Propagation features of evanescent acoustic waves

Abstract

Evanescent waves are not true propagating waves, so questions regarding their traversal time or propagation direction are ill posed. However, physical phenomena related to the time delay of pulsed evanescent perturbations, or refraction effects through a discontinuity, can be easily put into evidence in acoustics by proper choice of acoustic modes and geometrical configuration. In this paper, lateral displacement and time delay through forbidden barriers of Lamb mode waves with contradirected phase and group velocities are studied. Low noise detection achieved by cutting off spurious mode generation and a common objection made toward experiments of this type is avoided. This objection states that the highest frequency components of the pulse travel faster and are less attenuated than the main carrier frequency of the packet, distorting the original pulse to such an extent as to render measurements unrelated to barrier traversal times. Elastic waves generated with piezoelectric transducers are injected into aluminum plates through wedge-shaped Lucite waveguides so as to generate the appropriate Lamb mode which traverses a forbidden region and is picked up after it with a similar setup of transducer and waveguide. Various configurations of wave barriers have been measured and possible theoretical interpretation is given.