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 °C, and the third group was treated at 200 °C. All specimens were isothermally conditioned at 20 °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 (Eb), damping coefficient (tan δ), acoustic conversion efficiency (ACEL), and relative acoustic conversion efficiency (RACEL) were evaluated on every moisture content level. Treatment at 180 °C caused the EMC to decrease by 36% and the volume swelling to decrease by 9.9%. Treatment at 200 °C decreased the EMC by 42% and the swelling by 39.6%. The 180 °C treatment decreased the value of the longitudinal sound velocity by 1.6%, whereas the treatment at 200 °C increased the velocity by 2.1%. The acoustical properties EL′, Eb, ACEL, and RACEL were lower due to the higher moisture content of the samples, and only the tanδ increased. Although both treatments significantly affected the swelling and EMC, the treatment at 180 °C did not significantly affect the acoustical properties.

Bark Features for Identifying Resonance Spruce Standing Timber

Measuring the acoustic properties of wood is not feasible for most luthiers, so identifying simple, valid criteria for diagnosis remains an exciting challenge when selecting materials for manufacturing musical instruments. This article aims to verify whether the bark qualities as a marker of resonance wood are indeed useful. The morphometric and colour traits (in CIELab space) of the bark scales were compared with the structural (width and regularity of the growth rings and of the latewood) and acoustic features (transverse sound velocity, radiation ratio, impedance, and wood basic density) of the wood from 145 standing and 10 felled spruce trees, which are considered a resource of the resonance wood in the Romanian Carpathians. It has been emphasized that the spruce trees with acoustic and structural features that match the requirements for the manufacture of violins have a bark phenotype distinguishable by colour (higher redness, lower yellowness and brightness)—as well as by scale shape (higher slenderness and width). The south-facing side of the trunk and the external side of the scale are best for identifying resonance trees by their bark. Additionally, the mature bark phenotypes denote topoclinal variations and do not depend on tree age. Moreover, the differences among bark phenotypes are noticeable to the naked eye.

Investigation of Modal Behaviour of Resonance Spruce Wood Samples (Picea abies L.)

Results of experimental modal analysis of a resonance and non-resonance spruce wood (Picea abies L.) are presented. The resonance wood came from a tree from Poland and Bosnia and Herzegovina, while the non-resonance wood came from the vicinity of Olsztyn from the north-eastern Poland. The modal parameters (modal frequency, modal damping and mode shapes) of the wood samples were determined for the samples of 8 mm in thickness. Modal analysis was made by pulse excitation. The resonance and non-resonance wood differ in the fundamental modal parameters as well as in the number of potential modes. Additionally, calculated values of damping factor are presented. The values are much bigger for a non-resonance wood than for good quality resonance spruce.