J. M. Conoir
Laboratoire d'Acoustique Ultrasonore et d'Electronique URA CNRS 1373, Universite du Havre, Place Robert Schuman, 76610 Le Havre, France
The theoretical and experimental spectra, obtained from an infinite pipe insonified with a plane wave in oblique incidence, show resonances which are related to three types of helical waves: the circumferential waves (A[sub i] or S[sub i] waves), the transversal guided waves (T[sub i] waves), and the Scholte wave (A wave). The resonances of the Scholte wave are detected in a frequency window and their frequency slowly increases when the incidence angle increases, whereas the ones of the other waves increase towards infinity when the incidence angle tends towards the transversal critical angle. At oblique incidence, the experimental results obtained with the MIIR show resonances which are related to the helical waves S[sub 0], T[sub 0], T[sub 1], and A. This method uses a long pulse with many sinusoid periods, a steady state takes place in a part of the infinite shell. In this presentation, the excitation is a short pulse and the scattered echoes are detected when the helical wave emission is in front of the receiver. Between each echo, the helical wave has covered one step of the helix (L=2(pi)a/cos (gamma), a is the outer radius of cylinder, (gamma) is the angle of the helix). This method allows us to calculate the group velocity of the helical waves.