<p>The fixed acoustic characteristics of unit-cell cavities fundamentally limit the broadband sound absorption performance of cavity-type sound-absorbing metamaterials. This study identifies that the primary cause of the fixed acoustic characteristics in unit-cell cavities lies in the acoustic rigidity boundary conditions of the cavity walls. Based on this insight, a band-pass acoustic valve structure is proposed to replace conventional acoustic rigidity boundaries. Additionally, a framework of “band-pass acoustic valve–additional equivalent stiffness–impedance matching” is introduced to broaden the bandwidth of cavity-type sound-absorbing metamaterials. The method addresses the dual challenges of narrowband absorption limitations and the degradation of absorption peaks caused by cavity boundary gradually transforming states. Experimental results demonstrate that, compared to traditional cavity-type sound-absorbing metamaterials, the proposed metamaterial incorporating band-pass acoustic valves achieves a 65% increase in sound absorption bandwidth without an increase in size. This work provides a practical approach for achieving high-efficiency broadband sound absorption under spatial constraints.</p>