves musical values, the Clements RT-7 is a remarkable achievement.

(adapted from Audio magazine, Jan. 1991; Bert Whyte)

= = = =