Patent GB-1358057-A (GB)
Title: Electrostatic loudspeakers
IPC: H04R19/02
Filed: 1971-10-11
Abstract: 1358057 Loud-speakers L M BOBB and C C POND 11 Oct 1971 [9 Oct 1970] 47274/71 Headings H4X and H4J An electrostatic loud-speaker (Figs. 1, 2) includes a perforated relatively inflexible electrode 15 spaced from a membrane 18, 19 having one or more conductive layers 20, 21 by a spacer 16, 17 of compressible, acoustically porous dielectric material in engagement with both the plate 15 and the membrane 18, 19 and having points of contact distributed over the area of the membrane 18, 19. As described the loud-speaker is of push-pull type. Electrode 15 is of aluminium and its edges are engaged in a groove formed in a frame between outer strips 12, 14 (Fig. 3) and a narrower intermediate strip 13. The strips 12, 13, 14 are adhesively laminated to each other. The spacers 16 and 17 are then inserted. The membranes 18, 19 are adhered to the strips 12, 14. The membranes 18, 19 are smoothed to remove wrinkles but are not pretensioned. The overall thickness of the spacers 16, 17 equals the overall thickness of the strips 12, 14. One or more small apertures in one or both membranes 18, 19 away from a conductive layer ensures equalization between the inter-membrane space and the ambient air pressure. Electrical connection to the electrode 15 is made by a self-threading screw (not shown) passing through strips 12, 14 and lightly engaging electrode 15. Alternatively, aluminium strip is used having one end folded around an edge of electrode 15 and then extended to overlie and to be clamped to one of the outer strips 12, 14. The conductive layers 20, 21 on the membranes 18, 19 are divided into separated areas by erasing, abrading or etching in narrow strips 22, 23. Alternatively the layers 20, 21 are applied through a mask. Contacts 29, 30, 31 for one layer 21 and contacts 29a, 30a, 31a for the other layer are connected to a matching transformer T (Fig. 5a). Bias voltage HV is applied between a transformer tapping and the plate 15 so as to cause the membranes to be drawn towards electrode 15. Resistors R 1 , R 2  form a low pass network with the speaker capacitance and the total membrane area radiates low frequencies. At higher frequencies the areas 21b, 21c, 21d remain operational, while areas 21b alone radiate the highest frequencies. The conductive pattern shown effects good frequency radiation. Alternatively a circular membrane (not shown) has a central conductive area and concentric annular areas of progressively decreasing width. The membranes 18, 19 are preferably of polyethylene terephalate coated with aluminium having a resistance of 3 to 5 ohms per square. The spacers 16, 17 are open-cell foamed polyurethane or polyethylene sheet preferably embossed in a waffle pattern to provide lands 17a and valleys 17b. Alternatively the spacers 16, 17 are of uniform thickness or the valleys 17b are stamped out leaving only the lands 17a, which may be interrupted. The distribution of areas or points of support of the membranes 18, 19 by the spacers 16, 17 may be varied in selected areas of the membranes but within each selected area the support distribution is preferably uniform. The thickness of the spacers may alternatively be altered. A loud-speaker system (Figs. 9, 10) comprises a rectangular cabinet having a front wall 32 and side walls 33. A dynamic loud-speaker 34 is in the lower part of the cabinet below a horizontal partition 35. A push-pull electrostatic loudspeaker 36 in the upper part of the cabinet is exposed for sound radiation through the front wall. Two reflector walls 37 within the cabinet meet at an apex just behind the electrostatic loud-speaker 36. The side walls 33 of the upper portion of the cabinet are provided with apertures 38 for radiation of sound from the rear of loud-speaker 36. An alternative loud-speaker system has a dynamic loud-speaker 34 (Fig. 11) beside an electrostatic loud-speaker 36. A single inclined reflector 37a reflects sound through an opening 39 in a side wall. The reflector 37a divides the cabinet into two separate volumes.