Acoustic muffling of the Sonic Black Holes

Despite the growing interest in Sonic Black Holes (SBHs), existing research predominantly focuses on their sound absorption properties. However, real-life SBH structures, which consist of a finite number of concentric rings, inherently involve both sound absorption and reflection, which differ from the ideal SBHs designed for perfect absorption. Exploration of the muffling characteristics as well as the underlying mechanisms of SBHs are scarce, leaving their properties and underlying principles remain unclear. To bridge this gap, the above issues are investigated numerically and experimentally in this paper, in the context of a linear SBH connected to the main duct as a side branch. Numerical analyses reveal that SBH demonstrates multi-peak, broadband muffling characteristics, and outperforms the muffling performance of traditional quarter-wavelength tube. It was found that when the sound pressure distribution inside the SBH roughly matches odd quarter wavelengths pattern, both the absorption and reflection effects are maximized, resulting in the occurrence of muffling peaks. The SBH induced slow wave effect lowers muffling peak frequencies, creating multiple muffling peaks within the analyzed frequency band. The internal thermal-viscous dissipation effect ensures the broadband muffling performance. The numerically predicted noise attenuation results are validated through comparisons with experimental measurements. While shedding light on the underlying sound attenuation mechanisms, this study leverages the combined effects of partial absorption, reflection, cavity resonances and slow wave phenomena in practical SBHs to develop compact, broadband, low-frequency silencers, providing practical guidelines for future SBH-based muffler designs.