Summary: The performing spectral method, developed by

*P. Haldenwang, G. Labrasse, S. Abboudi*, and

*M. Deville* [J. Comput. Phys. 55, 115–128 (1984;

Zbl 0544.65071)], to solve multi-dimensional Helmholtz equations, associated to mixed boundary conditions with constant coefficients, is extended to boundary conditions mixing a first order normal derivative with a second order tangential derivative. The accuracy of the proposed algorithm is evaluated on two test cases for which analytical solutions exist: an academic problem and a physical configuration including an interface with shear viscosity. The procedure is also applied to the research of the Rayleigh–Bénard instability thresholds in closed cavities with thin diffusive walls.