Patent GB-1147381-A (GB)
Title: Improvements in or relating to electromechanical transducer assemblies
Inventor: PEARCE GEORGE
IPC: B06B1/08
Filed: 1966-09-02
Abstract: 1,147,381. Sound directing appliances; magnetostrictive transducers. SMITHS INDUSTRIES Ltd. 1 Sept., 1967 [2 Sept., 1966], No. 39445/66. Headings H4J and H4X. In an electromechanical transducer having a predetermined frequency of resonance for transmitting a beam of acoustic energy of said frequency for reflection from a reflector, the beam has associated therewith a cavity resonant at said frequency for propagating further acoustic energy originating from the transducer. A transducer 1 comprises six identical annular magnetostrictive members 2, separated by synthetic rubber rings 3, stacked together, secured by nylon cords 4 and conjointly embraced by a toroidal winding 5 of synthetic rubber insulated wire. A glass-fibre shell comprises a cup-shaped reflector 6 and an acoustically transmissive circular bottom plate 7 bolted to the reflector 6. A frusto-conical wall 8 of the reflector 6 extends downwardly from a circular top -plate 9 and has a reflective expanded ebonite core 10. Holes 12, 13, 14 ensure the shell is flooded when submerged. The transducer 1 is clamped between plate 7 and part 9 by a stud 15 and nut 16. The upper end of the transducer 1 is spaced from the part 9 by an expanded ebonite disc 17 and an expanded rubber ring 18. At the lower end is a glass fibre framework 19 and an expanded rubber ring 20. The lower end of the transducer 1 bears via the ring 20 on a shoulder of the framework 19, the shoulder being provided by a glass fibre annulus 21. Within the framework 19, the annulus 21 is spaced by struts 22 from an annulus 23 that bears on the plate 7. Part of the framework 19 extends into the transducer 1, this part consisting of a third annulus 24 spaced by struts 25 from the annulus 21. The space from the annulus 24 to the disc 17 is occupied completely by four expanded ebonite discs 26. The assembly is secured to a ship's hull by bolts 27. Each magnetostrictive member 2 is a scroll of a single length of ironcobalt-vanadium alloy tape impregnated, in vacuum, with epoxy resin base. The impregnated scroll is heated to about 160 C. and dipped briefly into epoxy resin powder to acquire a corrosion-resistant coating. The stacked members 2 are wound with the winding 5 and an electric pulse is passed therethrough to impart to the members 2 a permanent magnetic polarization. Pulsed electromagnetic stimulus of each identical member 2 produces radiated acoustic energy at the resonant frequency of oscillation of each member 2. The distance between the lowest disc 26 and lower end of the transducer 1 is approximately one half of the wavelength in water of acoustic energy radiated by the transducer 1. The expanded ebonite discs 26 provide a pressurerelease wall for acoustic energy, the cavity 28 bounded by the part of the framework 19 within the transducer 1 being resonant to radiated acoustic energy. The cavity length is not exactly one half wavelength but is modified by an end correction. Acoustic energy at this frequency is radiated from both the internal and external surfaces of the transducer 1. The introduction of the resonant cavity 28 produces fewer effective side lobes and a wider main beam. The resonant mode of the cavity 28 is complex, its direction being significant in relation to the wavelength of acoustic energy and conditioned by the elasticity of the cavity walls.