Abstract
Some of the most prized woods used for the backs and sides of acoustic guitars are expensive, rare, and from unsustainable sources. It is unclear to what extent back woods contribute to the sound and playability qualities of acoustic guitars. Six steel-string acoustic guitars were built for this study to the same design and material specifications except for the back/side plates which were made of woods varying widely in availability and price (Brazilian rosewood, Indian rosewood, mahogany, maple, sapele, and walnut). Bridge-admittance measurements revealed small differences between the modal properties of the guitars which could be largely attributed to residual manufacturing variability rather than to the back/side plates. Overall sound quality ratings, given by 52 guitarists in a dimly lit room while wearing welder's goggles to prevent visual identification, were very similar between the six guitars. The results of a blinded ABX discrimination test, performed by another subset of 31 guitarists, indicate that guitarists could not easily distinguish the guitars by their sound or feel. Overall, the results suggest that the species of wood used for the back and sides of a steel-string acoustic guitar has only a marginal impact on its body mode properties and perceived sound.
Highlights
There is some scientific literature relating to all these stages, reviewed briefly below, but to date there has been no clear synoptic effort to apply rigorous scientific methods to the overall question: “does the back wood choice of an acoustic guitar, in itself, produce repeatable and recognisable differences of sound quality?” That is the main task of this study, to be tackled by a combination of physical measurements and blinded perceptual experiments
A very experienced luthier built six steel-string acoustic guitars to the same dimensional and material specifications of all their parts, except for the back and side plates, which used different woods ranging widely in mechanical properties, popularity among players, and price
To further limit the possibility that participants could recognize the guitars by non-acoustic cues, they were asked to close their eyes both while being handed each guitar by the experimenter, and when the experimenter picked up each guitar from them; they were asked not to look at the body of the guitars during the test, and not to tap on the guitar body or inspect it in any other way
Summary
The acoustic guitar is one of the most popular musical instruments across the world. In the U.S alone, more than one million acoustic guitars are sold each year (Music Trades, 2016a). The attention of guitar players is often focused on the choice of wood used for the back and sides of the soundbox (Johnston, 2006, 2013). The scarcity and increasing cost of many traditional back woods has pushed guitar makers to explore alternative solutions. The maker makes some choices of detailed dimensions (shape, thickness, bracing pattern, etc.), and the result is a soundbox with certain vibration modes and associated resonance frequencies and damping factors. These modal parameters give the acoustical ‘fingerprint’ of the instrument, and determine many aspects of the sound. There is some scientific literature relating to all these stages, reviewed briefly below, but to date there has been no clear synoptic effort to apply rigorous scientific methods to the overall question: “does the back wood choice of an acoustic guitar, in itself, produce repeatable and recognisable differences of sound quality?” That is the main task of this study, to be tackled by a combination of physical measurements and blinded perceptual experiments
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