14 ADVANCED MATERIALS & PROCESSES | JULY/AUGUST 2023 Over a century ago, the Martin Guitar Company (Nazareth, Pa.) established four benchmark species as key ingredients of a high-quality instrument: old growth Adirondack Spruce (Picea rubens), Brazilian Rosewood (Dalbergia nigra), Honduran Mahogany (Swietenia macrophylla), and African Ebony (Diospyros crassiflora)[1-3]. Though derived from Old World luthier principles, they came to represent the ideal combination of raw materials that allowed a balanced tone to project from an attractive, portable, and playable instrument. It was so good in fact, that this quadrumvirate became the benchmark of what a great acoustic guitar should be. Other manufacturers, like Gibson, Fender, Yamaha, and Washburn also adopted this combination and the resulting scooped midrange frequency curve became the sound that allowed human voices to soar above hundreds of projected harmonics, yet at a volume that could compete with the popular mandolins and banjos of the day. The Martin D-28 exemplified this design and became the standard following its introduction in 1931[4] (Fig. 1). At present, nearly 2,000,000 acoustic guitars are sold annually in the U.S., a mere quarter of global sales[5]. However, this growth has constraints. Old growth spruce forests, the main source of coveted straight-grained soundboards, are being increasingly protected for their ecological, bio- diversity, and carbon sequestration benefits[6,7]. And other tonewoods have greater sourcing challenges. Brazilian Rosewood and Honduran Mahogany are currently listed as endangered by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES)[8]. In 2017, all rosewood species were likewise protected and the only instrument-grade ebony available is from Cameroon. Though these species continue to be offered in new instruments, unreliable availability has led to an ongoing search for substitutes[2,9-13]. Substitutes are often sought by trial-and-error among similar species in an effort to replicate the performance of the benchmarks. However, similar market forces can create similar problems, e.g., a Rosewood substitute species, Sapele (Entandrophragma cylindricum), has reportedly suffered from overharvesting[8]. This article describes an effort to identify tonewood substitutes using the ANSYS Granta Selector[14] based on derived performance objectives for physical and mechanical properties. These objectives are then combined with those for species morphology, producibility, and sustainability to arrive at sustainable and viable tonewood substitution options that meet manufacturer and customer expectations. Because of required larger starting stock dimensions, only the acoustic guitar top (soundboard) and body (sides and back) are discussed here. METHOD AND RESULTS The principles of multiple objective materials selection are well established and described elsewhere[15,16]. Prior work[2,17-20] has described formulae correlating with acoustic performance, typically incorporating density, modulus, and damping coefficient as key properties, but generally did not include process metrics such as ease of cutting, formability, bondability and finishability, mature tree form and size, and environmental sustainability. These factors are experientially considered by instrument manufacturers and there have been dozens of alternative tonewood species explored, many commercialized, based on similarity to benchmarks[2,13]. For example, Godin Guitars (Canada) transitioned from neotropical mahogany to local Black Cherry (Prunus serotina) in 2008 for the body of a classical model[21], and Breedlove Guitars builds with local Oregon Myrtlewood (Umbellularia californica) collected from downed trees[22]. Taylor Guitars cleverly upcycles local end-of-life trees removed from urban landscapes, such as Shamel Ash (Fraxinus uhdei) and Red Ironbark (Eucalyptus sideroxylon)[23]. Such efforts to source locally have had success and are consistent with sustainable practices. Fig. 1 — Traditional Martin D-28 acoustic guitar with benchmark tonewoods indicated. Fig. 2 — Flow diagram for tonewood selection.
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