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would confer higher distortion levels, but in practice, the distortion from such a large radiating area is quite negligible, even at realistically high sound-pressure levels as the excursion remains small. Due to their large area, the Duetta diaphragms radiate heat well, providing a high power handling which is necessary in view of the low sensitivity. A particular feature of the Apogee drive system is its very high inherent linearity, resulting in a freedom from the compression effects apparent in many moving-coil systems, as well as in some electrostatics when the peak handling limits are exceeded. Martin-Logan CLS Turning to the Martin-Logan CLS full range electrostatic, this open-panel speaker must also suffer the low-frequency roll-off limitation of a finite sized baffle or in this case open radiator, and copes with this by means of a designed resonance at 50Hz. (Quad similarly resorts to a medium Q low frequency resonance). The electrostatic principle is attractive in many respects, not least because it allows the use of an extremely light diaphragm - so light, in fact, that its vibratory contact with the air molecules may be used as a palliative, controlling, damping mechanism. Light diaphragms can thus move quickly, promising good high-frequency response, while their low inertia assures a low level of stored energy, promising an accurate transient response. In principle, the electrostatic driver is very efficient in its conversion of moving electric charge into acoustic power. Problems arise, however, in interfacing the necessarily large electrostatic and low acoustical impedance element to the room acoustic, as well as to the power amplifier. The science of electrostatics is the science of high voltages, with the polarizing field established by as much as 7kV applied to the inner-shielded diaphragm. Ideally, the fixed-mesh electrodes on either side of the diaphragm need to be driven in push-pull at upward of 500V, and require isolation from the user. (In the case of the Quad, the electrodes are protected behind grounded mesh screens.) A thick, tough, black plastic insulating coating is applied to the strong CLS steel plate electrodes; these simultaneously form the external grilles of the speaker. Highratio step-up transformers are used to couple the amplifier to the diaprhgm, while the reactive input characteristic is not one that allows for optimum power transfer. An electrostatic offers an input characteristic that is predominantly capacitive, hardly an ideal load. By the time these considerations have been taken into account, as well as the need to improve the narrowing directivity with frequency of a single large diaphragm, the much-vaunted efficiency has been whittled away. Nevertheless, most electrostatics have a basic sensitivity of around 85dB/W (8 ohms), which is rather better than that achieved by present large ribbon systems. Martin Logan have some interesting design features which are used to gain a more favourable directivity. The CLS diaphragm is unusual, for although it is only 0.004" thick, and consequently very light (the whole moving system is reckoned to weigh the same as one cubic inch or 16.4ml of air), it is formed from a pretty rigid plastic and is almost self-supporting. No surface damping is used in the diaphragm; its stretched “skin” and “plate” vibration modes are partially controlled in the first instance at the boundaries, where a
lossy foam-plastic mounting is used, and by the acoustic impedance of the air load imposed on the diaphragm. ( but also by the positioning of the further foam support strips) . For comparison, the Quad ESL-63 uses a rather thinner 0.00137" diaphragm, (with 0.001" plastic film for the dust-cover membranes.) The CLS uses a density of Mylar polymer similar to Quad, here giving a total mass of around 20 milligrams. (By comparison the ESL-63 is even more extraordinary, its diaphragm weighing just 0.003gm, or 3 milligrams, not too far removed from the tip mass of a pickup cartridge!) Another interesting comparison is with a typical wide-band moving-coil dome tweeter, where the moving mass is typically around 200mg. Quads enjoy higher levels of air-damping than Martin-Logans and, in addition, use an interlayer of the sheerest gauze to provide some resistive mechanical contact to the diaphragm surface, damping free modes. From its appearance it is obvious that the CLS diaphragm is broken up into discrete radiating areas by damping strips. It is possible to physically deflect these separate “cells” by blowing on them. As when air is blown over the necks of differently sized bottles, each one has its own distinct “sound”. By this differential multi-cell means, dominant resonances are moderated and dispersed by dimensioning each rectangular cell differently. The vertical side compartments of the CLS diaphragm are of fairly high Q, and are tuned to 50Hz. Above resonance, this level falls at 2 natural rate of 6dB/octave, partly compensating for the naturally rising 6dB/octave response of the panel as a whole. Beyond this frequency, range, the double section, frequency -compensated, step-up transformer applies a further reduction in mid/treble level. Finally, the distribution of the cells and the horizontal arc of the curvilinear diaphragm complete the frequency balancing while increasing the horizontal coverage. A full-range planar diaphragm like the CLS becomes increasingly directional as the wavelength of sound approaches the size of the panel. In the Magneplanars and Apogees, this is solved by allocating the subsequent frequency range to narrower and narrower line-source elements. For the concentric multi cell Quad, the diameter is subdivided no less than eight times, providing a controlled reduction of element size with frequency, and thus maintaining a good radiation angle. Martin-Logan deals with the problem in a different way: the electrostatic panel is formed into a nearparabolic contour with a prime radiation approaching a vertical half-cylinder. With this broader-angled distribution, some of the natural rise in frequency response is also offset. If the diaphragms were sufficiently well damped, and dimensionally stable to operate without the subdividing cells, this geometry would approach the ideal. In practice, the cells boundaries terminate the high frequencies in a non-uniform manner, resulting in an arrays of treble radiators rather than a uniform whole. Such an array is likely to result in an off-axis polar response with a complex fine structure at high frequencies. This will generate some level of random amplitude and phasing irregularities when a
Page 1 and 2: _____________________ THE FLAT RESP
Page 3: in a true ribbon form. The magnet s
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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