Response Variation in a Group of Acoustic Guitars
Mark N. French · Sound&Vibration · 2008
Dynamic testing was performed on a pool of guitars in the production facility of a major guitar manufacturer. The production process is highly automated, with the goal of improving quality and reducing build variation. However, no objective metric has yet been used to quantify the differences in the dynamic response of the instruments. We performed dynamic testing on the assembly line to measure the lower resonant frequencies of a pool of instruments. The standard deviations of the first two modes were a few percent of the mean, and the data suggest that a significant percentage of the variation is due to intentional differences in the species used for backs and sides. It is commonly accepted in manufacturing operations that build variation is inversely related to build quality. Efforts to improve the product and the manufacturing process are hampered if the effect of changes cannot be distinguished from build variation. So it is critical that good metrics for build variation are established. In guitar manufacturing, dimensions of components and completed instruments are routinely measured as part of the production process. Dimensions are certainly useful descriptors but are not directly related to the sound produced by the instruments. A better metric would be more directly related to the physical mechanisms that produce sound. The structural-acoustic interaction through which an instrument makes sound is strongly conditioned by the coupled resonant frequencies of the instrument. Thus, controlling variation in those frequencies is key to controlling variation in the tonal quality. Ideally, a measurement that relates dynamic response of the instrument to radiated sound could be used; an intriguing possibility is radiation efficiency, but current methods take far too much time and equipment to be practical in a production environment. Near-field acoustic holography is faster but is too expensive to be practical. A practical first step toward a more global metric is to use the lower resonant frequencies of the instruments. It is generally accepted that the lower frequencies strongly condition the sound and exhibit structural-acoustic coupling in a way that can be described by simple math models. Furthermore, these frequencies can be measured quickly using inexpensive equipment. A well-controlled build process should manifest itself in low variation in the lower coupled resonant frequencies. Resonant frequencies were measured on a pool of instruments during the assembly process at Taylor Guitars in El Cajon, CA (www.taylorguitars.com). Most of the instruments had completed and finished bodies but no necks. It was desirable for body structure to be complete, including the finish. However, since the neck installation involves hand fitting, it was important to be ‘upstream’ of that process. If build variation were large for the bodies, it would be necessary to move to earlier stages of the build process to identify the source. If the variation were small, testing could be conducted after final assembly. Taylor produces a range of different instruments in the same facility (Figure 1), and all instruments available at that stage of assembly were tested, regardless of the model. The data presented here are from a single design, called a Grand Auditorium Cutaway. There are various build levels for this design, but the differences are limited to the side and back materials and complexity of the trim. The basic design of the instrument is shown in Figure 2. All instruments had the same dimensions, the same bracing, the same bridge and the same soundboard material.