GB346646A - Improvements in and relating to sound reproducing devices - Google Patents

Abstract

346,646. Loud-speakers; circuits for electrostatic instruments. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London.-(Assignees of Kellogg, E. W.; 1185, Ardsley Road, Schenectady, New York, U.S.A.) Sept. 24, 1930, No. 28620. Convention date, Sept. 27, 1929. [Class 40 (iv).] An electrostatic loud-speaker, Fig. 1, comprises a diaphragm divided into a number of sections 10 interconnected through inductance elements 13 to the output circuit of an amplifier 11. The dia. phragm sections may co-operate with a rigid perforated electrode or may, as shown, be arranged between two rigid electrodes 14, 17 which are connected through batteries 16, 19 to one of the amplifier circuits which, for convenience, is designated as the grounded terminal. The rigid plate or plates are preferably formed in corresponding sections interconnected through conductors 15, 18. A damping resistance 12 is provided as shown. Fig. 3 shows a modification in which inductance elements 22, 23 are provided between the sections of the plates 14, 17, the diaphragm being either in the form of a continuous conducting sheet or of separate sections interconnected by conductors 21. In order to ensure maintenance of phase opposition, one or more auto-transformers 25 may be connected at intervals between the plates 14, 17, the mid-points of the windings being connected to conductor 27 to which the damping resistances 26 are connected and which is connected both to the cathode of amplifier 11 and to a mid-terminal on the secondary of transformer 24. The provision of inductance elements as shown serves to neutralize the capacitance between the electrodes and such an arrangement allows a much larger total capacity to be employed without overloading the amplifier and ensures that the impedance of the device is the same at all frequencies within the essential audio range. The phase differences between the several sections of the diaphragm may be utilized to produce a desirable distribution of sound. Fig. 4 shows an arrangement which may be employed for producing a beam of sound which spreads horizontally like a fan. The diaphragm is in the form of vertical strips a .. e, a wavelength or more in height and the strips are interconnected as shown so that the output from the amplifier passes through the strips a, b, c, d, and e in that order. Such an arrangement produces a convex wave front but a concave wave front may be produced by reversing the connections. A perfectly symmetrical arrangement can be obtained by connecting the strips on opposite sides of the middle strip in pairs. In addition or as an alternative to the method employing phase-difference effects, the direction of the sound can be controlled by disposing the radiating elements over a curved instead of a plane surface in which case the diaphragm might be given a cylindrical form.