GB967088A - Improvements in or relating to condenser microphones - Google Patents

Abstract

967,088. Microphones; vibratory diaphragms. K. SCHOEPS, [trading as SCHALL-TECHNIK DR.-ING. KARL SCHOEPS]. Jan. 25, 1962 [July 8, 1961], No. 2938/62. Heading H4J. The directional characteristics of a condenser microphone are altered selectively by sliding valves which alter the impedance of a back cavity. The valves may be operated electromagnetically. As shown in Fig. 1 a diaphragm 1 mounted adjacent a counter-electrode 4 has a cavity 10 in a plate 8 behind the counterelectrode which is perforated as at 7. The boundary portion 16 of the plate 8 is slotted so that the cavity communicates with an annular cavity formed by a slider 12 which in the position shown is sealed off. The cavity 10 also communicates, through apertures such as 15, with space at the rear of the microphone, such communication being made resistive by the constriction presented by the peripheral rebate 14 on a plate 11 secured to the rear of the microphone. A slide valve 18 mounted on the outside of plate 11 may be moved upwards from the position shown to seal off the apertures 15 in which position, provided valve 12 is also closed, the microphone is pressure responsive. If valve 12 is closed and valve 18 is opened to introduce resistance, the microphone is said to be directional. If valve 12 is opened then, regardless of the position of valve 18, the microphone is undamped rearwardly and has a figure-of-eight characteristic. In the simpler arrangement of Fig. 3, in which the front of the diaphragm is covered by a permeable plate 28, a single valve 25 is provided to damp the large breathing apertures 27 in the plate 21 which defines the back cavity 22. When the valve is moved downwards the cavity is given free access to the rear and the microphone is transferred from a directional to a figure-of-eight characteristic. Without the peripheral rebate the valve may be made to close the apertures 27 completely to provide a circular characteristic. In the construction of Fig. 4 the damping plate 31 is fixed and the plate 32 defining the back cavity is movable to open the large breathing apertures 33. In Fig. 5 the directional damping is permanently provided by a plate 42 and a single valve 46 opens or closes the breathing holes 44 giving access to the back cavity. In this embodiment the diaphragm works in pushpull between counter-electrodes 4 and 48. Fig. 6 shows the arrangement of Fig. 1 modified to operate the valves 12 and 18 electromagnetically. Coils 51 and 52 can be energized ,to displace the valves against rubber rings 53 and 54 which normally hold the valves shut.