A verification of the fundamental assumption used in ultrasonic pulse echo attenuation measurements through the immersion technique was carried out. The method assumes that a perfectly bonded interface exists between specimen and surrounding liquid. Experiments on hydrophobic polymers show that the assumption is broken and the common procedure to calculate real attenuation is no longer valid since reflection coefficients cannot be determined by means of the basic elastic wave propagation theory. For some cases the technique looses its cogency since no echoes can be received. Available methods using direct contact transducers only offer a partial solution to the problem. For cases where echoes are received, an alternative method of measuring both the reflection and attenuation coefficients is proposed in order to properly apply the immersion technique.
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