A plug-in sonic black hole for duct terminations

This work presents a plug-in sonic black hole (SBH) designed for insertion at the termination of circular ducts to achieve broadband sound absorption. Unlike standard SBHs, which require tailoring the duct wall geometry, the plug-in configuration consists of an independently manufactured, axisymmetric insert. This alternative approach offers several acoustic advantages. For the same free cross-sectional area, the plug-in SBH exhibits a lower reflection coefficient than the standard design. Moreover, while the latter relies on the rainbow trapping effect-associated with cavity resonances–over a sometimes dominant frequency range, the black hole effect–associated with axial Fabry–Pérot resonances–operates over a significantly broader frequency band in the plug–in SBH. The performance of the device is assessed through a progressive modeling strategy: starting from a continuous waveguide approximation, followed by a discretized model with a finite number of rings, and finally validated experimentally. A prototype was 3D printed and tested in an impedance tube, demonstrating both the feasibility and effectiveness of the proposed concept, with good agreement with numerical simulations. From a practical perspective, the plug–in SBH is easy to fabricate using low–cost 3D printing and can be adapted to ducts of arbitrary geometry and material, making it highly versatile and suitable for retrofitting.