The Quad ESL was the world's very first full range, commercially available electrostatic loudspeaker. In spite of its great antiquity (some 50 years old to date) it was voted the "Greatest Hi-Fi Product of All Time" by Hi-Fi News and record review in the January 2000 issue of that journal.
The Quad ESL was first introduced to the public arena in 1957, although it was demonstrated on several occasions prior to this. It remained in production for some 25 years, being replaced by The Acoustical Manufacturing Company, Huntingdon, England in 1982. For many listeners and music lovers, this speaker remains the marque against which all other speakers with pretention to high quality are measured. Even it's successor in the Quad stable, the ESL '63 has never attracted quite the same following.
This speaker is often referred to as the Quad 55 or the Quad 57. If you respect the wishes of the designer, you call it the Quad Electrostatic Loudspeaker - that's all. IF you would like to investigate the origins of this confusion then go to Hard Core.
If you are devotee of this particular loudspeaker, or the electrostatic format, you should find this site particularly interesting. Even if you are not a fan, the historical and technical details contained in this site could be of some interest.
The various site locations you can visit are: