Fig. 1: Instability raises absorption limit.
From: Acoustic blackbody through instability-induced softening
Cavity air behind MPP (black) acts as a spring (stiffness k = SB∞/d, where B∞ = γpatm is air’s static modulus, S is area, and d is depth). An unstable component (negative spring \({k}^{{\prime} }\)) reduces effective B∞, thereby increasing absorption’s causal limit, 4π2dρc2/B∞, in Eq. (3). The critical condition occurs when unstable energy (\({E}^{{\prime} }\) in red) cancels the original elasticity (E in blue), reaching a new neutral condition with B∞ = 0.
