Experimental assessment of the effect of an eventual non-invasive intervention on a Torres guitar through vibration testing

Guitar FE09 – MDMB 626 is one of the best-known Antonio de Torres instruments and is an excellent sounding example of a guitar with tornavoz. Although the instrument is in playable conditions, the back plate has a deformation and cracks which are undoubtedly the result of the pressure exerted by the tornavoz supports. Over the last hundred years experts have chosen not to have the cracks repaired as it might result in a change in the sound. Recently, professionals stated that the guitar sounded different with strips of masking tape covering the cracks. Although subjective evaluations and claims abound, no quantitative data is available to determine the effect of this modification. This paper provides the results of an experimental campaign aimed at assessing the effect upon the vibration response of this eventual non-invasive intervention. Vibration testing was performed on the top and back plates before and after adhering strips of masking tape along the cracks. The influence of tensioning the strings is also examined. Correlations were done in both modal and frequency domains. The results allow conclusions to be drawn regarding the influence of this simple non-invasive intervention that can lead to audible changes, proving the feasibility of using vibration-based NDT methods for damage or structural modification assessment of musical instruments.