Achieving broadband sound manipulation within deep-subwavelength, ventilated structures remains a fundamental challenge in acoustics. Here, we introduce a degenerate sandwich architecture that exploits the synergistic coupling of monopoles and dipoles. This platform enables simultaneous broadband control over sound reflection, transmission, and absorption. We experimentally validate this concept using a sandwich-structured device comprising two Fabry-Pérot resonator arrays and a central micro-perforated plate. Remarkably, featuring a deeply subwavelength cumulative interaction length of merely λ/107 at 200 Hz, the device achieves near-perfect sound absorption (99%) across an ultra-wide spectrum of 200–2000 Hz. Furthermore, the device provides robust sound insulation that significantly outperforms conventional mass-law predictions while maintaining permissible airflow. The industrial viability of this strategy is practically demonstrated through a prototype silencer for a household vacuum cleaner. This work overcomes inherent bandwidth limitations in sub-wavelength wave manipulation and paves the way for advanced noise-control technologies in open systems.