The patents discussed in the Links Above cover all the materials examined formally by Peter Walker in his Patent Application U.S. 3,773,984 for the speaker that became known as the ESL '63. Where relevant, both the GB and US patents are referred to, assuming that they exist. Some US patents have no GB equivalent, and vice versa. If any reader of this page has knowledge of other interesting and directly related technical papers on this subject I would be glad to hear of these. The series of references is very eclectic, covering not only feedback as it applies to amplifier systems (the Quad '63 uses a form of feedback), but one paper, by Stolaroff, on magnetic recording. There are, naturally, references to other electrostatic speaker patents, including the monumental Patent granted to William W. Wright of Dayton-Wright fame.
The Patents are arranged below in chronological order by date of grant, not date of application. This will create confusion in some people's minds as to why the speaker is referred to as the ESL63, as Peter Walker's patent was granted on November 20, 1973. For the unitiated, it is called the ESL '63 because that is when Peter Walker made the first entries in his note book about this new speaker. The technology has not really change in the 988-989 series of speakers, which still employ the ESL '63 annular ring schema and the constant current drive system, with the addition of high quality electrostatic woofers.