Acoustic properties of modified wood under different humid conditions and their relevance for musical instruments

In musical instrument making, there is a strong need to find alternatives to the traditional endangered and expensive tropical wood species used today. The present study examined whether different commercial and experimental modified wood materials have the necessary acoustic qualities under different humid conditions (dry, standard and wet) to contribute to the use of raw materials for wooden musical instruments. The materials were thermally-modified wood (ash, aspen and birch), acetylated wood (beech, maple and radiata pine), melamine- and phenol formaldehyde-treated beech and furfurylated Scots pine (Kebony Scots pine). Investigations involved physical (density rho, Equilibrium moisture content EMC, volumetric shrinkage) and dynamic elastic testing by a free-free flexural vibration method to determine various acoustic characteristics: specific dynamic modulus (MOEdyn/rho), damping coefficient (tan delta), speed of sound (c), specific acoustic impedance (z), sound radiation coefficient (R) and acoustic conversion efficiency (ACE). The modified materials and especially acetylated wood showed low EMC values and high dimensional stability at each humidity level, which are considered important factors for making quality musical instruments. Based on the obtained value ranges of all acoustical properties, the different modified wood materials could find uses in musical instruments where specific characteristics of sound are required. Furthermore, most of the modified materials showed an excellent acoustic performance in the three humid conditions based on a high ACE and low tans. Furfurylated Scots pine and phenol formaldehyde-treated beech showed an inferior acoustic quality with the lowest ACE and the highest tans, which is a less favourable combination for most of the musical instruments.