Forgive me if I go a bit overboard but I love talking about this stuff.
Discussions of ESL’s versus dynamic speakers usually touch on their respective narrow versus wide dispersion patterns. Some love the beaming (magical imaging) and others hate it (head-in-a-vice effect). Unfortunately; physics doesn’t permit having both the magical imaging and a nice wide sweet spot, simultaneously.
Personally; I love being in the focus of a big pair of flat panel ESLs… but not for long periods, and not at all when guests drop by (as not everyone can sit in my lap).
Both the curved ML’s and the sequentially phased Quads remedy the head-in-a-vice effect; although I prefer the Quads.
A lesser known but more versatile remedy for the head-in-a-vice effect employs electrically segmented stators. Some commercial versions have a separate narrow stator section for treble frequencies, and some have three stator sections for the lows, mids, and highs.
Some cutting-edge ESL’s in the DIY community use symmetrical, multi-segment wire stators to tailor the dispersion. This scheme is somewhat analogous to the Quad model 63, with some important differences explained below.
The Quad 63 uses a separate mid/high center panel, with concentric ring conductors powered sequentially thru an LC (inductor/capacitor) delay line, to emulate a point source projecting a spherical wave front.
Below is a schematic of a multi-segment wire stator ESL employing (15) six-wire groups driving the diaphragm from the center-line outward-- to emulate a line source projecting a cylindrical wave front.
In any driver circuit, inserting a series resistor, with a capacitor in parallel, forms a first-order low-pass filter. In the case of a segmented ESL; the separate wire groups, coupled with the opposing oppositely charged diaphragm, are the parallel capacitors. We need only insert the appropriate value series resistors between these wire groups to form a series of low-pass filters—as shown in the schematic below.
As driven by the segmented stators; the entire diaphragm radiates the lower frequencies, but the filter network limits the highest frequencies to only a narrow vertical band in the panel centerline. And the filter network progressively chops off the highs, moving toward the panel edges.
In addition to the frequency attenuation; the charging time of the wire group capacitors also imparts a progressive phase change which slightly curves the wave front. The effect is wide, smooth trending dispersion, as visually depicted in the directivity sonograms shown below. The sonograms compare a non-segmented flat panel, a curved panel, and a segmented panel.