Initial values for the integration scheme to compute the eigenvalues for propagation in ducts

It has been found that in the integration method for calculating the eigenvalues for propagation in two-dimensional ducts with flow care must be taken not to exclude roots which in the hardwall case are at infinity. When the real part of the wall admittance is positive starting values for the integration procedure to obtain these extra eigenvalues can be obtained by a limiting case analysis. When the imaginary part of the admittance is negative it appears that all the roots can be accounted for by the usual hardwall initial values κ_nb = nπ.