Patent GB-1414956-A (GB)
Title: Microphone units
IPC: H04R1/32
Filed: 1971-06-16
Abstract: 1414956 Microphones S Z M GABR 18 Sept 1972 [16 June 1971 28 July 1971 7 Sept 1971] 28331/71 35426/71 and 41710/71 Heading H4J A directional microphone arrangement in which pick-up of undesired external noise is minimized has the transducer diaphragm or diaphragms located substantially midway along the length of a duet, e.g. of circular or square cross-section and secured thereon by mounting means. A pair of like moving coil transducers 2, 4 (Fig. 1) are mounted one on each side of a baffle 6 and each is in a loading chamber 12 having four side walls 14 at right angles. The walls 14 have slits 16. Desired sound from a speaker facing directly either one of the transducers is picked up whereas ambient sound falls with substantially equal intensity on the two diaphragms 8 and is cancelled, the moving coils being connected together in phase opposition (Fig. 7, not shown). In an alternative arrangement (Fig. 2, not shown) two planar transducers (22, 24) are mounted coaxially on a baffle board (26) which is centrally apertured to provide acoustic coupling of the two diaphragms. The transducers (22, 24) have their coils adjacent to expose the full area of the diaphragms to sound. Two loudspeakers (28) are mounted on a side wall, one on each side of the baffle (26) and have their axes substantially at right angles to the common axis of the planar transducers (22, 24) to provide for accurate cancellation of feedback. This arrangement is used for two-way communication, e.g. as a loudspeaking telephone. A further arrangement (Fig. 4) has a single transducer comprising a planar diaphragm 30 and a pair of speech coils 31 co-operate with separate magnets 32. A baffle 35 has a circular aperture. The diaphragm 30 is clamped at its periphery between two rings 36 and two opposite bolts 40 extend inwardly through rubber or plastics grommets 41 from the baffle 35 to the rings 36 to provide the sole support for the transducer. The magnets 32 are supported by radial arms 38 extending from the rings 36. An annular aperture (34) Fig. 3 (not shown) defined between the baffle 35 and the rings 36 effects some sound equalization on opposite sides of the baffle. The baffle 35 is rectangular and apertures (42) Fig. 3 (not shown) at each corner minimize reflection effects at the corners between the walls 39 of the loading chambers and the baffle 35. Apertures may be provided in the diaphragm (Fig. 7) or in the diaphragm suspension. The effective area of these apertures is preferably made selectively variable and may be reduced to zero, e.g. by a blocking device. The transducer or tranducers may have a sound diffusing fabric cover to minimize noise effects. The duct is preferably built from a plurality of rings of at least three different materials, but of like inner and outer diameters, arranged in coaxial arrangement (not shown). The rings are of wood, metal, glass fibre, rubber or plastics material. Rings of different materials are next to one another so that sound transmission from one ring to the next is substantially attenuated at the interfaces. Preferably the rings are arranged in different order along the length of the duct, each ring having on each side a ring of a different material. The duct is preferably also formed radially of adjacent concentric rings, at least three in number and again of different materials (Fig. 9, not shown). The order of the concentric rings is not repeated in either adjacent ring set. Flat panels can similarly be formed from several layers of different materials. The various layers may be connected together by adhesive and/or by snap-fitting interconnections.  Each transducer (shown broken away in Fig. 10) may have a metal disc diaphragm 120 with a pair of speech coils (not shown) one on each side, and a frame supporting magnets (not shown) for co-operation with the coils. The frame consists of two like parts 121 which clamp a suspension 122 for the diaphragm 120 between them. Each frame portion 121 is of metal or rigid plastics and is coated with rubber or resilient plastics material 124. If desired, one speech coil and its magnet is omitted. The microphone is mounted within a tubular laminated housing as described above by providing the housing with a pair of inwardly projecting flanges of the same laminated construction, the microphone frame being secured between them. Internal sound reflections are reduced by providing rough, sound absorbent surfaces. Metal surfaces can be etched or embossed. All surfaces may be covered with flocking, suede or rubber compound.