Patent GB-672236-A (GB)
Title: Improvements in or relating to electro-mechanical transducers
IPC: H04R17/04
Filed: 1948-08-24
Abstract: 672,236. Telephone instruments. SONOTONE CORPORATION. Aug. 24, 1948 [Sept. 9, 1947], No. 22282/48. Class 40 (iv). [Also in Group XL (b)] An electro-mechanical transducer element includes at least one solid dielectric body composed of random oriented titanate particles and two electrodes united to opposite surfaces of the body, the particles of the body being permanently electrically polarized so that in the absence of an external polarizing potential the element exhibits a piezo-electric effect. Barium titanate gives the largest effect, but mixtures with similar titanates, notably strontium titanate, may be used. Fig. 6 shows two such titanate layers 16 each with electrodes 14, 15 soldered to opposite sides of a phosphor-bronze blade 20. On flexing the blade 20, one element is compressed and the other is stretched. Thus by polarizing the elements in the sense shown, by the temporary application of battery 25, the elements give voltages in aiding sense, and a push-pull output corresponding to the vibrations applied to the blade 20 may be taken across resistance 32. The spring blade may be of non-metallic sheet material with one electrode of each element cemented to it. Also, there need only be a single titanate element. The above devices may be used as a microphone or a phonograph pick-up by connecting the blade to an acoustic diaphragm or a stylus. Fig. 11 shows a pick-up in which the transducer element is substantially that of Fig. 6 and comprises a spring blade 1-16 on each side of which is fixed a suitably polarized and electroded titanate strip 1-11, 1-12. The free end of the blade is connected to a resilient needle arm 1-21 carrying a needle 1-23. One end of the blade assembly is held between resilient pads 1-52 fixed by a U-shaped yoke 1-54 to one half 1-41 of the cover, to which is also riveted at 1-43 the outgoing lead members 1-15, 1-25 of the element. A resilient pad 1-48 is also provided for the needle arm. A second part 1-42 of the cover is fixed on by a tubular rivet 1-34. Lugs 1-49 are provided for supporting the pick-up head from a tone arm. In a modification (Figs. 14-15, not shown) the needle arm turns the transducer element about its middle, the ends being held by resilient pads. Fig. 16 shows a torsionally operated pick-up in which the transducer elements 4-43 are mounted on opposite sides of a spring plate 4-46 connected by integral webs with a frame member 4-54 resiliently held in the casing. A needle arm 4-65 produces torsional movement of the titanate elements. As the torsional effect would be the same for small deviations on either side of the undistorted condition, it is necessary to give the element a large initial torsional bias, so that the movements during normal working are all to one side of zero. This is done by means of specially shaped resilient blocks 4-83. Alternatively, the titanate elements may be soldered on to a twisted backing plate which is held temporarily flat, or the backing plate may have offset lugs engaging the casing. Figs. 19, 20 show a microphone or sound emitter comprising three pairs of titanate elements 11, 12, each pair being mounted on a resilient bridge piece 5-42 fixed on a common base 5-41. The elements are connected to an acoustic diaphragm 5-49 by a member 5-48 which has two forked ends which engage lugs at the mid-points of the three bridge pieces to move them in unison. The individual elements are suitably polarized and connected in series so as to give voltages in aiding relation. In a modified instrument, Fig. 21 (not shown), a long titanate element is held between fulcra at the middle and one end; the other end being connected to the diaphragm. Also the electrodes are subdivided transversely to give five pairs of serially connected electrodes. A titanate strip may also be used for detracting or measuring strain in a metal beam to which it is attached (Fig. 22, not shown).