Some improved sound wave interference methods for measuring the longitudinal and transverse ultrasonic velocity in opaque as well as transparent solids may be simply carried out by using the ultrasonic light‐diffraction system (as arranged for making sound beams visible on a screen). The sonic unit of the system is arranged to produce two individual traveling‐wave sound beams, by use of two generators by splitting a single beam. Three simple arrangements are described in detail. In Case A one beam travels entirely in a reference liquid, while the other beam travels a parallel path in an immersed transparent test specimen. In Case B one beam travels entirely in a reference liquid, while the other beam travels an adjacent course through an immersed, transparent or opaque test prism, and on into the liquid at an angle to the first beam. In Case C the two beams are generated at the equal edge faces of a transparent or opaque isosceles test prism (only the base edge face contacting the liquid). The two beams traverse the prism to the base, where they are refracted into the test liquid as confluent beams.
In Cases A and B, simulated interference (by optical integration), and in Case C true interference, each give a light and dark band interference pattern on the screen, whose band spacing is used in calculating the velocity of sound in the test solid. The other required factors are the optical image magnification, the frequency of the sound, the angular disposition of the one or more acoustic surfaces of the test solid relative to the incident sound beams, and in Cases A and B the velocity of sound in the reference medium. Other variations of arrangement are suggested.
Advantages of the improved methods are simple preparation of test specimen, directness and simplicity of measurement and calculation, good accuracy, low sonic power requirements. A table of measured velocities (and attenuations) in two metals and in numerous plastics and polymers show the wide range of materials that may be measured by the new interference methods.