Deep-subwavelength ultra-low and ultra-broadband acoustic-black-hole metamaterials.

Acoustic black holes (ABHs) offer broadband wave-manipulation capabilities beyond conventional acoustic metamaterials (AMs) but are fundamentally limited by compromised structural stiffness, high-precision machining requirements and high cut-on frequencies. Here, we break these limitations by adopting a power-law density-tailored composite, ρ-ABH. Analytical derivation, wave-energy analysis, and coupled-system modeling demonstrate that both the cut-on and threshold frequencies of ρ-ABHs are reduced to one-fifth of those in conventional ABHs, enabling operation at deep-subwavelength scales (λ/11). This breakthrough arises from a remarkable wavelength compression and energy density amplification. The inertial-grading-induced amplitude decay also mitigates the fatigue and fracture risks inherent to conventional ABHs. The device experimentally entails efficient wave absorption at ultra-low and -broadband frequencies (25-1200 Hz) and with a 24.5 Hz threshold. Our approach overcomes fundamental frequency-scale constraint in AMs and vibroacoustic engineering, and circumvents manufacturing challenges via controllable material synthesis, offering a pathway for next-generation noise and vibration mitigation technology.