The patents shown in the Time Line below cover all the materials examined formally by Williamson & Walker in their Patent Application U.S. 8,008,014 for the speaker that became known as the ESL 57. 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 ESL 57, as the Williamson and Walker patent was granted on November 7, 1961. It is called the ESL 57 because that is when the speaker came onto the general market (Pat. Pend.).   The back plate on the speaker refers to the device as The Quad Electrostatic Loudspeaker (so there).