Coefficient of energy absorption, , as a function of the incident wave frequency (in Hz), f = ω/ 2π. a) Results when the incident waves on both sides of the slab are in phase (I₁ = I₂, black solid line) and antiphase (I₁ = −I₂, dashed blue line). b) Results when I₂ = I₁e^{iπ/ 2}. In both panels, the vertical dotted lines denote the approximate cavity resonance frequencies according to Eq. (45). In all cases we used ξ_i = 0.2 and the remaining parameters are the same as in Fig. 3.