Analysis of Classical Guitars' Vibrational Behavior based on Scanning Laser Vibrometer Measurements

One of the main goals in musical acoustics research is to link measurable, physical properties of a musical instrument with subjective assessments of its tone quality. The aim of the research discussed in this paper was to observe the structural vibrations of different class classical guitars in relation to their quality. This work focuses on mid-low-and low-class classical (nylon-stringed) guitars. The main source of guitar body vibrations come from top and back plate vibrations therefore these were the objects of structural mode measurements and analysis. Sixteen classical guitars have been investigated, nine with cedar and seven with spruce top plate. Structural modes of top and back plates have been measured with the aid of a scanning laser vibrometer and the instruments were excited with a chirp signal transferred by bone vibrator. The issues related to excitor selection have been discussed. Correlation and descriptive statistics of top and back plates measurement results have been investigated in relation to guitar quality. The frequency range of 300 Hz to 5 kHz as well as selected narrowed frequency bands have been analyzed for cedar and spruce guitars. Furthermore, the influence of top plate wood type on vibration characteristics have been observed on three pairs of guitars. The instruments were of the same model but different top plate material. Determination and visualization of both guitar plates' modal patterns in relation to frequency are a significant attainment of the research. Scanning laser vibrometer measurements allow particular mode observation and therefore mode identification, as opposed to sound pressure response measurements. When correlating vibration characteristics of top and back plates it appears that Pearson product-moment correlation coefficient is not a parameter that associates with guitar quality. However, for best instruments with cedar top, top-back correlation coefficient has relatively greater value in 1-2 kHz band and lower in the range of 2,5-5 kHz in comparison with low-class instruments. The study showed that variance, which is a measure of statistical dispersion, is a relevant parameter. The better the quality of the guitar the greater the variance value in 1-2 kHz band. It was observed that higher-quality instruments are characterized by stronger structural resonances of top plate in the range of 4-5 kHz, which means that luthiers should pay special attention to top plate vibrational behaviour in this particular frequency band. Additionally, the result analysis show that best spruce guitars are distinguished by greater top plate vibrations in the range of 2-3 kHz in opposite to best cedar instruments. It can be assumed that top plate vibrations in this particular frequency band may be associated with subjective impression of tone brightness, as commonly known opinion indicates that guitars with spruce tops sound relatively "brighter" in comparison to cedar-top instruments.