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Achieving broadband sound absorption in two-port open ducts is of fundamental importance in the acoustics, with wide applications ranging from noise control to duct sound mitigation. Yet the existing metamaterial designs are usually based on the use of Helmholtz-type cavities, posing limitations on the resulting absorption performance. Here, we propose and experimentally demonstrate a mechanism that uses coupling modulation of the resonance energy leakage and loss in ventilated metamaterials to realize optimal sound absorption. We design a slit-type unit cell as a practical implementation of the proposed mechanism and analytically prove its potential to obtain the desired leakage and loss factors simultaneously by properly adjusting the structural parameters. We benchmark our designed metamaterial with a conventional Helmholtz resonator-based design to demonstrate its advantage of sound absorption. Good agreement is observed between the theoretical predictions and experimental measurements. Our strategy represents a paradigm extending beyond classical models and opens up possibility for the design of high-efficiency acoustic absorbing devices and their applications in diverse scenarios especially broadband duct noise muffling. [ABSTRACT FROM AUTHOR]