Progress toward quantitative local ultrasonic measurements

The major part of experimental studies in ultrasonics deals with relative measurements of the acoustic field. It means that the absolute value of the ultrasonic pressure or of the acoustic displacements is unknown. Nevertheless, various methods exist for measuring the pressure in a fluid: use of hydrophones, reflection measurements and use of the inverse return principle, calorimetry, ultrasonic balance, etc. In most cases, they are quite difficult to handle, they induce some perturbation of the acoustic field to be measured, and, usually, give only a mean value of this field. The development of laser ultrasonics recently opened new perspectives in this domain. Laser probing of acoustic surface displacements gives very accurate local quantitative measurements. Such probing was limited to interfaces where the laser beam was reflected. This capability was recently extended to measurements of acoustic pressures inside fluids and acoustic dilatation inside solids [X. Jia, Ch. Matteï, and G. Quentin, J. Appl. Phys. 77, 5528–5537 (1995)]. This paper describes the method used and emphasizes the usefulness of such quantitative local ultrasonic measurements and its field of applications.