An environmentally friendly and efficient method to improve the acoustic vibration performance of bamboo for musical instruments: Nitrogen-protected heat treatment

Abstract

The traditional drying process is very important to the acoustical quality of bamboo musical instruments, such as Jinghu. However, this technique has inherent challenges such as a complicated process, long cycle times, and ineffective quality control. In this study, heat treatment under nitrogen protection was used to determine the effects of different temperatures (160 °C, 190 °C, and 220 °C) and durations (0.5 h, 1.5 h, 2.5 h, and 3.5 h) on the acoustic vibration performance of bamboo. The measured and calculated variables included specific dynamic elastic modulus E ’ / ρ , loss tangent tan δ , and acoustical converting efficiency ACE. With respect to microstructure and physical properties, the mechanism of how heat treatment affects the acoustic vibration properties of bamboo was determined. Results showed that (1) appropriate heat treatment under nitrogen protection can significantly improve the acoustic vibration properties of bamboo. Within the scope of this study, the optimal heat treatment conditions were heating at 160 °C for 3.5 h under nitrogen protection (type I). Compared to the control group, the E ’ / ρ and ACE values of type I heat-treated bamboo increased by 2.0% and 37.0%, respectively, and the tan δ decreased by 30.4%. (2) In terms of physical properties, the porosity of type I heat-treated bamboo increased from 31.6% to 35.2%, which increased the permeability along the longitudinal vibration of the bamboo. (3) In terms of chemical components, the concentration of the starchy substance in the parenchyma cell lumen of type I heat-treated bamboo was significantly less as it was degraded and volatilized. The content of hygroscopic hemicellulose was reduced from 25% to 21% due to pyrolysis, while the cellulose and lignin maintained their structural stability, thereby improving the bamboo’s ability to resist ambient humidity changes. (4) On the molecular scale, the microfibril angle (MFA) of type I heat-treated bamboo was reduced from 12.54° to 10.54°, and the crystallinity increased from 34.8% to 36.5%. Type I heat treatment improved the orientation degree of the bamboo cell wall fibrils and adjusted the distance between cellulose molecular chains. • Through heat treatment, the chemical composition, physical structure, and surface quality of bamboo have been changed. • The optimal heat treatment conditions for acoustic vibration performance improvement are 160 °C for 3.5 h under nitrogen protection. • The main mechanism of improvement include formation of porous pathways and increase of orientation cellulose microfibrils.