THE BASICITY OF THE SYSTEM OF EIGENFUNCTIONS OF THE TRANSVERSE CROSS-SECTION OF WAVE-GUIDES WITH FLEXIBLE WALLS

The eigenfunctions of the transverse cross-section of waveguides with oscillating walls are considered. The waveguides are filled with a fixed or moving acoustic medium in which the velocity of sound is either constant or depends on the transverse coordinate. It is shown that the eigenfunctions form a basis which is quadratically close to an orthonormalized basis of eigenfunctions of the waveguide with acoustically rigid walls.