Patent GB-409040-A (GB)
Title: Improvements in or relating to piezo electric microphones or speakers
IPC: H04R17/00
Filed: 1932-11-01
Abstract: 409,040. Loud-speakers; microphones. STEVENS, A. H., 5, Quality Court, Chancery Lane, London.-(Brush Development Co. ; Cleveland, Ohio, U.S.A.) Nov. 1, 1932, No. 30758. [Class 40 (iv).] A piezo-electric apparatus, more particularly a loud-speaker or a microphone, comprises a plate-like piezo-electric body adapted by flexing to convert electrical impulses into mechanical impulses and vice versa and having a flexible surface capable of direct radiation into orabsorption from a fluid of acoustic vibrations substantially non-selectively over a frequency range of upwards of one octave. Fig. 2 shows a construction of microphone, which may also be used as a loud-speaker, comprising a support 1 recessed on one side to provide a shoulder 3 against which the edge of the piezo-electric disc 2 is cemented at 4. The space behind the disc is filled with oil or petroleum jelly saturated felt 4<a> for damping purposes and a small aperture 1<a> extends through the web portion of the support so as to equalize the pressure on the two sides of the crystal diaphragm under changing barometric conditions. The disc 2 is composed of twelve pairs of sector-shaped plates 5, 5<1>, the plates of each pair being cut as shown from a plate 11, Fig. 4, previously cut as shown from the Rochelle salt crystal, Fig. 3, the electrical axis and transverse and longitidunal crystalline axes being shown at a-a, b-b and c-c respectively. One of the plates 5, 51 is then turned over and superposed and cemented to the other plate and, after the twelve pairs have been assembled, metal foil sheets 6, 7 are cemented to the opposite faces of the disc. According to a modified construction, two crystalline plates 11, Fig. 4, are employed, each plate having sector-shaped foils A, B, C, D cemented to its outer and inner faces as shown in Fig. 6, the two foiled plates being in turn cemented together with the bottom of one plate arranged adjacent the top of the other. The resultant structure is then turned down to the circular form shown in Fig. 6 and the sector-shaped foils connected in series, parallel or series-parallel. Fig. 11 shows a further modified form of piezo-electric body 22 consisting of a pair of rectangular plates 14, 15 each cut from a plate 11 as shown at 14<1> in Fig. 4 and having foil electrodes 16, 17 and 18, 19 cemented to their opposite faces, the two foiled plates being then cemented together. Two such bodies 22 may be mounted as shown in Fig. 14 in parallel spaced relation in a frame 23 of insulating-material, the bodies being held against rubber covered lugs 23<a> in the frame by thin paper sheets 24 cemented to the faces of the bodies and to the top and bottom faces of the frame. Fig. 15 shows a construction employing a plurality of such bodies 26 mounted with spacing blocks 27 on rods 29 on a base 28 and enclosed by a foraminous case 30, the four upper bodies being connected in parallel with each other and the four lower bodies being similarly connected while the two groups are connected in series. According to a further construction, Fig. 17, a piezo-electric body 32, constructed similarly to that shown in Fig. 11, is retained on shoulders 33<a> in a shielding case 33 by strips 34 secured to the case by screws 35, the back of the body being damped by felt 36 whilst according to a still further modification, Fig. 19, a single piezo-electric plate 37 has its end edges secured by cement 39 in a case 38. If desired, the piezo-electric body may be associated with a horn and diaphragm chamber.