Phenol formaldehyde resin impregnation of spruce wood to improve the dimensional stability and acoustic performance of harp soundboards

Abstract

To prevent cracks in the wooden soundboards of harps while maintaining its acoustic performance, Sitka spruce wood was impregnated with phenol formaldehyde (PF) resin. PF treatment significantly improved the dimensional stability of wood through the bulking effect of the PF resin introduced into the wood cell wall. Highly moistened wood was fixed at its ends and dried. The unmodified wood failed owing to drying stress, whereas the PF-treated wood did not fail because of its excellent dimensional stability. In addition, PF treatment was found to significantly reduce the internal friction of the wood, resulting in an improvement in the acoustic conversion efficiency. These results suggest that PF resin impregnation is a promising method for preventing the cracking of soundboards without degrading their acoustic quality. By contrast, the PF resin migrated around the surface, from which the solvent evaporated. If we need to modify the wood uniformly along the longitudinal (L) direction, its ends or cross-sections should be closed to prevent the migration of the PF resin along the L direction during solvent removal. It is practically difficult to prevent the condensation of the PF resin around the straight-grained surface when a large-dimensional soundboard is impregnated with PF resin. However, the results of a simulation using a multilayered model predicted that such condensation did not adversely affect the acoustic quality of the soundboard.