By Brian Applegate
The woods used to make guitars may soon be very different from what have been traditionally used. With the CITES (Convention on International Trade of Exotic Species) listing of all rosewoods, Honduran and Cuban mahogany, and Madagascar ebony, there will likely be a need to find suitable substitutions to replace some of the most popular woods used in guitar making. Unfortunately, the guitar community will likely be reluctant to readily accept substitutions for fear of using inferior quality woods compared to the tried and true materials of the past.
Since 2016 I have been engaged in a formalized study at the University of Edinburgh to try and quantify the vibrational properties of the guitar woods that are most threatened, rosewood in particular. The aim of this research is to prove that the vibrational properties of a guitar component is more important than the species to which it belongs. After testing hundreds of wood samples I’ve found two interesting things to be quite clear: 1. There is a vast difference of vibrational quality within a species, 2. There are unconventional woods that should perform as well as the traditional species that have become endangered. The hard part, of course, is convincing those reluctant to accept change.
Nearing completion in my shop are four guitars of identical design. The Sitka spruce tops were cut consecutively from a single billet. The Honduran mahogany for the necks came from the same tree. The ebony bridge and fretboard blanks were matched as to their physical and acoustical properties (mass, stiffness, damping coefficient, and fundamental tap tone). Two of the guitars have back and sides of East Indian Rosewood, and two were made with African Padauk.
The individual back and side sets were selected to match as close as possible their mass, stiffness, damping coefficient, and resonant peaks. Throughout the construction process, the top and back plates were tuned to similar resonances by shaving small amounts from the major braces.
The sound that we perceive when we appreciate the tone of a guitar can be measured in its acoustic resonance. The guitar body is tapped at the bridge location with an impulse hammer and a condenser microphone records the resulting sound vibrations that the guitar body produces. Each of the four guitars were measured in this way and the results are contained in the graph above. The Y axis measures the sound pressure level in decibels (dB) and the X axis indicates the frequency/pitch. In practice, when a guitar string is plucked, these resonances are also excited and will add overtones at these frequencies regardless of the note’s frequency. These overtones will influence the sound characteristics of an individual instrument. So far the success of this project has been to produce such acoustically similar instruments and matching these resonant frequencies is crucial for making the guitars sound the same. Upon completion there will be four guitars with physical and vibrational characteristics as similar as can be practical. The only variable is the species of the back and sides.
Forthcoming will be a blind test to determine if guitar players are able to discern whether the instrument is Rosewood or Padauk. If the results show the instruments are indiscernible, it would provide evidence that it is not the species of the wood but the characteristics of the wood that will determine its contribution to the instruments’ sound. If so proven, I would expect there to be a greater acceptance of alternative tonewoods – the woods used for sound producing components - without the concern of compromising quality. In addition to being included in my thesis, I expect to publish the results of this study once completed.
The art of guitar making is steeped in lore and tradition. Mere testimony is unlikely to change the minds of a community that has a high regard for the tonewoods built into these traditions. But as dwindling resources may necessitate, sustainable alternative tonewoods must be vetted in a way to ensure the guitar built tomorrow is as good as the guitar built yesterday. With the addition of science to the art of guitar making, I believe and intend to prove this is possible.
Brian Applegate has been a professional luthier since 2002 focusing primarily in making steel-string guitars at his shop in Chanhassen, Minnesota. Additionally, Brian is pursuing a Phd. at the University of Edinburgh, Scotland with a focus on wood acoustics.