A History on electrostatic loudspeakers

A history

Quad advert (date unknown): The first public demonstration of ES devices was in 1881, when Dolbear exhibited a complete ES telephone system at the Paris Electrical Exhibition. Between 1920 and 1935 several ES speakers and microphones were made, mainly in Germany. In this country the Primustatic speaker retailed for 25s. in the early 1930s. It consisted of a single perforated aluminium sheet, and a fluted metallised paper diaphragm. The insulation was partly a special lacquer on the perforated sheet and partly the fluted paper, which was waxed. Hairy strands of wool held the fluted diaphragm in Place but allowed movement. The whole was curved to produce wide-angle radiation of the higher frequencies. The speaker was usually connected as in fig. 2B, when the PD between anode and chassis partially collapsed the thin diaphragm on to the fixed plate. Signals increased and decreased this PD, causing more and less attraction respectively, and corresponding movement of the diaphragm and the air surrounding it. It worked very well at high frequencies, but as the programme material in those days had no high frequency content other than microphone hiss and record surface noise, it never became popular. We will appraise its technical capabilities later.

For the next twenty years, the only ES audio devices in general use were condenser microphones. Some of these were superb, and one of them undoubtedly played a vital role in the success of "ffrr" in its early days.

While the mechanical simplicity remained a tantalising attraction to acoustic engineers, the known high distortion when used as a loudspeaker slowed further development. Soon after, the Second World War provided other and more urgent problems.

In 1954, Prof. F. V. Hunt's book "Electroacoustics" provided the key to the distortion problem. He gave a very rigorous mathematical examination of the ESL in various forms. His calculations showed that in push-pull form, like fig. 1, it was capable of handling large signals without much distortion if the diaphragm was given a constant charge instead of a constant potential or voltage. The push-pull form was not new; models were on sale in Germany around 1920 ! But the constant charge idea was new and once more work began in ESL design.

By 1955, P.J. Walker of the Acoustical Manufacturing Co. (later Quad) had produced two laboratory models, and these were demonstrated at the B.S.R.A. Annual Exhibition in May, 1955. One was open back and front and handled middle and high frequencies with amazing clarity. The other covered the whole frequency range with equally good quality and used a large closed cabinet or infinite baffle to maintain its bass response. The completely open type was further developed to cover the whole frequency range, and in March, 1956, it was demonstrated to the Physical Society in this form. This was the world's first full-range ESL in open or doublet form.

Between these two events, H. J. Leak had demonstrated a very successful ES tweeter (above 1,000 c.p.s.) and in America several ES tweeters appeared. By April, 1956, at the Audio Fair three ES units were being demonstrated; the newcomer was another fullrange model by Goodmans. The journey from laboratory model to factory production is seldom easy, and this one is no exception, since it covers completely new ground. There are no one else's mistakes to benefit from, and no one else's success to copy from ! It is good going to reach production at acceptable high-fidelity standards in a mere 4 years-the moving coil speaker has had a 30-year start !

Taken from the review of the Quad ESL 57 in the November 1957 edition of HiFi News. The concept of electrostatic speakers has been known about since the discovery of electricity. The first electrostatic speaker was designed by Dolbear. This device was incorporated into an electrostatic telephone system which was demonstrated at the Paris Electrical Exhibition of 1881. Electrostatic speakers continued to be made during the 1920's and 1930's mainly in Germany. In the UK the Primustatic loudspeaker was available for 25 shillings (£1.25). For the next 20 years the only electrostatic (ES) devices available were condenser microphones. Development of ES devices stopped during the 1940's and wasn't started until the mid 50's.

The main stumbling blocks to successful ES devices were the fact that the theory hadn't been worked out properly and the theoretically massless diaphragm couldn't be made from the materials available at the time. In 1954 Prof. F. V. Hunt's book "Electroacoustics" provided the solution to the theoretical design and a year later DuPont patented a new material called Mylar. All the previous main hurdles to a successful ES speaker had been overcome and several companies started to build prototypes.

By 1955, Peter Walker who owned Quad was demonstrating two prototype designs of ES speakers. One had an enclosure similar to the majority of all loudspeakers, and the other was an open back device. This second type was further developed to cover the whole audible frequency range and was first demonstrated as a working design in 1956.

The design of the ESL 57 remained virtually unchanged for its entire life in production. How many other products can this be applied to ? The ESL 57 was finally phased out after a production run of 25 years and 55,000 units produced.