Patent GB-303828-A (GB)
Title: Improvements in or connected with sound reproducing devices
IPC: H04R1/42
Filed: 1928-12-10
Abstract: 303,828. British Talking Pictures, Ltd., (Assignees of Forest, L. de). Dec. 10, 1927, [Convention date]. Condenser telephones and other electrostatic instruments.-In a sound reproducer in which the flow of compressed air is controlled electrostatically, the whole of the gas stream is caused to pass between plates or screens subjected to varying potentials and closely disposed so that onlv a lamina or film of gas is contained therebetween. The air or other gas may be ionized prior to and/or during its passage through the electrostatic, field. In the construction, Fig. 1, one end of a metallic casing member 1 is closed by a plate 7 of insulating material so as to form a chamber 3 to which compressed gas is supplied through the tube 4. Apertured plates 5, having their inner edges formed into a V-shape., are clamped between the casing 1 and a cover piece 6, the whole forming one electrode of the electrostatic device. The other electrode is formed by a series of plates 9 mounted on the reduced end of a rod 8 mounted on the plate 7. A terminal 11 on the casing 1 is connected by a wire 12 to ground at 13, whilst the rod 8 is connected to one end of the secondary 14 of a Tesla coil, the other end of which is connected through a blocking condenser 15 to wire 12. A radio frequency oscillator for supplying the high voltages to the condenser electrodes is connected across the primary 16 of the Telsa coil as is also an audio frequency amplifier for delivering the speech currents to the device.. The Tesla coil may be replaced by any suitable source of direct high voltage, such as a, high voltage rectifier system. Any suitable form of sound wave directing arrangement, such as a born, may be attached to the cover piece 6. Fig 3 shows a modification in which a number of apertured plates 25 are mounted in spaced relation within a unitary hollow: casing 17, a series of. plates 24 mounted on the rod 23 being interleaved with the plates 25. In this modification, an audio-frequency transformer 31, 34 is connected between the secondary 30 of the Tesla coil and the blocking condenser 33, a radio-frequency bye-pass condenser 32 being shunted across the coil 31. A modification of the arrangement shown in Fig. 1 is described comprising a single spiral strip wound around the central spindle 8 together with a similar outside spiral strip attached to the inner surface of the chamber forming member. Fig. 4 shows a construction in which a conical metallic ; member 40 is mounted on the plate 38 and spider 42 within a correspondingly shaped casing 36. , In the arrangement, Fig. 6, the electrodes comprise plates 45, 46 having perforations 47, 48 which are out of alignment. The plate 46 carries projections 49 in alignment with the holes 47 and air is forced through the plates by a motor 50 and fan 51. In the modification, Fig. 7, a casing comprising portions 80, 80<1> and 82 is employed, the portion 80 being provided with the gas connecting tube 81 and accommodating a heating coil 83<1> for ionizing the gas. A rod 83 is mounted within the portion 80<1> so as to be in electrical connection therewith and is surrounded by a thin corrugated cylindrical casing 84 which is insulated from the rest of the device. The terminal 84<1> on the portion 80<1> is connected through a source of high potential current 85 to one side of the secondary 86 of a transformer, the other side of which is connected to the casing 84. In a further modification, Fig. 9, a source of compressed air 70 is connected to a casing 69 in which is mounted a perforated conducting wall 71 and a screen 73 covered with metallic flakes 74. The wall and screen, which form the condenser electrodes, are connected up as shown with a source of current 78 and transformer 77, 79. A.neutral insulating protecting screen 75 is provided across the front of the casing. Fig. 11 shows an arrangement employing two screens 88, 89, air from a fan 85 being ionized by a Nernst glower 86 before reaching the screens. In a modified arrangement one of the screens itself serves to ionize the air whilst in a still further modified arrangement, three screens are employed one of which serves to ionize the air whilst the others form the modulating air condenser. In the further arrangement, Fig. 15, the fan 121 forces air through the perforated metallic plate 123 mounted within the insulating casing 122. The perforations are arranged in rows and a light metallic member or valve 124, either in the form of a long strip or individual flakes, is hinged or flexibly mounted in front of each row. A metallic screen 124<1> is mounted in front of the plate 123, the screen and plate being connected with the high voltage source, 125 and the secondary 126 of the transformer 126, 127 as shown. Fig. 16 shows a construction in which two screens 132, 134 are mounted on metallic frames 131, 133 respectively, the space between the screens being filled with a porous, fibrous insulating material 140, such as silk threads, cotton wool, glass wool &c. An air source, 129, and, if desired, a heater 130 are provided and the, frames are connected to a high voltage source 135 and transformer 136, 137. In operation, the screens are more or less attracted towards one another under the electrostatic action so as to compress more or less the porous material therebetween, thereby control ing the passage of air therethrough in accordance with the telephonic currents. Fig. 18 shows a modified construction of screen in which the transverse wires 142 are connected to the positive terminal of the high voltage source 143 through the secondary 144 of the transformer 144, 145, the longitudinal wires 141 being connected to the negative terminal of the source. The varying potentials across the two sets of wires cause them to be drawn together to a varying extent thus altering the porosity of the screen. The two sets of wires may be interwoven with flexible insulating fabric thread. Fig. 17 shows a further modified form of weave, the wires 150 and 160 being connected up so as to be of opposite polarity and interwoven with transverse threads or wires 161. Specification 254,032 is referred to. The Specification as open to. inspection under Sect. 91 (3) (a) comprises also a construction similar to that shown in Fig. 1 but in which a single apertured plate closely surrounds a sharp edge on a conical head on the end of the central rod 8. A further modification, Fig. 10 (Cancelled), is also described employing a framework 61 supporting a woven wire screen 62, fine, light metallic flakes 63 being attached to the screen at their edges so as to leave them free. The screen is connected by a wire 64 to a high voltage source of current 65, the other terminal of which is grounded through the secondary 66 of the transformer 66, 68. The screen thus becomes an electroscope, the flakes being repelled therefrom by the force of electric repulsion, the degree of which varies with the varying telephonic currents applied to the primary 68. A fan &c. is mounted behind the screen, the passage of air through the screen being controlled by the speech currents. According to a still further modified construction, Fig. 16 (Cancelled), a blower 100 is connected through an insulating coupling 101 with a metallic casing 102 provided at its other end with a horn 103. Mounted within the casing are a number of Nernst glowers 104 connected in series to the current terminals 107 shunted across the feed wires 106. A high potential source, 110 and the secondary 108 of an audio frequency transformer are connected in series with each other and are in turn connected to one of the terminals of the Nernst glowers and to the casing 102 as shown. This subject-matter does not appear in the Specification as accepted.