Relationships between different parameters at the hybrid resonant frequency with perfect absorption. (a) Dimensionless dissipation parameter (black curve) and dimensionless thickness (red curve) of the sealed air layer, plotted as functions of the dimensionless relative frequency location of the hybrid resonance. (b) Deformation component of the normal displacement at the hybrid resonance frequency, plotted as a function of . Impedance matching means that the surface-averaged displacement has to be equal to that of the incident sound – that is, 〈W〉 = 〈W₁〉 − 〈W₂〉 = W_s. Proximity to the anti-resonance also means that 〈W₁〉 ≃ 〈W₂〉. These two conditions imply a large W compared with its surface average, and account for its large variance component of the deformation . In particular, the small dissipation coefficient and the subwavelength scale dovetail with the large W in a self-consistent manner so as to achieve total absorption. The dot-dashed blue lines in the two figures indicate the parameter values relevant to the experimental results shown in Figure 5.