Patent GB-1305456-A (GB)
IPC: H04R7/14
Filed: 1970-02-18
Abstract: 1305456 Electromagnetic sound transducers O HEIL 18 Feb 1970 [19 Feb 1969] 7676/70 Heading H4J In an electro-acoustic electromagnetic transducer, such as a loud-speaker or microphone, a diaphragm arrangement consisting of diaphragm portions or diaphragm members is such that the portions or members define between themselves a plurality of narrow air spaces. The air spaces are substantially closed all around with the exception of one side which remains acoustically open. These open sides of adjacent air spaces face in opposite directions. A conductor in the magnetic field of a permanent magnet arrangement is formed by or is fixedly connected to at least some of the diaphragm portions or diaphragm members. During passage of an audio-frequency current through the conductor the at least some diaphragm portions or diaphragm members cause sound waves to be emitted through the open sides of the air spaces. Alternatively, during vibration of the at least some diaphragm portions or diaphragm members by sound waves impinging thereon, audiofrequency currents are induced in the conductor. One form of diaphragm is a metal foil 2 (Fig. 3) bent into meander form. Thin plastics foils 4 close the air spaces at the front and rear. The metal foil 2 is located in a strong magnetic field 6 and sound is emitted in the directions 8. To provide greater flexibility near the longitudinal edge of the diaphragm it may be made of plastics tape plated with metal except in the region of the longitudinal edges. Alternatively, the air spaces formed by the meander diaphragm foil 2 are closed by plastics sleeves (18), Fig. 4 (not shown), which substantially close the air spaces at opposite sides while extendings lightly beyond the opposite end edges of the foil 2. These sleeves (18) may be connected at the outer edges (18<SP>1</SP>) to a carrying structure (not shown). Another diaphragm arrangement (Fig. 5) consists of metal foil strip 10 bent into a wavy configuration, adjacent portions being cemented but insulated from each other at regions 10<SP>1</SP> where they abut. Rows of air spaces are formed by thin plastics web closures 12 connected alternately at the upper and lower edges of the foil 10. A magnetic field 16 extends normal to the direction in which an audio current is passed through the metal foil strip 10. Sound is emitted in the directions of the arrows 16, i.e. in the direction of the magnetic field. The poles (not shown) producing the magnetic field are formed of spaced iron laminates or perforated sheets so as to be sound transparent. A loud-speaker (Fig. 8) consists of rectangular strips 24 of permanently magnetized ferrites. Sets of laminated soft iron polepieces 26 engage the strips 24. Diaphragms 10 in the form of conductive strips are arranged in the air spaces and are of the form shown in Fig. 5 with upper and lower closures 12. Sound waves are emitted between the laminated polepieces 26. An alternative loud-speaker (Fig. 10) has a circular magnetic field generated between two permanent magnetic ferrite rings 28, 30. Each ring has a soft iron pole shoe 28<SP>1</SP>, 30<SP>1</SP>. The folded diaphragm 2 is of ring form and is of the type shown in Fig. 4 having interdigitated sleeves 18 of polyethylene terephthalate. The diaphragm 2 is held by coaxial rings 34 of rubber or plastics material. The magnetic circuit may be magnetically closed on the inside and/or outside of the ring structure by iron bridges (not shown) to increase the field strength. A sound reflecting cone 32 is arranged in the opening of the magnet ring 30. An outwardly projecting horn 36 may be connected to the ring 28. Another loud-speaker (Fig. 14) has two parallel spaced permanent magnets 42 and a pair of end plates 46 and 50 formed of soft iron. Each end plate 46, 50 has rectangular openings (48), Fig. 15 (not shown), for sound propagation. A meander shaped diaphragm 60 is cemented along its longitudinal edges to laminated polepieces 54, 54<SP>1</SP>. The inner ends 58 of the laminates extend up to the longitudinal edges of the conductive portion 62 of the diaphragm 60. The polepieces 54 are offset relative to the polepieces 54<SP>1</SP> to close successive air spaces at the top and bottom of the diaphragm 60. The laminates 54 are connected at their upper ends to the end plate 46 whereas the laminates 54<SP>1</SP> are slightly spaced from the inner face of the end plate 50 to provide an air gap 53. The magnetic force created in this air gap creates a continuous tension in the diaphragm 60. In a modification (Fig. 16, not shown), the diaphragm (60<SP>1</SP>) is attached to both end plates 46, 50 but is  corrugated. Metal strips (62<SP>1</SP>) are connected to flat portions of the corrugations. A loudspeaker (Fig. 17) has a number of diaphragms 78 and a pair of permanent magnet voice coil units 64. The diaphragms are separated by alternately facing U-shaped spacers 80. The upper voice coil is connected by rods 82 to the upper diaphragm 78 and through holes in the second diaphragm to the third diaphragm &c. The lower voice coil is likewise connected to other, alternate diaphragms. Alternatively a single voice coil is connected by rods 82 to every second diaphragm, the other diaphragms being not driven and made heavier. The diaphragms may be double cones as shown, or have semispherically or other curved surfaces. An earphone or microphone (Fig. 19) has soft iron slotted cover-plates 88 sandwiching a permanent magnet ring 92. A non-magnetic baffle 94 provides an inner space 94<SP>1</SP> Two sets of polepieces 96 are respectively connected to the upper and lower cover-plates 88. A meandershaped diaphragm is cemented at the upper and lower longitudinal edges to the respective polepieces 96. Alternatively, flat sleeves parallel and spaced from each other are each provided with a spiral winding, the windings being connected in series or parallel. The upper polepieces may close the upper ends of the sleeves while the lower polepieces close the spaces between adjacent sleeves. Other diaphragms may consist of a narrow metal tape or wire 40 (Fig. 12a) wound in a close spiral on a tape ring 38 which is subsequently folded into meander shape. To facilitate folding to meander shape cut-outs 39 (Fig. 13a) may be formed at longitudinally spaced portions where the tape is to be bent. The wires 40 may be flattened in these regions. After folding the tape into meander shape the flaps 38a between the cut-outs 39 are bent over to close the cells at opposite ends. The wires may be sandwiched between two thin plastics tapes which are heat sealed to each other. Instead of winding wire in a spiral, a plurality of parallel wires or tapes may be provided on the tape before it is formed into a ring. The ring, when formed, has the end portions slightly offset so that one end of a wire on one of the tape ends is connected to the end portion of the adjacent wire so that a spiral is formed. The ring is then formed into meander shape. Adjacent diaphragm portions may be more widely spaced at their open ends than at their closed ends. By coating the diaphragm portions with ferromagnetic material, the magnetic polarization induced in the coating by the magnetic field keeps the portions at equal distances from each other and elastically stretched.