Optimal model for the diffraction effect in the ultrasonic field of pistontransducers

In this work, the analytical and experimental examination of the problem of diffraction effect is treated. In the laboratory, the diffraction phenomen a have been mainly due to the beam spread of the ultrasonic plane wave prop agating through a viscoelastic material. In fact, this effect has been foun d to be related, essentially to the attenuation and dispersion losses on a viscoelastic material. In this work, a frequency domain system identificati on approach is applied to determine an optimal function correcting the beam spread effect in both the normal and oblique incidences for a large freque ncy band (300 kHz-3 MHz). The Maximum Likelihood Estimator is applied to th e magnitude and phase of the measured beam patterns of the used transducers in order to determine the model parameters. The calibration procedure is a lso discussed. Once the proposed model is established, the propagation thro ugh a viscoelastic plate is described and a comparison with measurements is done to validate the investigated model. The obtained longitudinal and she ar attenuation and dispersion of the ultrasound in the viscoelastic plate a re compared with those obtained by applying the complex harmonic plane wave s combination model.