Experimental validation for the diffraction effect in the ultrasonic fieldof piston transducers and its influence on absorption and dispersion measurements

The aim of this work was to study the diffraction effects in the ultrasonic field of piston source transducers and their importance for accurate measu rements of attenuation and dispersion in viscoelastic materials. In laborat ory measurements, the diffraction phenomena are mainly due to the beam spre ad of the ultrasonic wave propagating in viscoelastic materials. This effec t is essentially related to the estimated attenuation and dispersion in the material. In this work, a frequency domain system identification approach, using the maximum likelihood estimator (MLE), was applied to the measured data in order to determine a function for correcting the diffraction losses in both normal and oblique incidences for a large frequency band (300 kHz to 3 MHz). The effective radius of the used transmitter was determined by t he inverse problem when ultrasonic beam propagation was investigated in a w ater medium. Using the estimated radius, the propagation through viscoelast ic materials was established, and the acoustic parameters of these material s were estimated, Attention was paid to the determination of the attenuatio n and dispersion in the materials, These quantities were compared to those obtained without diffraction correction in order to sec the influence of in troducing the diffraction correction into the propagation model.