Measurement of acoustic absorption coefficient with phase-conjugate ultrasonic waves

Experimental results on measurements of the acoustic absorption coefficient in test objects that were obtained with two methods, i.e., a standard insert-substitution method and a modification thereof using phase-conjugate waves, are given. Samples of gelatin and biological tissue in vitro (porcine muscle fibers) were used as test objects. Gelatin objects were manufactured that were both homogeneous and with inhomogeneities in the form of a rough surface or inclusions (air bubbles) distributed over the volume. A rough surface leads mainly to phase distortions of a probe beam, while bubble inclusions cause additional field scattering. For all homogeneous samples, both compared methods produce identical results. In the case of inhomogeneous samples including biological tissues, absorption measurement by a standard method may lead to significant errors. It is demonstrated that the use of properties of phase-conjugate waves provides an opportunity to eliminate almost completely the measurement error connected with phase distortions and reduce the error in the case of a medium with scatterers.