Improvements in or relating to the generation of mechanical waves of supersonic frequency

Abstract

Abstract 466,212. Piezo - electric vibrators. SCOPHONY, Ltd., and JEFFREE, J. H. Nov. 21, 1935, No. 32258. [Class 40 (iv)] [Also in Group XL] In a device for generating mechanical waves of supersonic frequency in a body and comprising a piezo-electric crystal assembly having one face associated with the, body and adapted for excitation by electrical currents of the required frequencies, there is interposed between the face and the body an intermediate layer having an acoustic refractive index greater than that of the body and less than that of the crystal assembly, the layer having a thickness of one quarter wave length, or of an odd number of quarter wave lengths, at the frequency of resonance of the crystal element, and being in direct contact with the body. The acoustic refractive index of a material is defined as the product of its specific gravity and the velocity of propagation of sound in the material. The piezoelectric element may be a quartz crystal or the combination of a quartz crystal with steel plates. The intermediate layer may be of a single substance, such as that known under the Registered Trade Mark " Cellophane " or maybe constituted by interposed layers of Cellophane" and aluminium or by tin foil sandwiched between two layers of " Cellophane." The intermediate layer may be separated from the crystal assembly by an even number of layers, each having a thickness of one or of an odd number of quarter wave lengths, the material of the odd-numbered layers having an acoustic refractive index less than that of the even-numbered layers and of the crystal assembly, and the even-numbered layers having an acoustic refractive index greater than that of the substance forming the layer in contact with the body. For example, a combined quartz crystal and steel plate element may be separated from a layer of aluminium in contact with the body by layers of aluminium and steel. The piezoelectric device may be used in the light-modulating arrangement described in Specification 439,236, [Group XL].

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