Liu et al. J Wood Sci (2020) 66:14 https://doi.org/10.1186/s10086-020-01862-7 Journal of Wood Science ORIGINAL ARTICLE Open Access Properties of common tropical hardwoods for fretboard of string instruments Meihong Liu, Limin Peng, Shaoyi Lyu and Jianxiong Lyu* Abstract Fretboards of string instruments are usually made of rare woods that commonly have a high density, strength, and hardness; further, they are wear resistant, uniform in texture, and feature an elegant color. To reduce the consump- tion of scarce timber resources, especially of endangered tropical hardwood species, suitable replacement materials should be identifed. The substitute can be either common tree species having similar characteristics, or fast-growing plantation wood that has undergone modifcations to match the performance of precious woods. This study com- pares the anatomical structure, physical features, mechanical properties, and surface color of three precious woods traditionally used in fretboards (ebony, Indian rosewood, and African blackwood) against maple, which is used for the backboard, ribs, and necks of string instruments. Based on the data, a set of performance evaluation indices for select- ing alternative materials for fretboards is proposed. In specifc, the replacement wood should be a difuse-porous tropical hardwood with few vessels and a smaller diameter, thick fbrous walls, and a cell wall rate of more than 50%. In terms of physical properties, it should have low swelling coefcients for moisture and water absorption, and dimen- sional stability. The replacement should also display hardness values greater than 9.0 kN in the cross-section and greater than 6.0 kN in the tangential and radial sections. Further, it should have a high modulus of rupture (> 149 MPa) and elasticity (> 14.08 GPa), good impact bending strength, and good wear resistance (80–150 mg/100 r). To satisfy the traditional aesthetics, the wood surface color should be black, dark brown, or dark purple-brown, with colorimet- ric parameters in the range of 0.0 < L* < 30.0, 0.0 < b* < 6.0, and an a* value as small as possible. The evaluation indica- tors used for searching potential high-quality alternative tree species are not the same as those for replacing tradi- tional fretboard materials using modifed fast-growing plantation wood. The physical and mechanical properties and the surface color of traditional precious fretboard wood are important evaluation indicators for whether the modifed fast-growing plantation wood can replace the traditional fretboard wood. Keywords: Tropical hardwoods, Fretboard, String instruments, Wood anatomy, Mechanical properties, Color parameters Introduction have a long growth cycle, the yield is low, and the wood Te fretboard of traditional string instruments is mainly is easily damaged during production. In addition, Indian made of tropical hardwoods, such as ebony, Indian rose- rosewood and African blackwood used for fretboard wood, African blackwood, and other precious woods are included (in Appendix ΙΙ of the CITES) [3], so these that are characterized by a high density, high strength, woods and their products are restricted in world trade [4, and hardness, good wear resistance and dimensional sta- 5]. Ebony’s long growth cycle, low yield, and vulnerability bility, and a beautiful color [1, 2]. However, these trees also limit its applications in musical instruments. Tere- fore, it is urgent to seek sustainable alternative woods with similar properties. Another option is improving the *Correspondence: [email protected] Research Institute of Wood Industry of Chinese Academy of Forestry, function of fast-growing plantation wood using physical, Hunan Collaborative Innovation Center for Efective Utilizing of Wood & chemical, and biological treatments [6–9] to match the Bamboo Resources, Beijing 100091, China performance of precious wood. © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. Liu et al. J Wood Sci (2020) 66:14 Page 2 of 11 When musicians play string instruments, they adjust similar in color and texture that it is difcult to distin- the pitch by pressing the strings to the fretboard. Te guish them. Tis is an important reason why African fretboard is often marked by frets to determine the pitch blackwood has been widely used in the market [1]. While positions [10]. Te poor contact between the strings Indian rosewood is also a popular choice for fretboards, and the fretboard can afect the tone and pitch, such as Sproßmann et al. considered its performance only in the noise and wolf sounds [11]. Te rigid fretboard has bet- soundboard. In the previous experiments, it was found ter resistance to bending and torsional deformation that the acoustic