Causal-constraint broadband sound absorption under isothermal process

Causality, a cornerstone of physical laws, fundamentally links a system's structural characteristics to its wave interaction properties, such as the minimum thickness of acoustic absorbers required for specific absorption spectra. Traditional causality principles for sound absorption are derived under the assumption of adiabatic sound propagation. In this Letter, we propose a generalized causal framework that incorporates isothermal processes, accounting for nonslip boundary conditions at the fluid-solid interface. These conditions introduce velocity and temperature gradients, challenging the conventional adiabatic assumption. To validate our framework, we analyze two distinct absorber types: a metamaterial with multiresonant units and a metafoam with multilayer double-porosity structures. Our theoretical and experimental studies reveal that absorber thickness can exceed the adiabatic limit, being instead governed by isothermal constraints. This paradigm shift deepens the understanding of sound absorption mechanisms and paves the way for designing high-performance acoustic devices that approach fundamental performance limits.