Interaction between wave number pairs for a capped cylinder

When acoustic-structure interaction is analyzed according to the wave numbe r-based version of the surface variational principle (SVP), considerable co mputational effort is required to generate each coefficient in the quadrati c sum forming the variational quantity. Examination of the wet-surface impe dance, which represents the spectrum of pressure amplitudes generated by a specified spectrum of surface velocity amplitudes, reveals that many cross- impedance terms are very small. This suggests that some coefficients need n ot be computed. The article introduces a priori criteria for selecting the impedance terms to be omitted, based on the supersonic cutoff wave number. Each truncation scheme is assessed by comparing its predictions to the conv ergent SVP solution. It is shown that, for the nonsymmetric azimuthal harmo nics, the field quantities and the radiated power are well predicted if sub sonic waves are ignored. In contrast, for the axisymmetric component, subst antial errors (6 dB or more) for radiated power arise, unless a broad spect rum of subsonic waves are included in the formulation. The complex power is mostly reactive, being associated with an evanescent field, even for the s upersonic spectrum. The study shows that small interactions between subsoni c waves in the axisymmetric case can result in the high wave number spectru m being associated with substantial radiation contributions. (C) 2000 Acous tical Society of American. [S0001-4966(00)02904-0].