the flat response - Hificritic.com

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should not be taken too literally in terms of the analysis of sound quality; if audiophiles were fully briefed on the limitations of most designs, many would refuse to buy a speaker at all. What does matter is the subjective judgment of the final result, governed by price and expectation. As the report developed, the Duettas and the Logans begin to position themselves at extremes of technology and specific performance, while the Magneplanar emerged as the classic, balanced, middle-of-theroad contender. Design Compromises Although radically different in appearance, the Duetta and the CLS are both mediumsized, 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 2 composite multiple-ribbon systems for the mid and treble ranges. 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 - i.e., 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 wellintegrated full-range system this difficulty greater than the increase in complexity. Apogee Duetta Details 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 aluminium foil/Kapton ribbon with three series connected conducting paths is suspended between the poles of a powerful, linear, open magnet. The radiation is bidirectional and dipolar, 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 save at 90 degrees to the speaker, 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, also a dipole, and quaintly 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 the audible range. At present, it is inconceivable that a loudspeaker could be made to operate full range
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 infrasonic frequencies. Apogee has overcome, in a number of ways, many of the fundamental resonance problems of such a stretched diaphragm. The moving element of their woofer is a laminated combination of aluminium foil and Kapton plastic tape with good selfdamping properties. The element is generally pleated in the horizontal direction to increase its resistance to bending in that plane, while the shape is semi-trapezoidal to maximize its geometric asymmetry and thus control the natural upper frequncy resonant modes. Finally, the upper and lower sections of the diaphragm are differentially tensioned by adjustable clamps to provide a broad, double-tuned lowfrequency 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 from quite a high frequency, here about 150Hz and is generally augmented by some intended diaphragm resonance(s). After passing through a fairly level region, the output begins to fall naturally with increasingly frequency 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 roll-off acctually begins at over 1kHz, and the crossover overlap is deliberately broad to help fill in the energy. 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 does decline at the top of its range. In the case of the Duetta, this inflection is at frequencies above 12kHz. The Scintilla's more extended top-end response is assured by its vertically arranged line of four half-inch wide ribbons of wider directivity, 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. The virtual line source, but narrow form achieves good horizontal directivity. However, owing to the wide magnet gap, necessary to accommodate the freely moving 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 obvious penalties to pay for this low sensitivity in terms of required amplifier power. The Duetta's low-frequency driver is necessarily subject to some intrinsic resonance modes (as does the Magneplanar) and, in addition, suffers from some acoustic obstruction due to the perforated steel-plate baffle back plate used to support the massive array of single sided planar magnets. These provide the distributed magnetic field associated with the zigzag current flow of the diaphragm conductor pattern. The acoustic “window area” of the bass section is not very high, though if this were increased, rigidity would suffer. The bass driver, having a one-sided magnetic field, is driven in single-ended rather than the preferred push-pull mode. In theory, this
Page 1: _____________________ THE FLAT RESP
Page 5 and 6: lossy foam-plastic mounting is used
Page 7 and 8: I shall leave sound quality rather
Page 9 and 10: This CLS was better balanced than b
Page 11 and 12: For the record, the Quad ESL-63, wh
Page 13 and 14: Duetta can be rated well enough . F
Page 15 and 16: 50Hz, and, in my view, showed exces
Page 17 and 18: FIG.10 The CLS offered a below-aver
Page 19: * Provisions have been made for the

the flat response - Hificritic.com

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