vibration performance of the fretboard caused by the tension of diferent strings and the com- wood is not an important factor, and the corresponding pressive force of the strings and fngers [12]. Over time, requirements are much lower than those for the sound- the fretboard’s surface can become abraded, leaving pits board wood. or grooves especially in the high-pitched parts. Moreo- Tis paper systematically analyzes the anatomical ver, the fretboard is glued to the neck and increases the characteristics, physical and mechanical properties, and overall stifness of neck. Te stif fretboard can inhibit surface color of three commonly used fretboard woods the reduction in the resonance frequency of the sound- [21], and summarizes the important criteria for fretboard board, making the instrument sound stronger and more selection. Tese results will not only help identify alter- uniform [13]. Tus, the wood used for fretboards should native timbers for fretboards in the future, but also pro- have good wear resistance, high hardness, and high stif- vide the guidelines for improving the surface color and ness while maintaining excellent fexibility. physical and mechanical properties of fast-growing plan- Te researchers have comprehensively examined the tation wood to match the precious woods. performance requirements and evaluation indices of soundboard wood. However, corresponding studies on Materials and methods the fretboard are still insufcient and unsystematic [14]. Materials Paté et al. [15, 16] studied two guitars that only difer in Te heartwood of three tropical hardwood species and the fretboard material (ebony or Indian rosewood). Te one control was analyzed: ebony (Diospyros crassifora well-known “dead spot” phenomenon was observed, J. Kenig ex Retz., from Indonesia), Indian rosewood where a frequency resonance of the string and the (Dalbergia latifolia Roxb., from Indonesia), African main structure at the coupling point lead to an abnor- blackwood (Dalbergia melanoxylon Guill. & Perr., from mal damping of the note. Te two guitars showed strik- Tanzania), and hard maple (Acer saccharum, from the ingly diferent behavior in the dead spots in terms of the United States). Te wood (1.5 m in length) was taken afected notes as well as how much they were afected from the main trunks of three trees approximately 50 cm [17]. Trough a comparative study of sound and driving- in diameter at breast height. Te wood was provided by point conductance of these two guitars, it was observed a string instruments’ manufacturer. Te sizes of samples that the fretboard wood had an infuence on the location were 5 × 5 × 5 (all dimensions are presented as the radial of and the susceptibility to dead spots. When a frequency (R) × tangential (T) × longitudinal (L) in units of mm) resonance of the string and the main structure at the cou- for measuring anatomical features; 20 × 20 × 20 for den- pling point occurs, the string damping may be bigger for sity; 20 × 20 × 300 for the modulus of elasticity (MOE), guitars with rosewood fretboards [18]. Tis indirectly modulus of rupture (MOR), and impact toughness; proves that fretboard wood will afect the timbre of the 50 × 50 × 70 for hardness tests; 7 × 7 × 15 for the hard- string instrument. Hiziroglu et al. used ebony, Brazilian ness test of wood cell walls, 20 × 100 × 100 for tribologi- rosewood, and Indian rosewood in violin fretboards and cal properties; and 20 × 50 × 50 for wood color analysis. compared the resultant tonal qualities. Tey found that the diferent woods led to diferent sound characteris- Measurement of anatomical features tics: the timbre was clear and harmonious for the ebony Before microstructure observation, three samples of each fretboard, while it was delicate and soft for the rosewood species were softened in a boiling mixture of water, glyc- fretboard [19]. Sproßmann et al. studied the use of Afri- erin, and ethanol (1:1:1) for 30 days. Ten, the samples can blackwood (Dalbergia melanoxylon Roxb.) instead were cut into 20-μm-thick slices along the tangential and of ebony (Diospyros crassifora Hiern.) in the fretboards cross-directions using a sliding microtome. Tese slices of classical guitars, and that of ziricote to replace Indian were stained using safranin to make permanent sec- rosewood in the soundboard. African blackwood was tions for the measurement of anatomical characters. We found to be similar or superior to ebony in terms of the used a Leica DM 2000 microscope connected to a digital physical, mechanical, and acoustic properties; and there- camera (Leica DFC 295) and attached to a personal com- fore, the former is more suitable for use in the fretboard puter to acquire the microscopic images.