Controlling low-frequency noise in ventilation ducts remains challenging with conventional porous materials, whose sound attenuation capability is constrained by the weak tunability of their acoustic impedance. Here, we report a dual-porosity meta-silencer that addresses this constraint by embedding a millimeter-scale cylindrical perforation array into melamine foam. The resulting metafoam integrates micron-sized intrinsic pores with engineered perforations, enabling wide-range modulation of the surface acoustic impedance. By expanding the accessible range of both acoustic resistance and reactance, the metafoam achieves efficient broadband sound attenuation at low frequencies while preserving structural simplicity. Based on this principle, two non-intrusive silencer configurations, single-sided and double-sided, are designed. Over the 100–1600 Hz range, they raise the average transmission loss to 9.3 dB and 18.7 dB, respectively, with a peak enhancement of up to 12.5 dB compared with a uniform foam silencer of the same total thickness. This work provides a practically viable meta-silencer design paradigm for low-frequency duct noise control, with potential applications in duct systems.