|Leak High Frequency
The Leak Electrostatic loudspeaker for high frequencies was never put into production - but it came very close indeed, and it is likley that many "pre-production" units were made.
The photo at the left is of a prototype unit that was discussed in a paper referenced below. This paper foucsses on Leak's search for high-quality transducers that feature "piston action", and along with the ribbon speaker, the electrostatic speaker was a contender (as the actuating force is applied evenly across the surface of the electrostatic diaphragm, thus preventing diaphragm "break-up" found with cone speakers at high frequencies).
In developing the prototype electrostatic speaker, Leak considered the chararcteristics of both the single-ended type, and the push-pull type (referencing F.V Hunt, "Elctroacoustics" from Harvard University press for much of the fundamental acoustic work).
The prototype was of the balanced push-pull variety, and featured very low distortion. In push-pull types of electrostatic speakers, the distortion is reduced by charging the central diaphram via a large series resistor. Leak considered that it was possible to achieve distortions as low as 0.1%. The unit showed ultra-smooth frequency response from 1.5kHz to 20kHz, +/- 2dB, both on-axis and 30 dgrees off-axis.
In conclusion H.J Leak stated that (in December 1955):
"At present the author cannot produce an electrostatic bass loudspeaker having both the necessities of reasonable size and great reliability. This is because materials of suitable form and physical properties have not yet been made. The problems are technological, and they will be solved in time - probably within a year."
The role of development of a full range electrostatic speaker was also being invetsigated by QUAD around the same time. This rival audio-pioneering company was highly successful with its research into electrostatic types, and marketed such a speaker in the mid/late 1950's. This speaker is still very highly regarded 40 years on.
Leak went on to develop, and very nearly commercially release a "high-frequency-only" electrostatic loudspeaker. Leak were advertising this electrostatic loudspeaker in pamphlets from around 1956 (the photo at the right of the production model is from a Leak pamphlet that also described the Varislope II pre-amplifier, and the Leak moving coil pickup). Actual full production appears to have been halted. The reason for this is not known.
Leak susequently took a new approach to loudspeaker design as a result of a paper published by D.A.Barlow in Wireless World ("Rigidity of Loudspeaker Diaphragms", Dec 1958). This paper discussed the advantages in using stressed-aluminium skins for fabricating loudspeaker diaphragms that displayed "piston-action" over a wide freqeuncy range. It was "piston-action" that Leak had been pursuing from the mid-1950's, and whilst ribbons, and electrostatic types were considered, it was to be this new developement that would give Leak the high-quality "piston-action" speaker they had been searching for.
Reference and Acknowledgments:
Technical information and photo of Leak Electrostatic prototype speaker from Journal of British Institute of Radio Engineers, December 1955: "High Fidelity Loudspeakers: The Performance of Moving Coil and Electrostatic Transducers" by H.J.Leak.
Photo of Leak Electrostatic pre-production speaker taken from Leak advertising leaflet kindly supplied by Philip Thornton-Evison of the United Kingdom.
Photos enhanced by kVs.