Apparatus and a method for indicating variations in acoustic properties on an interface

Claim 1

Apparatus for indicating variations in acoustic properties at the interface of a first material in contact with a second material, which first material is capable of transmitting elastic waves therethrough, which apparatus comprises transducer means for injecting an elastic wave signal pulse into the said first material so as to be repeatedly reflected back and forth across the said first material between a surface of predetermined reflection properties and the interface between the first material and the second material, the first material being such that the attenuation with distance of elastic waves traversing therethrough is substantially constant, an amplifier for providing electrical signals corresponding to received reflected signal pulses, detector means for detecting the electrical signal corresponding to an early reflected signal pulse (N), automatic gain control means so controlling the amplifier as to set to a predetermined level the amplitude of the said electrical signal (N) and to determine the gain of the amplifier for subsequent electrical signals corresponding to later reflections of the pulse, comparison means for comparing with a reference standard the amplitude of an electrical signal corresponding to a selected later reflection (N+M) of the pulse, response means for indicating in response to the comparison a change in the attenuation of pulses upon reflection at the said interface, the said response means being inhibited from operating in response to signals corresponding to reflections of the pulse between the said early reflection (N) and the said selected later reflection (N+M).