Apogee Duetta II loudspeaker Martin Colloms
Panel speakers are back in fashion. After years of determined effort on the part of a few established manufacturers, the number of these designs has seen a marked increase, and many new exotic models have appeared. Stereophile has recently reviewed two leading examples of the genre, the $2780/pair Apogee Duetta and the $2490/pair Martin-Logan CLS, (both in Vol.9 No.7). To judge by the tone of letters arriving at the magazine's offices, the reviews generated heated controversy. John Atkinson asked me, therefore, to conduct an in-depth examination of the two models, to give a fuller picture of what these speakers are capable of.
Although radically different in appearance, the Duetta and the CLS are both medium-sized, floor-standing, open-panel speakers. The Duetta is a two-way system, with a moderately sized, broad-band ribbon tweeter working through the upper mid and treble ranges. It is related to the three-way Scintilla, which uses a composite multiple-ribbon system for the mid and treble range.
The number of "ways" is crucial to a design, since no single transducer can properly encompass the breadth of the audible frequency range. From the design viewpoint, the more ways—ie, the greater the number of divisions of the frequency range—the more freedom there is for the engineer to trim and balance the frequency response to provide a natural sound. Conversely, with a low number of "ways," the designer has to fight for continuity and an even power over the range, not always with complete success. Increasing the "ways," however, makes it more difficult to design a well-integrated full-range system out of proportion to the increase in complexity.
Apogee has pioneered the use of modern "ribbon" technology, and In the case of the Duetta, this description is certainly true for the HF unit. Here, a tall, pleated aluminum foil/Kaptan ribbon with three conducting paths is suspended between the poles of a powerful, linear, open magnet. The radiation is bidirectional and bipolar, the energy behind being out of phase by 180 degrees with that for the front. Unless placed very close to a back wall, this will not result in specific cancellation, since by the time wall-reflected rear waves have combined with the direct radiation, their phase is randomly indeterminate. Suppose the panel is typically placed at five feet from the back wall, and the listener is 15 feet from the speaker. At the low frequency end of the range handled by the tweeter ribbon, say 1kHz, the listener is 15 wavelengths away from the front-panel sound, and 25 wavelengths from the reflected sound to the rear. Given normal room reflections, specific cancellations will be diffused and of small significance; with reducing frequency, however, this factor will assume increasing importance.
The lower frequency range of the Duetta is handled by a stretched film diaphragm, tensioned and supported over its whole boundary perimeter, and referred to by Apogee as a "ribbon." Its motion is akin to a drum skin, as is that of the Magneplanars and the vast majority of electrostatics. This is quite distinct from the piston-like, push-pull, action of a freely suspended ribbon whose fundamental to-and-fro resonance may be so low as to occur below audibility. At present, it is inconceivable that a loudspeaker could be made to operate fullrange in a true ribbon form. The magnet system would be so huge that no-one would be able to move it. It is also likely that such a design would overload prematurely at subsonic frequencies.
Apogee has overcome, in a number of ways, many of the fundamental resonance problems of a stretched diaphragm. The moving element of their woofer is a laminate combination of aluminum foil and Kaptan plastic tape with good self-damping properties. The element is generally pleated in the horizontal direction to increase its resistance to bending, while the shape is semi-trapezoidal to maximize its geometric asymmetry and thus disperse resonant modes. Finally, the upper and lower sections of the diaphragm are differentially tensioned to provide a broad, double-tuned low-frequency resonance, rather than one of a higher Q.
Several factors control the frequency response of an open panel. At low frequencies, the bass rolls off at 6dB/octave and is generally augmented by some deliberate diaphragm resonance. After passing through a fairly level region, the output begins to fall naturally and the drive signal must be crossed over to the next driver before this happens—in the case of the Duetta, around 500Hz. For the mid-treble ribbon element, the low-frequency rolloff begins at over 1kHz, and the crossover overlap is deliberately broad to help account for this. Once again, given some consideration of the acoustics of the vertical slot in which the tall ribbon tweeter operates, and the mild canting of the element to improve the vertical directionality, the treble ribbon's output falls at the top of its band. In the case of the Duetta, this is at frequencies above 12kHz. The Scintilla's more extended top-end response is assured by the final set of four half-inch ribbons, which augment the output from the mid-treble ribbon.
The Duetta's treble ribbon is a wide-range transducer, covering a range from some 500Hz to 22kHz, and is of exceptional sound quality. Advantages of this large ribbon include a virtual absence of resonance or coloration, with great acoustic transparency—no physical obstruction before or behind the driver element, plus direct coupling of the applied electro-mechanical force to the air. Good horizontal directivity is achieved by the virtual line source form. However, owing to the wide magnet gap which is necessary to accommodate the conductor, the tweeter does not offer much sensitivity. Running the three conductor elements of the ribbon in series brings the load impedance to 4 ohms, but the sensitivity remains very low, in speaker terms, at typically 78dB/W (this is an 8-ohm watt), which is some 10dB below the average. There are penalties to pay for this low sensitivity in terms of required amplifier power.