Investigating Sound Absorption of Oil Palm Trunk Panels Using One-microphone Impedance Tube

Sound absorption coefficient of oil palm trunk was explored using an impedance tube. Palm samples were taken from the central part of oil palm trunks with cut directions parallel and perpendicular to vascular bundles. Sound absorption was evaluated for palm panels with blind-holes with multiple radii and depths, as well as perforated and grooved panels and a panel with perforated holes at different distances from a solid backing. Measurements of sound absorption within the frequency range of 300-2000 Hz indicated that the sound absorption coefficient of the cross-cut biomass, ~0.15, was slightly greater than that of the parallel-cut panel, ~0.10. Samples with different depths of blind holes showed slight improvements in sound absorption coefficients as compared to the unmodified cross-cut panel. There was a significant improvement for 5-mm hole diameter with 10-mm depth, ~25% improvement as compared to that of 5-mm depth. The combination of the through-hole panel and grooved board allowed ~80% of sound to be absorbed for 1750 to 2000 Hz. Finally, the grooved board was removed and an air cavity backing was introduced by placing the through-hole panel 2-, 4-, and 6-mm away from the tube end. The sound absorption coefficients were then measured to be greater than 80% near the resonance frequencies, as calculated using the distributed Helmholtz resonator model.