Improved Low-Frequency Broadband Absorption Using 3D Folded Cavity Acoustic Liners

Airborne noise with a low dominant frequency content (<~500 Hz) has detrimental effects in many applications, but is as yet beyond the scope of conventional acoustic noise mitigation techniques using liners, foams or claddings owing to mass and volume considerations. It contributes significantly to environmental noise pollution, and unwanted structural vibrations causing diminished efficiency, comfort, payload integrity and mission capabilities. An alternative approach using liner configurations with realistic mass and volume constraints having innovative folded core geometries is investigated to ascertain its low-frequency noise absorption characteristics. The relative performance of various candidate folded core designs is compared using a metric that we term the Low-Frequency Performance (LFP) factor, 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 3D printed test articles in a normal incidence acoustic impedance tube yield good correlation with simulations. More than 100 Hz of continuous bandwidth with an absorption coefficient greater than 0.6 is shown to be possible in the 250 to 400 Hz range with a 38.1 mm (1.5 inch) thick liner. 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.