Electronic speckle pattern interferometry has been used to study the deflection shapes of a piano soundboard. A design for an interferometer that can image such an unstable object is introduced, and interferograms of a piano soundboard obtained using this interferometer are presented. Deflection shapes are analyzed and compared to a finite-element model, and it is shown that the force the strings exert on the soundboard is important in determining the mode shapes and resonant frequencies. Measurements of resonance frequencies and driving-point impedance made using the interferometer are also presented.

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