Abstract
The design of deep-subwavelength structures for low frequency sound perfect absorption is challenging. Subwavelength perfect absorption implies increasing of the density of states at low frequency while maintaining impedance matching to the surrounding medium. Therefore, the study of the eigenvalues and eigenvectors of the scattering matrix in the complex frequency plane appears extremely powerful to analyze such systems and derive the critical coupling condition. The latter consists in exactly compensating the leakage of the structure by using the intrinsic losses. Two simple structures, the thicknesses of which are 88 times and 40 times smaller than the perfectly absorbed wavelengths, are presented for reflexion and transmission problems, respectively.
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