Building acoustic performance of stepped modular rice straw–paper pulp absorbers: In situ EApu measurements and effects of surface finishing

Standard laboratory tests of sustainable acoustic materials are often conducted on flat, homogeneous specimens and may not represent the installed behavior of modular surfaces. This study evaluated the in situ acoustic performance of stepped rice straw-paper pulp composites using the Ensemble Averaging Pressure-Velocity (EApu) method. Sound absorption coefficients were obtained in 1/3-octave bands over 100–3150 Hz and compared with flat reference panels. The modular stepped panel showed improved mid- to high-frequency absorption relative to a 10 mm flat panel, but lower low-frequency effectiveness than a continuous 30 mm panel, consistent with thickness-dependent and assembly-related effects. Natural dyeing did not produce a clear change in absorption, whereas waterproof coatings caused a slight reduction toward the upper frequencies. The results indicate that rice straw-paper pulp composites are promising bio-based acoustic elements and that EApu is a useful tool for evaluating modular acoustic surfaces under architecturally realistic conditions.