Patent GB-1380930-A (GB)
Title: Electromagnetic transducer
IPC: H04R9/04
Filed: 1971-12-22
Abstract: 1380930 Electromagnetic transducers MAGNEPAN Inc 22 Dec 1971 [23 Dec 1970] 37589/74 Divided out of 1380928 Heading H4J An electromagnetic electro-acoustic transducer 30 (Figs. 1 to 3), such as a loud-speaker, includes a diaphragm 31 having on one or both faces a current conductor 36 consisting of a number of spaced and physically parallel conductor strip portions. A permanent magnet arrangement 34 is arranged to produce a magnetic field in and orthogonal to the conductor strip portions so that an alternating current flowing through the conductor 36 produces movement of the diaphragm 31 normal to itself. The permanent magnet arrangement 34 comprises magnetized particles embedded in a non-magnetic matrix (e.g. synthetic rubber bonded barium ferrite). The permanent magnet arrangement 34 preferably lies flush against a perforate, stiff panel 35 of ferromagnetic material (e.g. light gauge galvanized sheet iron). Panel 35 is secured by adhesive to frame strips 32 of polystyrene or other rigid material. The permanent magnet arrangement 34 has perforations 34a which align with the perforations 35a in the panel 35 to permit movement of air through the perforations as the diaphragm 31 vibrates. Instead of the permanent magnet arrangement 34 being a single piece, as shown, it may be made up of numerous strips of the flexible rubber bonded barium ferrite material. The magnet arrangement 34 is magnetized in its thinnest dimension (Fig. 3) in elongate zones, in the same configuration as the configuration and spacing of the runs of the conductor 36 on diaphragm 31. The magnetized zones define alternate N and S poles in the areas between adjacent runs of the conductor. The magnetized zones do not necessarily coincide with lines of perforations 34a, 35a. The diaphragm 31 is preferably tensioned (within its elastic limit) and may be made of polyester film, "Saran" (Trade Mark), rubbery material, paper, catgut or polyethylene. Alternatively the diaphragm 31 is of foamed polystyrene sheet which may be corrugated around the periphery where the diaphragm will flex. Where the diaphragm is under tension it may be stretched in both directions, but in one direction the tension may be merely sufficient to eliminate wrinkles. Where the diaphragm 3 is square it may be uniformly stretched throughout its entire area. In a long narrow diaphragm, tension may be applied across the breadth of the diaphragm, the tension being uniformly varied from one end to the other. As a result various portions of the diaphragm will resonate at different frequencies, thereby giving good output at low frequencies while maintaining a substantially uniform frequency response throughout the audio frequency range. The conductor 36 is applied after the diaphragm has been stretched and may be a metal foil. If a stretchable conductor 36 is used it may be applied before tensioning the diaphragm. The conductor 36 may be a printed wire or may be formed by printed circuit or vapour deposition or may be simply adhered. The diaphragm 31 may be a circle, ellipse, triangle, kidney shaped or leaf shaped. As shown in Figs. 1 to 3 the panel 35 (and magnet arrangement 34) is dished along a diagonal to accommodate the maximum excursions of the diaphragm 31. Alternatively the ferromagnetic panel (35.1), Fig. 5 (not shown), and permanent magnet arrangement (34.1) are flat and the spacing between the magnet arrangement (34.1) and diaphragm (31.1) is constant. Another embodiment (Figs. 6, 7, not shown) is similar to that of Figs. 1 to 4 but the diaphragm has a conductor (36.2) on each surface of the diaphragm. A plurality of conductors (36.3), Fig. 8 (not shown), may be stacked upon each other and insulated from one another on each surface of the diaphragm. If desired, where the sheet magnet (34.4) and panel (35.4) are flat (Fig. 9, not shown) an additional sheet magnet (34.4<SP>1</SP>) is applied to the rear side of the panel (35.4) and a second diaphragm (31.4<SP>1</SP>) carrying a current conductor (36.4<SP>1</SP>) confronts and cooperates with the sheet magnet (34.4<SP>1</SP>). Both diaphragms (31.4 and 31.4<SP>1</SP>) move in synchronism. Another construction (Fig. 10, not shown) dispenses with the ferromagnetic panel (35) and a flexible, perforated, sheet magnet (34.5) is held taut by a frame and spacer strips (32.5). There may be single diaphragm, or two diaphragms one on each side of the sheet magnet (34.5). Where there is a single diaphragm the sheet magnet (34.5) may be magnetized so that the magnetic fields are predominantly adjacent the upper surface of the sheet magnet (Fig. 29, not shown). The vibratable area of the diaphragm (31.6), Fig. 11 (not shown), may be maintained spaced from the sheet magnet (34.6) by cylindrical plugs (32.6<SP>1</SP>) of foam rubber or batted cotton fibres. An upright panel structure 30.7 (Fig. 12), which may be a room divider, has a frame 32.7 of rigid metal or rigid