Transforming Room Acoustics with Causality‐Driven Dual‐Function Passive Metamaterials

Abstract Recent advances in acoustic metamaterials have overcome narrow‐band limitations, allowing for optimal tuning of key functions such as sound absorption and diffusion. However, most existing designs focus on single functions, leaving a gap in multi‐functional solutions that can address complex acoustic requirements. To fill this gap, a Dual‐Function Passive Acoustic Metamaterial (DFPAM) is proposed that effectively balances absorption and diffusion within the same structure by leveraging principles of causality and passivity. The DFPAM achieves high broadband sound absorption (〈α〉 ≈ 0.85) and diffusion (〈δ〉 ≈ 0.8), covering more than one octave in each configuration. With a compact thickness of only 12 cm, nearly half that of traditional treatments, this design uses just a fraction of the available space (1/3 for absorption, 2/3 for diffusion) while maximizing performance. This novel approach not only optimizes sound control but also provides a highly space‐efficient solution. The DFPAM offers transformative possibilities for room acoustics and can be adapted to diverse environments, including building and aerospace applications, where multi‐functional acoustic solutions are in increasing demand.