A new method for sensitive and automatic measurements of sound velocity and attenuation

Publisher Website
Google Scholar
Abstract
A new automatic method using the pulse‐echo technique has been developed to sensitively measure ultrasonic wave velocity change and attenuation in liquids or solids. This method relies on two principal ideas: (i) All the frequencies (the ultrasonic frequency fus, the local oscillator flo, the mixed frequency fmx, and repetition rate frep) are derived from a single master oscillator. Therefore, they are commensurate, and there exists some relationship between their phases. (ii) A frequency down conversion results in a phase sensitivity enhanced by the conversion ratio fus/fmx. A computer‐controlled apparatus has been built on this principle with phase‐locked synthesizers working in the 10–2000 MHz range. An experiment performed on a diluted magnetic crystal shows that the sensitivity on the sound velocity is better than 5.0×10−9 at 1000 MHz. An intrinsic characterization of the sound velocity sensitivity may be defined as the measurement performed on a reference path length equal to an acoustic wavelength λ. This gives the sensitivity at 1000 MHz as (δV/V)λ=3.0×10−5. The method performs simultaneous amplitude measurements with an accuracy of ±0.01 dB and a dynamic range larger than 60 dB. The idea could also be used to improve the sensitivity of other measurements; for instance, that of dielectric constant or magnetic susceptibility.