GB911866A - Condenser microphones - Google Patents

Abstract

911,866. Condenser microphones. AKUSTISCHE U. KINO-GERATE G.m.b.H. Feb. 23, 1960 [March 4, 1959], No. 6259/60. Class 40 (4). In a directional condenser microphone having at least one diaphragm exposed to a sound field on both sides and a phase-shifting element placed between one side of the diaphragm and the sound field, the diaphragm has an area of at least 10 cm. and one of the sides, or the diameter, of the diaphragm is much larger than the wavelength in air of the highest frequency to be transmitted. The microphone has a uni-directional pattern that is essentially frequency independent although it has a high self-capacity (10<SP>2</SP>-10<SP>3</SP> pF.) and consequently may be connected to an associated amplifier by long cables (10 metres), a bidirectional pattern can be obtained by the addition of a frictional acoustic resistance between the diaphragm and an electrode: it is also suitable for use as a loud-speaker. A perforated plastic trough 3 (Fig. 3) is placed on a sheet-like acoustic frictional resistance 2, e.g. filter paper, contained in a perforated sheetmetal casing 1. The diaphragm 7 which has been vapour-coated with a metal is tensioned by an adhesive connection at its rim to an electrode 5, being separated therefrom by parallel threads 6 consisting for example of 20 microns thick, beads made of the synthetic filamentary material known under the Registered Trade Mark " Perlon." A perforated cover 8, Sa acts as a housing for the instrument. Fig. 4 shows a vertical cross-section of the assembled microphone, the cavity 13 and the resistance 2 performing the phase-shifting for the frequencies below 2 kc/s. which impinge on the back of the diaphragm after traversing the sides of the microphone. Two diaphragms, only one of which need be electrically effective (Fig. 5, not shown) and having the frictional resistance between them can be used to produce a different directional pattern, or a mechanically-operated perforated slide (Fig. 8, not shown) could be used. A slack, thin, resilient diaphragm (Fig. 7, not shown), may be provided to protect the instrument from moisture. An assembly of two diaphragms each with its own electrode, having the acoustic resistance between the electrodes (Figs. 5 and 9, not shown) may be used for stereophonic recording. The diaphragm may have any shape, e.g. circular, oval or brickshaped, as in Fig. 16 (not shown). Alternatively the microphone may be shaped like a hexagonal pencil (Fig. 18, not shown); a perforated hexagonal sleeve acts as the electrode, while a diaphragm covers, although it is separated from, three sides of the sleeve, an acoustic resistance lining the remaining three sides.