A NEW METHOD FOR SENSITIVE AND AUTOMATIC MEASUREMENTS OF SOUND-VELOCITY AND ATTENUATION

A new automatic method using the pulse-echo technique bas 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 f(us), the local oscillator f(lo), the mixed frequency f(mx), and repetition rate f(rep)) 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 f(us)/f(mx). 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 X 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 lambda. This gives the sensitivity at 1 000 MHz as (deltaV/V)lambda = 3.0 X 10(-5). The method performs simultaneous amplitude measurements with an accuraey 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.