Patent GB-318934-A (GB)
Title: Improvements in piezo-electric devices
IPC: H04R17/00
Filed: 1929-09-12
Abstract: 318,934. Russell, E. W. C., (Assignee of Sawyer, C. B.). Sept. 12, 1928, [Convention date]. Piezo-electric instruments; loud-speakers.-In a piezo-electric unit for use as the energizing movement of a loud-speaker comprising one or more pair of plates of piezo-electric material having their surfaces perpendicular to the electrical axis of the material and subject to the action of an electrostatic field parallel to this axis, the mutually perpendicular directions of expansion and contraction of one plate of the pair under the applied electrostatic field substantially coincide with the directions of contraction and expansion respectively of the other plate so that a twisting movement is imparted to the juxtaposed plates, means being secured to a free edge portion of the plates for actuating the diaphragm by this twisting movement. According to one construction, plates 2 are cut from a crystal 1, Fig. 8, with the extended plane surfaces substantially perpendicular to the electrical axis a. When the plates are so cut from a Rochelle salt crystal, the direction of expansion and contraction of the crystal when subjected to an electric field is substantially at 45 degrees to the major longitudinal axis. The plates 2, 2<a> and associated electrodes 3, 3<a> are assembled as shown in Fig. 9, the inner electrodes being connected to one lead and the outer electrodes to the other lead as shown. The arrows 4, 5 in this Figure indicate the direction of expansion and contraction respectively of the plate 2 when the electrical field is in the direction indicated by arrows 6, the arrows 7, 8 indicating the direction of expansion and contraction of the plate 2<a> when the field is in the direction of arrows 9. The plate 2 thus tends to contract in the same direction as the plate 2<a> tends to expand and vice versa, so that if the lower edge 10, Fig. 10, of the assembled plates is held rigid, the upper portion 11 carrying the actuating arm tends to move as shown in. dotted lines. The plates and electrodes are closely cemented together and the external electrodes electrically connected as shown in Fig. 4, by a piece of gauze 15 and clip 13 carrying a lead 14. The internal electrodes, which are brought out over the edges of the plates and insulated at 16 from the external electrodes, are similarly connected by gauze 19 and clip 17 to which is connected the lead 18. The unit is then mounted between sponge rubber pads 32 in a frame 21 and cover 30, Fig. 3, the lower edge of the unit being cemented at 22 to the base 20 of the frame, whilst its upper edge is cemented at 24 to an arm 23 for actuating the diaphragm 27. An adjustable spring 83 is preferably provided as shown between the arm 23 and a screw 36 carried bv an extension 35 of the frame 21. In order to prevent the reactive forces of the diaphragm from displacing the crystalline element laterally, a resilient metal reactor rod 40 is provided between the arm 23 and extension 35. According to a modified construction, plates e, e<1>, f, f<1> may be cut from a crystal slab 2<1> as shown in Fig. 12, the plates and electrodes 46 - - 50 being assembled and electrically connected as shown in Fig. 13.