The ESLs work in a very different way to conventional loudspeakers. For those who don’t know, here’s a brief explanation of the principle. Whilst most speakers work with a cone type drive unit, the ESLs use a very thin membrane, one-tenth the thickness of a human hair. This MYLAR diaphragm has a special conductive coating and is stretched between two electrode plates. Relative to earth, both electrode plates have a high positive charge (+5000v), but there is a slight difference between the charge on each electrode. It is this difference that causes the negatively charged membrane to be more attracted to one than another, thus causing the movement of the diaphragm.

The diaphragm is mounted on an open frame and is thus free of any coloration from a cabinet. Because the diaphragm acts as a full range driver, there is no need for a crossover. There are no discontinuities in the frequency response curve, so you get the full music spectrum, as it was recorded.

The ESLs do much more than reproduce the full range - and this is where all other electrostatic speakers stop. Acousticians have recognised for a long time that an ideal loudspeaker should be a point source from which sounds waves ripple like a pebble in a lake. Ordinarily, an electrostatic would fall at this hurdle, but not the QUAD ESLs.

By using a series of concentric anodes, rather than just two plates, the Quads are able to produce a spherical sound pressure pattern. A series of electrode rings are fed with delay lines, so each ring responds to the change in current a split second after the previous ring, creating movement in the diaphragm identical to the ‘ripple in the pond’. The 988s and 989s both use this principle, with the 989s having additional panels to give a robust bass performance.

The Quad ESL is the only speaker to use this method so if you want the finest, most accurate sound in existence - better choose QUAD.