Application of Holographic Interferometry to Vibrations of the Bodies of String Instruments

Abstract

Some eigenmodes of both the top and back plates of the bodies of string instruments are known from measurements made with capacitance transducers. But this method is rather tiresome and therefore holographic interferometry is applied. It is not possible to use the “real-time” holographic interferometry because even the short time necessary for the photographic development of the hologram is long enough for the surface of the wooden body to change, perhaps because of small moisture changes. On the other hand, time averaging gives satisfying holographic pictures. Examples of those at low, middle, and high frequencies are given for excitation over the sound post and the bass bar, as well as on the bridge. Although the interference fringes on time-average holograms can be interpreted quantitatively, only qualitative conclusions are made, especially those which seem to be valid not only for this particular specimen but also in general.