One wall of a parallelepiped monobloc of steel, of about 1 cm thick, is used as an acoustic wave guide, to study Lamb wave attenuation at a frequency of 1.7 MHz, when the pressure, due to a gas or to a liquid in contact with only one surface of the plate, is changed. We report that, if a gas is used, each excited mode shows, in the examined range from 1 to about 800 bars, a linear dependence between the received pulse amplitude and the pressure values, the first diminishing when the second increases. Lame mode has the highest sensitivity, due to its maximum ratio between vertical and tangential displacement of the plate surface. An effect, contrary to expectation, is found when a liquid is in contact with the metallic surface: with increased hydrostatic pressure, a decrease in attenuation is observed, for low order plate modes, while attenuation increases for higher order modes.
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