Rigid inclusion design strategies for enhanced sound absorption of natural fiber composites

Natural fibers are increasingly explored for sound absorption, with Oil Palm Empty Fruit Bunch (OPEFB) offering a sustainable alternative to mineral absorbers. Yet, OPEFB composites often underperform, particularly at low frequencies due to impedance mismatch and long acoustic wavelengths. While rigid inclusions have been studied, the role of their position and orientation, especially in simple rectangular forms, remains limited. This study uses 2D finite element models based on the Johnson–Champoux–Allard (JCA) equivalent fluid model and employs Nelder–Mead optimization to determine inclusion specifications for broadband absorption in OPEFB composites. Experimental validation was conducted for two non-optimized cases up to 2.7 kHz, while optimized designs were assessed numerically. Results show inclusions near the front surface enhance low-frequency absorption, while those near the rigid backing improve high-frequency response. Orientation adjustments broaden the bandwidth by about 500 Hz toward lower frequencies and increase the absorption coefficient by about 0.46 near 1.6 kHz compared with the same-thickness porous material. These findings highlight inclusion placement and orientation as effective strategies to improve OPEFB absorbers, with potential applicability to other porous materials.