Microscopic constitutive freedom in metamaterials
Metamaterials expand the range of accessible macroscopic responses through architectural design. Here, a cross-scale design strategy is introduced that combines microscopic control of static constitutive properties with spectral engineering through macroscopic architecture. Beyond shaping frequency-dependent responses, this strategy modifies the corresponding broadband performance bounds in passive systems by engineering the static parameters entering causal constraints. The concept is realized in granular activated carbon, where reversible sorption enhances equilibrium compressibility. Extrapolation of a multiscale model supported by low-frequency measurements to the zero-frequency limit yields an effective static bulk modulus approximately half the conventional nonsorbing isothermal reference value. This reduction substantially lowers the corresponding causal minimum-thickness bound for rigidly backed absorbers. A cascaded hierarchical composite architecture further redistributes absorption toward the target frequency band, achieving near-perfect absorption with a normal-incidence absorption coefficient above 0.9 across most of the 40--6400 Hz range. Together, static constitutive control and macroscopic spectral design enable ultrabroadband sound absorption beyond the conventional isothermal thickness benchmark without violating causality. This strategy offers a new approach to extending the attainable performance of metamaterials.