Acoustic wave generation in viscoelastic rods by time-gated microwaves

A one-dimensional model is developed to predict the acoustic waves generated in viscoelastic rods by electromagnetic microwave pulses. The wave generation is due to the rapid thermal expansion caused by microwave absorption. The distribution of the temperature rise along the rod is taken to be either uniform or linear. The effects of this distribution and of the microwave pulsewidth are investigated numerically. Some tests are carried out on PVC rods instrumented with accelerometers. The presence of the accelerometer mass at the end of the rod is taken into account in the model. The comparison between the experimental and the theoretical accelerations leads to the evaluation of the maximum temperature rise during one pulse. Although this temperature rise is found to be extremely low, it produces acoustic waves easily detectable with the accelerometer. The inverse problem to estimate the viscoelastic properties of the rod and the temperature rise is approached by means of a numerical minimization technique. (C) 1999 Acoustical Society of America. [S0001-4966(99)04507-5].