A classical way of improving acoustical absorption performances of porous materials is the use of corrugated surfaces; this use can obtain lower cut-off frequencies and also improve the overall absorption over a wide frequency range. An analytical approximation is presented for the calculus of the absorption on this kind of surfaces, where the thickness gradient is represented as a series of steps. Reflection coefficient of every step is obtained and will contribute to the net reflection coefficient. Theoretical results will be presented and shown to agree with experimental data.

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