Broadband sound absorption and slow wave in truncated sonic black hole

Abstract

The sonic black hole (SBH) exhibits significant potential for trapping and dissipating acoustic waves. In this article, we propose a truncated-tail SBH (TSBH) composed of 20-unit cells, with heights increasing according to an exponential law. The truncated-tail SBH enhances low-frequency absorption peaks below the first-order cut-on frequency by promoting global resonance along the entire length of the structure. This results in an expansion of the sound absorption bandwidth towards lower frequencies, while maintaining the overall structural length. At low frequencies, the high-quality factors of resonance impede the formation of broadband sound absorption peaks. To address this, porous materials are integrated into the unit cells, effectively mitigating the inherent radiation leakage at frequency below 850 Hz. This approach significantly enhances sound absorption in the low-frequency range, ensuring better energy dissipation and addressing the challenges posed by strong dispersion effects in this regime. Slow wave phenomena are also observed in the same frequency range by calculating the effective bulk modulus and sound speed. Test samples are manufactured using 3D printing and tested in a square cross-section impedance tube. An average sound absorption coefficient greater than 80 % is measured over a broadband range from 250 to 1700 Hz with a structure thickness of 100 mm. The proposed slow sound theory and the properties of truncated SBH with added porous materials may pave the way for perfectly absorbing SBH, potentially broadening its application.