Pacific Rim Tonewoods has partnered with Taylor Guitars to produce a unique research study on the grading of spruce tonewood by acoustic criteria.
The study, linked here (PDF 568 KB), was published on October 23, 2019 in the prestigious Journal of the Acoustical Society of America.
This article may be downloaded for personal use only. Any other use requires prior permission of the author and the Acoustical Society of America.
For some time, it has been recognized that there is little connection between the aesthetic criteria used to grade tonewood, and the physical characteristics of the wood that give rise to tonal quality of the guitar. PRT has adopted methods developed by the University of Montpelier, France, to measure acoustic properties of tonewood, such as density, Young’s modulus ( a measure of “intrinsic stiffness”), and damping. We have measured several thousand soundboard and bracing blanks of both Sitka and Lutz spruce, to fully characterize the natural range of the tonewood that flows through our mill, and is delivered to the acoustic guitar industry. The study is the latest of several research projects currently underway to better understand how the measurable properties of spruce tonewood impact the tonal quality of the acoustic guitar.
The overall goal of such studies is to determine the optimal suite of acoustic properties of the soundboard and bracing for a given guitar design (in this case, the Taylor Grand Auditorium 814ce), as determined by trained listeners under “blinded” conditions. Determining an optimum is the first step toward the ultimate goal of selecting and delivering a consistently optimized product from material that inherently possesses all of the variety of nature. Even after careful selection and milling, the spruce tonewood we presently deliver to the acoustic guitar industry demonstrates a wide variability in both density and stiffness (see Figures 1 and 2 ).
This first study succeeded beyond expectation, if success can be defined as a clearly demonstrable and useful result. Partnering with Taylor Guitars, we built two groups of 5 identical instruments, all on the same day, by the same team of builders, with component materials (backs/sides, bridges, etc.) of identical density. The instruments differed only in the density and stiffness of the spruce soundboard in one group, and the soundboard and bracing in the second group.
The instruments were then subject to careful recordings, which were presented to listeners under blinded conditions, and ranked by the method of systematic pairwise comparisons, a statistical method that allows ranking and an estimate of the degree of difference. The study was performed by Dr. Sebastian Merchel of the Technical University of Dresden , Germany.
We found that, for this particular design, the sound quality produced by guitars made with tops of low density and low stiffness Sitka spruce were consistently and significantly preferred over spruce of higher density and higher stiffness.
Such a result is true for this particular design, but should not be expected to consistently apply to all guitars. Just as nature is diverse, so too is the acoustic guitar, and so too is human preference. What this study does demonstrate, however, is that acoustic properties of spruce topwood make a consistent, important, and (we dare say) predictable difference in sound quality of the finished guitar. This is a much discussed topic among luthiers and guitar aficionados. This is the first study, to our knowledge, that demonstrates this in a scientifically robust fashion for the steel stringed guitar. More generally, this study demonstrates the importance of integrating guitar design with the acoustic properties of the soundboard and bracing. Stay tuned for more work, specifically on the impact of damping, which was kept constant in this study.
Please do peruse this study, and please do direct any questions to Dr. David Olson, Research Director of PRT, email@example.com. Comments are more than welcome, and feedback does improve the usefulness of future study. PRT is currently working with our local community of luthiers, providing acoustically graded tonewood on spec. We plan to do so formally for the general public in the near future, thru this website.
The official online abstract can be found here, and official reprints of the article are available for download through the Journal of the Acoustical Society of America. Copyright 2020 Acoustical Society of America. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the Acoustical Society of America.