Document Type : Research Paper
Authors
- Shahrzad Omrani Ghahjaverestani 1
- Amir Lashgari 1
- Mehran Roohnia 2
- Ajang Tajdini 2
- Seyyed Khosrow Hossein Ashrafi 2
1 Department of Wood Industries Engineering and Lignocellulosic Products. Ka.c. Islamic Azad University, Karaj, Iran.
2 Department of Wood Industries Engineering and Lignocellulosic Products. Ka.c.Islamic Azad University, Karaj, Iran.
Abstract
Abstract:
Background and objectives: Wood remains the preferred material for acoustic resonators and soundboards due to its orthotropic behavior, low density, high stiffness, and exceptional low vibration damping properties. To simultaneously optimize surface protection and acoustic efficiency, wood surface modification using natural spirit )alcohol-based( multilayer coatings has been proposed as an efficient solution. However, the type of formulation and layering of the coating can have different effects on the vibration properties. This study aimed to evaluate and compare the effects of two natural multilayer spirit coating systems (orange spirit coating and dragon’s blood resin spirit coating) on the physical, mechanical, and dynamic-acoustical properties of spruce (Picea abies) wood to identify the optimal coating formulation.
Methodology: Ten healthy, defect-free samples of spruce (Picea abies( wood were prepared, measuring 150×12×2 mm (longitudinal, radial, and tangential) along the grain timber. After surface treatment, the samples were stored in a climate-controlled room (21°C and 65% relative humidity). Non-destructive forced flexural vibration testing was performed before coating application and the samples were classified based on standard criteria. The samples were divided into two groups of five; the first group was coated with multi-layer orange spirit coating and the second group was coated with multi-layer dragon’s blood spirit coating (including treatment steps, filler, middle layer (sealing) and final coating (topcoat)). The traditional French polishing technique was used to apply the coatings. After dring and condition of the samples at the end of each coating stage, the forced vibration test was repeated. The acoustic signals were recorded using Audacity software and analyzed using the NDT-lab® system. The parameters of density, dynamic elastic modulus, vibration velocity, acoustic coefficient, damping coefficient, acoustic conversion efficiency and mechanical impedance were calculated and the data were compared with Excel software and regression analysis.
Results: The results showed that the application of both types of coatings caused changes in the physical and acoustic properties of spruce wood, but the stability and trends of these changes differed between the two coatings. In the orange spirit coating, the density and vibration velocity increased with a high coefficient of determination (R2 = 0.94) and the specific elastic modulus was maintained. The acoustic coefficient decreased relatively in both coatings, but in the orange spirit coating this decrease was more limited and accompanied by a higher coefficient of determination (R2 = 0.90). The damping coefficient evaluation showed that the orange spirit coating provided a more uniform distribution and better stability in the vibration behavior without causing sudden drastic changes (R2 = 0.96), while the dragon’s blood spirit coating showed heterogeneous fluctuations in damping. Furthermore, the acoustic conversion efficiency of the orange spirit coating remained closer to the raw wood values at different stages (R2 = 0.91). Mechanical impedance also showed a more uniform increase and better stabilization in samples coated with orange spirit coating compared to dragon’s blood spirit coating.
Conclusion: Both natural spirit coatings were able to maintain the lightweight structure of the wood. However, the orange spirit coating, due to the formation of a more uniform layer and more controlled penetration, provided substantially better performance in maintaining and improving mechanical stability, vibration velocity, acoustic coefficient, and acoustic conversion efficiency. This coating caused minimal adverse changes in the damping coefficient. Overall, the spirit alcohol coating is recommended as a superior natural coating for optimizing the acoustic performance of spruce wood soundboards in musical instrument construction and architectural acoustic applications.
Summary: These findings indicate that the use of appropriately formulated multi-layer natural coatings can improve the surface protection of resonator wood without significantly degrading its acoustic properties. Therefore, the results of this study provide a valuable basis for the design and development of sustainable natural coating systems for use in musical instruments and other acoustic engineering applications requiring high vibrational stability and durability.
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