For centuries luthiers strived to enhance the sound quality of their musical instruments. Some copied the methods and techniques of their predecessors, while others tried new techniques, but the results were not always successful. This endeavour continues today as luthiers are trying to understand why some of the older instruments have a better sound quality. Most of those attempts were directed towards the violin.
Wood used to manufacture instruments is referred to as tonewood, or resonance wood. The quality of the produced sound depends on the elastic properties of the wood and how well it can oscillate. The sound quality of tonewoods can be best described through the radiation ratio (R) which is defined as R = √ MOE/ρ3. A significant difference can be observed between high- (15) - and low- (9) quality soundboards. Some of the famous Stradivari violins have R-values reaching 16 and above, whereas even the best quality modern violins tend to reach a maximum R of 12–15. The reason that old instruments have a much higher R value can only be explained by partial wood degradation, which reduced the density, without affecting the MOE, or speed of sound, too much. Various attempts were made to improve the sound quality of wood through artificial ageing. The ones showing the most promise is heat treatment and fungal degradation (Danihelová et al. 2022; Rang et al. 2016; Zhu et al. 2016). Previous studies have shown that the most important wood characteristics for good sound quality are MOE – which directly affects the speed of sound - and ρ (Zerbst et al. 2018). An increase in the MOE and lowering of ρ results in a higher R value. Soundboards easily transmit vibrations to the edges and are good sound radiators and typically have a low ρ and high MOE with a high sound radiation ratio and tend to resonate lower frequencies much clearer and louder than other wood species (Yoshikawa, 2007 and 2014).
Various attempts were made to improve the sound quality of wood through artificial ageing i.e., heat treatment and fungal degradation (Danihelová et al., 2022; Rang et al., 2016; Zhu et al., 2017). Previous studies have shown that the most important wood characteristics for good sound quality are MOE – which directly affects the speed of sound - and ρ (Zerbst et al., 2018). An increase in the MOE and lowering of ρ results in a higher R value.
In previous studies it was observed that low and moderate thermal exposures (110°C– 175°C) do not change acoustic properties significantly (Gadd and D’Arcy, 1986; Zerbst et al., 2018). Thermal treatment between 160°C and 220°C was found most suitable to degrade hemicelluloses and lignin, as desired for acoustic changes (Esteves and Pereira, 2009; Výbohová et al., 2018; Zerbst et al., 2018), because at these temperatures the desired density decrease is obtained. At temperatures above 250°C cellulose chains start to degrade, which then also affects the MOE. However, exposure to high temperatures for a short time may prove to be beneficial in improving the acoustic properties and was included in this experiment.
Obataya (2017) studied the effect of artificial hydrothermal ageing on the acoustic properties of wood. Treatment between 1 and 7 days at a RH of 57–64% was found to increase the specific dynamic Young modulus (E’/ρ) and decrease tan (δ), which indicates that heating at intermediate RH can improve the acoustical properties.
Schwarze et al. (2008) discovered the presence of fungi, such as Physisporinus vitreus and Xylaria longpipes in the soundboard of old violins, which led to a decrease in hemicellulose content that might be responsible for the higher radiation ratio. Fungal degradation is a possible approach for the improvement of acoustical properties in wood, as the degradation of polysaccharides, such as hemicelluloses and lignin can have a positive effect on the acoustic properties. Fungi are known to deposit large amounts of organic acids as part of their natural metabolism that accumulate as oxalate salts along their hyphae. These salts form crystalline structures along the hyphae (Cromack et al., 1977), which leads to an increase in c and MOEL. These findings suggest that extended incubation periods with fungi can improve c up to a point where degradation of the cell wall occurs. It can therefore be concluded that the appropriate fungus capable of secreting a large amount of oxalate salts will improve acoustic properties of wood.
Wood degrading fungi are typically grouped into white rot and brown rot, depending on the wood components they attack. White rot fungi degrade cellulose, hemicellulose and lignin, while brown rot degrades only hemicellulose and cellulose. The main objective of this study was to decrease the wood density by removing hemicelluloses and part of the lignin, without compromising the strength provided by the cellulose. Therefore, the effect of degradation caused by both brown and white rot fungi was analysed.
Not much research exists dealing with chemical treatment of sodium hydroxide (NaOH) and sodium sulfite (Na2SO3) for improvement of acoustical wood properties. The selection of these two compounds for this study is based the fact that recent studies found high levels (1000 ppm) of Na and other elements (e.g., K, Cl, Ca, Cu and Zn etc.) residing in the cell wall structure of violins made from Stradivari and Guarneri (Taia et al., 2017; Su et al., 2021). A further assumption is that timber companies made use of salt as preservative in the waterways used to move logs, which could have, at some point, had a positive implication on the chemical structure of musical instruments (Gug, 1988). The above findings suggest that some minerals added via chemical or preservative treatment may have been used that improved the acoustical properties of historical musical instruments over time. Studies have found that alkaline treatment increases the crystallinity index of cellulose (Xu et al., 2020), which might result in improved acoustical properties.
Both sodium compounds were found to reduce the density of wood by degrading hemicellulose and lignin, while leaving the cellulose intact (Wachter et al., 2019; Li et al., 2021). Compared to acids (i.e., H2SO4), alkaline solutions do not damage the S2-layer, but loosen the middle lamella, where the largest proportion of hemicellulose resides especially, if combined with thermal treatments of 100°C and 120°C. Moreover, the combination of the two compounds was found to degrade hemicellulose more effectively (Maturana et al., 2022; Kim et al., 2020).
The benefits of an increased crystallinity index of the cellulose were shown to improve the acoustical properties of wood (Miao et al., 2021; Wang et al., 2019). This implies that any substance capable of increasing the crystallinity in wood may have a beneficial impact on the vibro-acoustical properties of wood, as soundwaves can move more easily through a crystalline structure than amorphous materials. NaOH was found to improve the crystallinity index of cellulose by selectively removing amorphous hemicellulose (Qi et al., 2023; Barman et al., 2020). However, alkaline treatments may decrease the crystallinity in wood at higher concentrations and longer exposure times (Xu et al., 2020; Nakano, 2010).
The aim of this study is to modify and improve the acoustic properties of wood through various treatments: heat treatment, fungal exposure, UV exposure, sodium hydroxide (NaOH) and sodium sulphate (Na2SO3) treatment. These treatments are expected to decrease the hemicelluloses, lignin and extractive content and therefore the density of wood, without significantly altering the MOE and therefore improve R.