15 ADVANCED MATERIALS & PROCESSES | JULY/AUGUST 2023 The next stage selected for species with a mature tree form amenable to economical extraction of quartersawn panels for modern soundboard production, typically, 21.6 x 53.3 cm, with long dimension parallel to the grain. This is supported by a straight growth habit and minimum mature bole diaThe present exercise identifies potential tonewood substitutes among the 214 tree species contained in the ANSYS Granta Selector database by characterizing acoustic performance of benchmark species commonly used in acoustic guitar soundboard and body components. Though a small fraction of over 73,000 estimated tree species[24], it is about one seventh of the 1575 commercially significant timber species[25]. After identifying species with similar acoustic performance to benchmark species, both soundboard and body subsets were filtered separately through three additional stages, tree form, sustainability, and producibility, using commercial spreadsheet functions. Tree form selected only for species with a mature size and form amenable to economical acoustic guitar manufacture. Sustainability was treated by eliminating any species with a documented extinction risk in the CITES Checklist[8] or the International Union for Conservation of Nature and Natural Resources (IUCN) Red List[26]. Finally, the remaining species were scored for producibility via two metrics, workability (ease of cutting) and finishability (ease of bonding, staining, etc.), converted to “good, average, or poor” ratings based on data from The Wood Database[27]. Species with a poor rating for either resulted in rejection. The spreadsheet output provided separate lists of down-selection tonewood species for both soundboard and body applications. A flow diagram for the selection process is shown in Fig. 2. Soundboard (top). In the case of soundboards, Bremáud[28]—based on the work of Barlow[17]—proposed a metric termed the Acoustic Conversion Efficiency (ACE), defined as: where E is Young’s Modulus, ρ is density, and tan δ is the damping coefficient (all properties were assessed as averages in the longitudinal tree growth direction). Figure 3 plots ACE ranges calculated for all 214 species, ordered from high to low, with higher values deemed to be better. The box selects the range bounding Sitka Spruce (Picea sitchensis), a benchmark acoustic soundboard tonewood, and contains 47 species. A second sub-selection (not shown) from this subset confirmed the compressive strength of all 47 would meet or exceed that of Sitka Spruce and thereby support full string tension. TABLE 1 — DOWNSELECTED SOUNDBOARD AND BODY TONEWOOD TREE SPECIES Soundboard (top) species Body (back & sides) species Noble Fir (Abies procera) Alaskan Yellow Cedar (Chamaecyparis nootkatensis) European Spruce (Picea abies) Douglas Fir (Pseudotsuga menziesii) White Spruce (Picea glauca) Black Cherry (Prunus serotina) Red Fir (Abies magnifica) Western Hemlock (Tsuga heterophylla) Silver Fir (Abies alba) Ekop (Didelotia brevipaniculata) Western White Pine (Pinus monticola) Pilon (Hyeronima alchorneoides) Sitka Spruce (Picea sitchensis) Bigleaf Maple (Acer macrophyllum) Western Red Cedar (Thuja plicata) Determa (Sextonia rubra; Ocotea rubra) White Fir (Abies concolor) Slash Pine (Pinus elloittii) Eastern White Pine (Pinus strobus) Radiata Pine (Pinus radiata) Douglas Fir (Pseudotsuga menziesii) Norway Pine (Pinus resinosa) Yellow Poplar (Liriodendron tulipifera) Scotch Pine (Pinus sylvestris) Sugar Pine (Pinus lambertiana) Swamp Tupelo (Nyssa aquatica) Norway Pine (Pinus resinosa) Angelique (Dicorynia guianensis) Ponderosa Pine (Pinus ponderosa) Fig. 3 — Acoustic conversion e iciency (ACE) box selection plot for soundboards. The box represents the selection of 47 species close to the benchmark species, Sitka Spruce.
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