Identification of wood structure related vibrational characteristics of wooden plates from Norway spruce (Picea abies [L.] Karst.) by high resolution laser scans

Abstract

The vibrational behaviour of wooden plates of Norway spruce (Picea abies [L.] Karst.) was studied by means of a high resolution laser scanning system, which allowed vibration measurements with an accuracy of 0.6 μm, a spatial resolution of the measuring spots of 28.6 μm, and a sampling frequency of 10,000 Hz. In total the vibration spectra of 10,760 measurement spots were analysed in terms of the number, frequencies, amplitudes, and damping of the overlapping natural oscillations. Special regard was given to the influence of the anatomical direction and the wood structure on the vibrational behaviour of the plates. In more than 70% of the vibration spectra two natural frequencies were present, while the other spectra showed indistinct vibration patterns. The anatomical direction of the wood had a significant influence on the vibrational behaviour in terms of the frequencies in absolute numbers as well as the amplitudes and the damping of the natural oscillations. Vibration spectra obtained from measurements in earlywood and latewood zones of the tree rings showed very similar characteristics, while measurements in zones with strong changes in tree ring widths and the presence of resin canals were often correlated with an increase of overlapping natural oscillations of indistinct frequencies, amplitudes, damping, and phase offset. The results indicate that the homogenous and highly organized cell structure in the wood of Norway spruce leads to distinct vibrational characteristics with two distinct overlapping natural frequencies. With regard to the use as resonance wood changes in tree ring widths and accumulations of resin canals are local defects, which have a negative impact on the vibrational properties of the wood.