Lignin-Driven Hydrogen Bond Network Rearrangement Enables Lightweight, High-Stiffness Kenaf Fiber Acoustic Diaphragms

Loudspeakers are essential in consumer electronics, medical devices, and automotive systems; however, their acoustic performance is often limited by a weak high-frequency response and excessive low-frequency distortion. In this work, kenaf bast fibers were extracted using a sodium hydroxide/anthraquinone (NaOH/AQ) solvent, which partially removed lignin while redistributing it at the micro- and nanoscales. During hot pressing, softened lignin acted as a natural adhesive, binding the fibers into a lightweight yet stiff composite diaphragm. The obtained diaphragm shows a density of 0.61 g/cm³ and a stiffness of 3.37 GPa, enhancing the high-frequency response while reducing low-frequency distortion. It delivers a wide-frequency response spanning 20 to 20,000 Hz, high sensitivity of 83.2 dB, and low distortion below 0.5%. Theoretical calculations reveal that lignin contributes to stiffness enhancement by shortening hydrogen bond lengths. Practical integration into automotive speakers further validates performance. This sustainable approach provides a pathway for next-generation high-fidelity acoustic devices.