Possibilities of Decreasing Hygroscopicity of Resonance Wood Used in Piano Soundboards Using Thermal Treatment

This article presents the possibilities of decreasing moisture sorption properties via thermal modification of Norway spruce wood in musical instruments. The 202 resonance wood specimens that were used to produce piano soundboards have been conditioned and divided into three density groups. The first specimen group had natural untreated properties, the second was thermally treated at 180 degrees C, and the third group was treated at 200 degrees C. All specimens were isothermally conditioned at 20 degrees C with relative humidity values of 40, 60, and 80%. The equilibrium moisture content (EMC), swelling, and acoustical properties, such as the longitudinal dynamic modulus (E'(L)), bending dynamic modulus (E-b), damping coefficient (tan delta), acoustic conversion efficiency (ACE(L)), and relative acoustic conversion efficiency (RACE(L)) were evaluated on every moisture content level. Treatment at 180 degrees C caused the EMC to decrease by 36% and the volume swelling to decrease by 9.9%. Treatment at 200 degrees C decreased the EMC by 42% and the swelling by 39.6%. The 180 degrees C treatment decreased the value of the longitudinal sound velocity by 1.6%, whereas the treatment at 200 degrees C increased the velocity by 2.1%. The acoustical properties E-L ', E-b, ACE(L), and RACE(L) were lower due to the higher moisture content of the samples, and only the tan delta increased. Although both treatments significantly affected the swelling and EMC, the treatment at 180 degrees C did not significantly affect the acoustical properties.