Development of eco-friendly spiral-shaped sound absorber made from handcrafted fibrous paper enhanced with spent coffee waste for broadband noise control

To address the non-degradability and toxicity of conventional acoustic materials, this study proposes a sustainable spiral-shaped sound absorber composed of plant fiber-based fibrous paper and recycled coffee waste (CW). The strong mechanical bonding between CW and Kozo fibrous paper in this composite acoustic material was observed using metallurgical microscopy, resulting in an environmentally friendly structure capable of controlling broadband noise. A prediction model based on parallel-slit theory was developed to evaluate the influence of key structural parameters-CW layer mass density, fibrous paper length, and absorber width-on sound absorption coefficients. Optimization reveals that wide spiral-shaped geometry paired with a high-density CW layer (0.04-0.05 kg/m2) enhances low-frequency noise reduction (<1000 Hz), whereas narrow configurations with a medium-density CW layer (0.03-0.04 kg/m2) improves high-frequency attenuation (>2000 Hz). The sound absorption coefficients of five prepared samples were measured using the two-microphone impedance tube method. The sound absorption coefficient showed significant improvement with the addition of an appropriate amount of CW in the mid- and high-frequency range. This work advances the development of lightweight, efficient, and sustainable acoustic solutions, providing a scalable strategy for the next generation of eco-friendly materials in line with circular economy principles and low-carbon manufacturing practices.