Quad Electroacoustics
The first fully successful full-range electrostatics, and also among the most respected, was produced in 1957: the Quad Electrostatic Loudspeaker (Quad ESL, later ESL-57) from Quad Electroacoustics, of Huntingdon, England. These were shaped somewhat like a home electric radiator curved slightly on the vertical axis. They were widely admired for their clarity and precision, but can be difficult to run while achieving low frequency bass output.
The Quad ESLs were designed by Peter Walker, founder of the company, and David Williamson. The first in the series was the ESL-57, influenced by U.S. Patent 1,983,377 developed by Edward W. Kellogg for General Electric in 1934. It was introduced in 1955, put into commercial production in 1957, and discontinued only in 1985. In 1981, Quad introduced the ESL-63 as a successor to the ESL-57. It attempted to address both the deficiency in bass reproduction of the ESL-57 and its extreme directionality at high frequencies. The latter goal is achieved by splitting the stators into eight concentric rings, each fed with a slight time delay compared to the ring immediately inwards, thereby attempting to simulate a point source.
The ESL-63 remained in production until 1999, when Quad introduced the ESL-988 and the ESL-989 – both of which the company still produces. Quad Electroacoustics introduced two new models in 2005, the smaller 2805 and the larger 2905. These models return to the slightly back-tilted stance of the original designs, albeit user-adjustable. Largely retaining the larger bass panels of the 98x models and concentric ring design of the ESL-63, the 2x05's feature heavier and far more rigid construction, and several electronic and transducer refinements.
Read more about this topic: Electrostatic Loudspeaker, Commercial Speakers