Patent GB-1522710-A (GB)
Title: Low frequency electro-acoustic transducer assembly
IPC: H04R9/06
Filed: 1975-11-19
Abstract: 1522710 Loudspeakers; diaphragms O HEIL 19 Nov 1975 [25 Nov 1974] 47691/75 Heading H4J A low frequency loudspeaker (Fig. 2) or other transducer comprises a stack of spaced apart, generally parallel fixed diaphragms 30a ... 30e and movable diaphragms 23a . . . 23f, the fixed and movable diaphragms being disposed in alternation with one another and defining therebetween a plurality of narrow air spaces, adjacent ones of the air spaces having open sides facing in opposite directions. A driver 11, in use, moves the movable diaphragms 23 towards and away from the fixed diaphragms 30. As shown, the driver 11 comprises a driving coil 19 to which are attached drive rods 21, connected along their lengths to movable diaphragms 23a to 23f. The diaphragm arrangement 12 (Fig. 2) comprises upper and lower blocks 27 and 26 in between which are disposed modular transducer assemblies 14 comprising diaphragm spacer blocks 28a to 28l connected on one side to movable diaphragm 23 and on the other side to fixed diaphragm 30, thereby defining a plurality of narrow air spaces alternately facing in opposite directions. A flexible membrane 33 is connected between spacer blocks 28 and the periphery of movable diaphragms 23. Drive rods 21 may be made from light weight rigid material such as drinking straws. Holes 34 provide for free passage through the fixed diaphragm 30. The diaphragms 23 and 30 are circular (see Fig. 3, not shown) but are in the form of two concentric conical sections to provide rigidity. To dampen longitudinal resonance in rods 21, a resilient viscous termination (43) Fig. 4 (not shown) is connected between their upper ends and upper block 27. To counteract any tendency for the unit to vibrate, two acoustic devices of Fig. 2 may be provided end to end (Fig. 2A, not shown) with the two driven units 11 connected to operate in push pull. An alternative transducer 50 (Fig. 6) uses rectangular movable diaphragms 79 and modular transducer assemblies 54 between upper and lower assemblies 51a and 51b. Each drive assembly 51a and 51b has two moving coils 60, 61. Fixed driver units 66 are provided each comprising a fixed diaphragm 68 defining a plane which divides unit 66 into upper and lower halves 69,70 with each half enclosing oppositely disposed upper and lower openings 73 and 74 on opposite sides of fixed diaphragm 68. A flexible membrane is connected between diaphragm 79 and surfaces 76 and 77. Movable diaphragm 67 comprises a plurality of alternately upward and downward protruding rigidizing nodes 81 and 82 (Fig. 8) arranged so that drive rods 63 and 64 are attached at the apex 83 or nadir 84 of each node 81, 82. Nodes 81 and 82 preferably define conic sections but may be hyperbolic paraboloids. The diaphragm 67 may be of layered cellulose fibres or paper pulp or polyethylene, acrylic or other plastics material. Fixed diaphragm unit 66 may be made by injection moulding. To achieve even greater rigidity of the movable diaphragm, two such diaphragms 67 as shown in Fig. 8 may be connected together with upward protruding nodes of the lower diaphragm attached to downward protruding nodes of the upper diaphragm (Fig. 9, not shown). Drive rods 63 and 64 connect to coils 60 and 61 and to nodes of the movable composite diaphragm. Alternatively, rigidity is obtained by moulding a conical section 105 (Fig. 10), of foamed polystyrene or other plastics, having peaks 107 and valleys 108 corresponding to the positions of the nodes of the diaphragms. The lower end of conical section 105 is attached to drive coil 110. The flexible membrane 80 (Fig. 6) or 33 (Fig. 2) may be dispensed with. In one arrangement the diaphragm 103 (Fig. 11), for use with the transducer of Fig. 6, has a peripheral reinforcing bead 115, and a lip 116 is provided, attached to acoustic barrier 71 and 72. The spacing of bead 115 from lip 116 is less than 1 mm. The length of lip 116 is determined by the maximum displacement of movable diaphragm 103. In another arrangement (Fig. 12, not shown) the movable diaphragm 103 is provided with a peripheral reinforcing wall, rather than a bead, which further rigidizes the diaphragm and provides a barrier to prevent air leaking past the lip of acoustic barriers 71 and 72. The spacing, again, should be less than 1 mm. In another arrangement (Fig. 14) a cylindrical housing 214 comprising a plurality of radially disposed, semi-cylindrical slots 215 facing in opposite directions. The movable diaphragms 223a to 223e each comprise six alternately upwardly and downwardly rigidized nodes 281 and 282 arranged in a circular array. The nodes extend upwards and downwards from a plane of the diaphragm 273. Undesirable resonances may be eliminated by straight slots (235) Fig. 15 (not shown) filled with viscous material and located between the nodes. Flag diaphragms may be of polystyrene, diameter 3 inch and weight about I gm. The section 212 of Fig. 14 may be modified so that the fixed diaphragm units 310 (Fig. 18) are of modular form which form a slack (Fig. 19, not shown). Between each adjacent pair of units 310 is disposed a movable diaphragm 223 as in Fig. 14. The fixed diaphragm unit 310 comprises a fixed plate having formed therein six equi-angularly spaced alternately upwardly and downwardly protruding nodes 314, 315 arranged in the same relative location as nodes 281, 282 of movable diaphragm 223 so that the nodes 314, 315 of the fixed diaphragm 310 will tend to nest with the nodes 281, 282 of movable diaphragm 223. At the apex of nodes 314, 315 is a hole 316 for passage of drive rods 324. A semi-cylindrical acoustic barrier 320 is terminated at each end by an upper spacer 321 and has intermediate spacers 322a, 322b. Similar barriers 320' and spacers 321<SP>1</SP>, 322a<SP>1</SP> and 322b<SP>1</SP> are on the underside. The difference in height between spacers 321, 322a, 322b, and barrier 320 determine the width of slot 215 (Fig. 14).