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Možnosti snížení navlhavosti dřeva a přestupu vlhkosti do dřeva užitého v konstrukci rezonanční desky hudebního nástroje
Zatloukal, Petr
This theses is focused on possibilities of sorption properties decrease of musical instruments soundboards. This task should be solved with respect to sustainability of previous acoustics properties or their improving. The resonance wood specimens has been fabricated and conditioned to the relative humidity of 40 % The specimens were divided into three groups. The first one with natural properties, the second one treated by the temperature of 180 °C and the last one was treated by 200 °C. Specimens were conditioned all together to the relative humidity of 60 % and at the last step at 80 %. The conditioning was isothermal with the temperature of 20 °C. Swelling and hygroscopicity establishment was based on dimensions and weight measurement during conditioning of specimens. The first, second and longitudinal natural oscillation frequency of the specimens were determined by free-free resonant frequency method. Longitudinal and transversal sound propagation velocity were measured using Fakopp Ultrasonic Timer. Dynamic Young’s modulus (E´L,E´RT), dynamic bend Young’s modulus (Eb), acoustic constant (KL, KRT), characteristic impedance (ZL, ZRT), acoustic conversion efficiency (ACEL, ACERT), relative acoustic conversion efficiency (RACEL, RACERT) and damping coefficient (tan δ) were determined using the measured data. The treatment by 180 °C caused the decrease of volume swelling, which was 7,4% lower after the treatment. The hygroscopicity decreased by 2,8%. The treatment by 200 °C decreased swelling by 8,5% and the hygroscopicity by 4,3%. The different results of both treatments were observed in case of longitudinal sound velocity. In case of 180°C treatment the value of longitudinal sound velocity decreased by 1,6 %, whereas after treatment by 200 °C the increase by 2,1% was found. Acoustical properties E´, Eb, K, Z, ACE, RACE were decreasing due to the higher moisture content and tan δ was increased. Heat treatment by 180 °C has not significantly affected acoustical properties and sound velocity but the treatment by 200 °C caused their increase.
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