polystyrene to which is adhered a diaphragm 31.7. The diaphragm 31.7 is stretched both longitudinal and transverse directions or in only one direction. A perforate flexible sheet magnet 34.7 (Fig. 14) is supported by a ferromagnetic panel 35.7. The diaphragm 31.7 is divided into separate vibratable areas 31.7b, 31.7c, 31.7d, 31.7e and 31.7f each of which is of different length and area. These areas are defined by rigid spacer strips 37.7. Each vibratable area has a different set of resonant frequencies. The conductor 36.7 extends the entire length of diaphragm 31.7. Panel 35.7 and sheet magnet 34.7 are dished adjacent each vibratable area. The frequency response curve (Fig. 13, not shown) is relatively flat over the audio spectrum because of the averaging effect of the vibratable areas and their resonant peaks. An alternative construction (Fig. 16, not shown) has the vibratable areas separated by integral divider bars of the frame (32.8). A ferromagnetic panel (35.8) is dished opposite each vibratable area of the diaphragm. Flexible sheet magnets (34.8), sized to conform to the size of the particular vibratable areas, lie against the panel (35.8). Separate conductors on the diaphragm permit different signals to be applied to different vibratable areas via a crossover network. The divider bars of the frame (32.8) may be omitted and the same signal may be applied to all the conductors on the diaphragm or the signal may be fed through a crossover network to apply the highest audio frequencies to the smallest vibratable area, the lowest audio frequencies to the largest vibratable area and the mid-range audio frequencies to a vibratable area of intermediate size. In another transducer (Fig. 18, not shown) the frame (32.9) defines five substantially equal openings (32.9a) Separate conductors (36.9) are provided, one for each opening (32.9a). A rigid spacer strip (38.9) at each opening divides the diaphragm at each area into two separate areas and the sizes of these two separate areas are different at each opening (32.9a) resulting in different resonant frequencies. Alternatively (Fig. 19, not shown) the conductor (36.10) is arranged as in Fig. 12. In another variation there is a different degree of stretching of the diaphragm at different areas and, if desired, the several vibratable areas are separated from each other so that they can be individually stretched. A transducer 30.11 (Fig. 20) has a frame 32.11 with several different sizes of openings 32.11a, 32.11b, 32.11c, and a uniformly tensioned diaphragm having a conductor 36.11 traversing all of the vibratable areas. To obtain a relatively flat frequency response the magnetic field strength around the smallest vibratable area 31.11c is increased (relative to areas 31.11b and 31.11a) by using a stronger magnet and placing it closer to the conductor on the diaphragm. Increased magnetic strength is obtained by wider spacing of the perforations in the sheet magnet. In a variation (Fig. 22, not shown) separate conductors are provided for the three areas 31.11a, 31.11b and 31.11c the conductor for area 31.11c being of considerably less mass than the other conductors. Separate signals are applied to the different conductors by a divider network. A separate diaphragm, thinner and lighter than the rest, may be used for vibratable area 31.11c. By using separate conductors for the three vibratable areas the cross-bars may be eliminated. Another variation is to use narrower and closer conductors for the area 31.llc. The transducer (Figs. 23, 24, not shown) may be cylindrical, the ferromagnetic panel (35.13) being cylindrical and dished slightly between its ends. The diaphragm (31.13) is also cylindrical, is outside the panel, (35.13) and carries a conductor (36.13) in conformity with magnetized zones of a perforate flexible sheet magnet (34.13) of cylindrical form touching the panel (35.13). Another transducer 30.14 (Fig. 25) has flexible sheet magnets 34.14a, 34.14b on opposite sides of a common diaphragm 31.14 carrying conductors 36.14. In a variation, a pair of diaphragms are used, confronting each other and spaced slightly from each other. The two diaphragms may have different tensions applied to them. One diaphragm may carry stacked conductors and be relatively heavy, and the other diaphragm may carry a small, thin conductor to be highly responsive to high frequency signals. Figs. 26, 27 (not shown), illustrate a diaphragm (31.15) with conductors (36.15a and 36.15b) on the opposite surfaces thereof. The conductors (36.15a) run transversely to the conductors (36.15b). Perforate flexible magnets (34.15a, 34.15b) one on each side of the diaphragm have elongate magnetic bands which match the corresponding conductor. Large areas of the diaphragm (31.15) are set in motion because of the criss-cross arrangement of conductors. An elongate transducer 30.17, Fig. 32 has the diaphragm 31.17 attached only at its periphery. Conductors 36.17a and 36.17b are isolated from each other to receive two stero