Vibration measurement of musical instruments using digital speckle pattern shearing interferometry

The structure and material of a musical instrument decisively influence its vibration modes. In turn, the precise measurement of vibration aiding in analyzing the structural and material properties of the musical instrument, and ultimately provides a basis for evaluating the performance of the musical instrument. This paper leverages the superiority of the digital speckle pattern shearing interferometry which also named digital shearography in optical dynamic measurements to determine the vibration distribution of musical instruments. The combination of digital shearography and time-average method is used to qualitatively determine the surface vibration modals of a musical instrument under different orders of intrinsic frequency excitation. The phase shift technique introduced by the time-average method removes the straight flow to improve contrast and make the modal streaks on the surface of the instrument clearer. Different modes of image subtraction are modulated by various expressions of the Bessel function, reflected in the changes of brightness and darkness of the fringes. Furthermore, in light of the properties of shearography within the field of non-destructive testing, this technique is adept at detecting flaws in the structural integrity of the instrument itself. Such deficiencies may encompass variations in the uniformity of surface material composition, the existence of cracks, irregularities in solder joints, and other associated concerns. This research expands the application of optical dynamic measurements in analyzing musical instrument vibrations and provides theoretical and methodological support for future research, thus introducing new technological tools to the industry.