"Robert Macy"
"Phil Allison"

Good electrostatic speakers ( eg Quad) can have THD levels of 0.03%, even
at
96dB SPL at 1 metre.

True, electrostatic speakers seem to provide the best performance for
the buck. Back in the 70's there was an engineer that lived in the
eastbay, Fremont, CA area that made sets of electrostatic speakers
using readily available materials, standard tools, and kitchen
appliances. His resulting speakers were 'metalized' mylar shrunk in
his oven over a framework.
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** The diaphragm of an ESL needs to have a VERY high resistance coating -
so that charge spreads slowly across the surface and does not move when an
external electric field at audio frequency is applied.

The resistance needed is in the hundreds of megohms per square.

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As part of his development during the project, he had taken his
speakers to JBL(?) in the midwest for testing in their lab and found
that at most levels distortion was barely measurable, at 120dB(?)
something like 0.1%, and at 140dB just above 1%. The main problem he
ran into was the 'beaming' effect caused by the size of the diaphragm.
To minimize that beaming, he actually split the audio into several
spectral bands and then drove strips of different widths on the
speakers in order to minimize the effect. Higher frequency narrow
strip.

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** Ever see a Quad ESL57 ?

The tweeter is a 3cm wide strip, mids come from a similar pair of strips
each side and there are two, fairly large rectangular bass panels outside
them. Horizontal dispersion is at least 15 degrees at the highest
frequencies but vertical dispersion is much narrower.

Crossovers are simple 6dB/octave and there is some overlapping - despite
which transient response is near perfect at any frequency.


.... Phil