The effect of traditional hygro-thermal pretreatments on the acoustical characteristics of white mulberry wood (Morus alba)

Abstract

The wood used in making musical instruments usually undergoes pre-treatments or conditioning. Some processes have resulted from laboratory research, while many treatments are applied by craftsmen using traditional methods that have not yet been scientifically assessed. This paper is based on laboratory methods which simulate hydrothermal pre-treatments traditionally applied by Iranian lute makers to mulberry (Morus alba), an important wood for lutes from the Middle East to Japan. By applying a cyclic process, drying (60°C) and ambient re-conditioning, which mimics wood seasoning and short-term aging, the damping coefficient (tanδ) was decreased down to 10% without negatively affecting specific modulus of elasticity (E'/ρ). Long-term (up to 4 months) soaking in cold water removed extractives, and decreased density as well as E'/ρ, but did not affect tanδ. Short-term hot water treatment removed as many extractives, but caused a smaller decrease in E'/ρ than cold water, and significantly increased tanδ. This paper helps to clarify some traditional processes used by instrument makers, and can provide a basis for necessary cross-cultural scientific studies in the future.

Reversible and irreversible effects of mild thermal treatment on the properties of wood used for making musical instruments: comparing mulberry to spruce

“…Another hypothesis would be that, as Morus is a ring porous species (Karami et al 2010), a very light wood as the one studied here would include a high proportion of vessels and parenchyma compared to that of fibres. This is consistent with previous observations that mulberry specimens with narrower rings (i.e., for this ring-porous wood, with wider proportion of vessel-rich earlywood and small proportion of fiber-rich latewood) tend to have lower values of both γ and E' L /γ (Se Golpayegani et al 2015). Across several studies (present results, Roohnia 2005, Se Golpayegani et al 2012, 2015 there is a positive correlation between γ and E' L /γ, although E' L /γ is theoretically determined at cell-wall scale and thus is generally little dependent of specific gravity (Norimoto et al 1986).…”

“…The studied material of white mulberry wood had a specific gravity (γ) of 0.49 on average (from 0.43 to 0.53) and an equilibrium moisture content at 20 °C and 65%RH (EMC) of 9.3 ± 0.4%. These values can be compared to previously published studies by Se Golpayegani et al (2012Golpayegani et al ( , 2015, who reported a significantly higher γ of 0.54 on average (from 0.45 to 0.60) and a slightly lower EMC of 8.0%. The wood used in the present study had narrower growth ring and consequently lower late wood percentage, which can explain the difference considering the ring-porous nature of this species.…”

“…Reduced anisotropy for white mulberry (L/R ratio of moduli of 4) is already reported (Se Golpayegani et al 2012), but the anisotropy for the specimens in the current study was, interestingly, even lower. In a comparison of several studies on vibrational properties of mulberry (Se Golpayegani et al 2015), the lowest values of E' L /γ were found for a wood considered as of "superior quality" for instruments (Yoshikawa 2007). It can be suggested that the extremely low value of E'/γ in L direction, accompanied by rather high values in R direction, may reflect an exceedingly high microfibril angle.…”

“…Accordingly, the interaction between the different wood species and acoustical properties, variations of musical instrument structures, and differences in musical tastes across different cultures lead to wood selection. Some instrument makers sometimes use traditional (hygro-) thermal pre-treatments (Se Golpayegani et al 2015). Currently, there is a growing interest on research and development studies in a more industrial scope, for general uses of wood of course, but also specifically for instrument making (Wagenführ et al 2006, Pfriem 2015, Zauer et al 2015, Krüger et al 2018.…”

“…A majority of studies on thermal treatments concern softwoods, whereas their effects could be significantly different when applied on various hardwoods (Matsuo et al 2013, Krüger et al 2018, Brémaud et al 2021. The acoustical consequences of low-temperature (<100 °C) hygro-thermal pre-treatments traditionally applied by instrument makers have been studied on mulberry (Morus alba L.) wood by Se Golpayegani et al (2015), with some interactions between treatments and extractives.…”