Vibrational properties of wood in frequency ranges including ultrasonic waves: Frequency dependences of dynamic young's modulus and loss tangent

The frequency dependence of dynamic Young's modulus and loss tangent of different species on frequencies were investigated at 20°C by means of the longitudinal vibration method. The dynamic Young's moduli up to 100 kHz and the loss tangents up to 70 kHz were obtained by the tap method and the electromagnetic method, respectively. The main results were as follows: (1) The dynamic Young's moduli in the longitudinal direction were almost the same from 5 kHz to 100 kHz, and this tendency was not related to species and treatment methods. (2) The loss tangent of untreated wood increased with the frequency, and the increasing rate was less than that of flexural vibration. In addition, there were some differences among species for the loss tangent and its increasing rate. (3) There were differences between heat-treated and untreated wood for the frequency dependence of loss tangent. The loss tangent of the wood that was treated at 150°C in a vacuum, in a moisture content of about 0%, was less than that of untreated wood in the frequency range of 10 kHz to 32 kHz. However, when treated wood increases to the equilibrium moisture content from 0%, its loss tangent was nearly the same as that of untreated wood. The loss tangent of wood that was treated at 180°C and 200°C in air dried conditions, was more than that of untreated wood in the frequency range of 10 kHz to 70 kHz, and the variations in the frequency dependence of the loss tangent were observed in the ultrasonic range. (4) It was known for the characteristics of formaldehyde-treated wood that the loss tangent was less than that of untreated wood in a lower frequency, and became the same gradually with increases of frequency so that the difference between them could not be observed in the frequency range of 17 kHz to 50 kHz.