Thermally Programmable Two-Port Non-Hermitian Acoustic Metastructure for Broadband and Direction-Dependent Absorption.

A thermally programmable two-port non-Hermitian acoustic metastructure is proposed to realize broadband and direction-dependent sound absorption by exploiting temperature as a non-geometric air-property tuning variable. Within a unified transfer-matrix and electro-acoustic circuit modeling framework, the influence of thermal variation in air density, viscosity, and speed of sound on impedance matching and loss-leakage coupling is quantitatively described. The model reveals that thermal modulation shifts the scattering-matrix eigenvalues and adjusts the critical-coupling condition, enabling asymmetric absorption associated with exceptional point behavior. Numerical analyses show that, under thermally induced parameter changes, the structure achieves an effective bandwidth of 321 Hz at a deep subwavelength scale, while maintaining robust one-sided suppression of reflection. Temperature-driven transitions between under-damped, critically damped, and over-damped states further demonstrate how non-Hermitian coupling can be programmably controlled without any geometric modification. The proposed framework consolidates theoretical modeling and numerical prediction, clarifies the mechanism of thermally programmable asymmetric absorption, and provides a compact route for broadband sound control in extreme or variable-temperature environments. These results offer fundamental guidance for designing programmable acoustic absorbers and establish a foundation for future material and high-temperature implementations.