Patent GB-1350465-A (GB)
Title: Transducer
IPC: H04R9/02
Filed: 1971-05-07
Abstract: 1350465 Electromagnetic transducers; headsets RANK ORGANISATION Ltd 3 May 1972 [7 May 1971 30 July 1971] 13746/71 and 36060/71 Heading H4J [Also in Division H1] An electromagnetic electro-acoustic transducer includes a diaphragm 1 (Fig. 1) having on one or both faces a current conductor 2 consisting of a number of spaced and physically parallel conductor strip portions 2A to 2H. A permanent magnet system 3 is arranged to produce a magnetic field in and orthogonal to the strip portions 2A to 2H so that an alternating current flowing through the conductor 2 produces movement of the diaphragm 1. The permanent magnet system 3 comprises magnetized particles embedded in a non-magnetic matrix such as neoprene rubber. The transducer may be embodied in a loud-speaker, microphone or earphone. A magnet strip 4 extends opposite and along each gap between adjacent conductor strip portions 2A to 2H. The magnet strips 4 are attached to a backing plate 5 of soft iron. Perforations 6 are shaped, sized and positioned to provide sound damping, thereby controlling unwanted resonances and providing good low frequency response. Adjacent magnet strips 4 are of opposite polarity (Fig. 2), the leakage flux between adjacent pairs of poles on one or each side of the diaphragm 1 providing the required magnetic field. Where magnet strips 4 are on each side of the diaphragm 1 those on one side act to attract the conductor 2 and thus the diaphragm 1 at the same time that magnet  strips 4 on the other side repel it during passage of current through conductor 2. The diaphragm therefore moves uniformly. The diaphragm 1 may be of magnetic or non-magnetic material. Where the diaphragm 1 does not act as a magnetic screen it preferably carries a conductor on one face only, while magnet strips 4 are provided on opposite sides of the diaphragm. The diaphragm is supported at opposite ends 7 but not at opposite sides 8. Low frequency response is improved by corrugating the diaphragm 41 (Fig. 4) in a direction parallel to the ends 7 (see Fig. 1), the corrugations being of angle 120 degrees. A second diaphragm 42 (Fig. 4) is in use close to the front magnet strips 43 and adjacent the corrugated diaphragm 41. Diaphragm 42 acts as a treble diaphragm and is held in tension, no electrical cross-over being necessary. By asymmetric placing of conductor strip portions 2A to 2H and the corresponding magnet strips 4, the driving force on the diaphragm 1 may be made inhomogeneous. A complementary effect may be obtained by shorted conductor loops (not shown) positioned on the diaphragm 1 to provide eddy current damping. The conductor strip portions 2A to 2H may be connected in parallel or in a seriesparallel combination to provide a desired impedance. Preferably, however, the conductor 2 is turned back on itself at regular intervals to provide a set of series connected strip portions 2A to 2H. A required polar response of the transducer may be obtained by curving or bending the magnet structure, the diaphragm being correspondingly curved. Headsets.-For quadraphonic use, a pair of earphones is provided (Fig. 5), each earphone consisting of a front transducer F and a rear transducer B, each earphone in use being spaced from the head by an air gap 54.