Design and Optimization of 3D Folded-Core Acoustic Liners for Enhanced Low-Frequency Performance

Airborne noise with a dominant low-frequency content has detrimental effects in many applications, but it is as yet beyond the scope of conventional acoustic noise mitigation techniques using liners, foams, or claddings owing to mass and volume considerations. An alternative approach using liner configurations retaining realistic mass and volume constraints and having innovative 3D folded core geometries is investigated to ascertain its low-frequency noise absorption performance. The relative performance of various candidate 3D folded core designs is compared using a metric termed the low-frequency performance (LFP) metric, which is derived from Zwikker-Kosten Transmission Line (ZKTL) theory–based numerical studies. An LFP-based software tool is developed to optimize packing of 3D folded cavities. Experimental verification of absorption coefficient spectra conducted using additively manufactured test articles in normal incidence acoustic impedance tubes yielded good correlation with simulations, verifying the feasibility of this approach. For an optimized design, more than 100 Hz of continuous bandwidth with an absorption coefficient greater than 0.6 is shown to be possible in the 250–400 Hz range with a liner 38.1 mm (1.5 in.) thick. With current additive and hybrid manufacturing techniques attaining critical commercial maturity, 3D folded cavity liners could provide a promising practical solution to mitigate low-frequency airborne noise, especially in aerospace applications.