Low-frequency enhancement of acoustic black holes via negative stiffness

Acoustic black holes (ABHs) attract increasing attention in the field of vibration suppression, sound radiation control and energy harvesting. Though the ABH effect usually works well in the mid-high frequency range, it cannot be triggered below the cut-on frequency. Thus, enhancing the vibration reduction at low frequencies is an urgent task. Inspired by the impressive low-frequency performance of the negative stiffness, we propose a novel idea that combines an ABH beam with a negative stiffness element (NS-ABH beam). To characterize such a system, a semi-analytical model is developed based on the Gaussian expansion method (GEM) in the framework of the Rayleigh-Ritz method. The modal loss factors (MLFs) and mean square velocity (MSV) are calculated to estimate the damping performance. The results show that the proposed NS-ABH beam can achieve remarkable vibration reduction below the cut-on frequency without sacrificing the ABH effect at higher frequency. The coupling mechanism between negative stiffness and ABH beam and the reduction mechanism behind are studied based on the modal projection method to better understand the observed phenomena. Finally, experiments have been implemented to validate the theoretical model and the NS-ABH performance.