Unidirectional reflection induced by multiple exceptional points in a non-Hermitian sonic black hole

Exceptional points (EPs) have attracted widespread attention due to their anomalous scattering and asymmetric wave manipulation properties induced by non-Hermitian degeneracy. However, the controllable generation of multiple EPs is typically constrained by high-dimensional parameter spaces and stringent eigenstate merging conditions, which poses particular challenges in subwavelength passive acoustic structures. This paper proposes a passive non-Hermitian platform based on a sonic-black-hole (SBH) gradient cavity to approach multiple-EP conditions at compact subwavelength scales. This platform leverages the synergy between gradient impedance evolution, localized thermoviscous dissipation, and ring-cavity coupling to control the scattering matrix eigenvalues and the eigenstate merging process. To efficiently search for structural parameters that satisfy the multi-EP conditions, an inversion algorithm design framework based on physical mechanism constraints was further established, thereby improving the search efficiency and stability of multi-EP configurations in complex parameter spaces. The results demonstrate that an SBH structure with a thickness of only λ/11 exhibits four pronounced near-EP states in the 600–2000 Hz frequency band, exhibiting significant directional dependence in the scattering response at the corresponding frequencies: near-perfect absorption for forward incidence and near-total reflection for backward incidence. This work reveals the physical mechanisms by which ring-cavity coupling and thermoviscous non-Hermitian drive the system toward multiple-EP conditions in SBHs, providing new avenues for the design of subwavelength passive non-Hermitian acoustic systems, low-frequency directional absorption, and integrated asymmetric acoustic devices.