1993 年 14 巻 6 号 p. 429-439
The soundboard of a piano without strings vibrates in almost the same way as in an isotropic plate in a low-frequency region, because the anisotropy of its wooden material is compensated by its ribs, while the soundboard vibrates more locally in a high-frequency region. The boundary condition of the soundboard in a low-frequency region is a mixture of the fixed and simply supported conditions, whereas it becomes a fixed-edge state in a high-frequency region. Wher strings are added to the soundboard, they increase the resonance frequency of each vibration mode on the soundboard, and reduce its Q value. Characteristics of a soundboard can be quantitatively shown by measuring an admittance at the driving point of each string on the bridge. The admittance measured is also important to indicate the degree of influence of strings on the sound-board. The soundboard vibrates in a low-frequency range, reducing the amplitude of the fundamental vibration, and emphasizing its partials since the driving points are near the edge. In an intermediate-frequency region, the fundamental mode contributes very little to vibrations since it is low than the frequency range. The admittance in a high-frequency region is high due to local vibrations occurring between ribs. Other acoustic characteristics of the soundboard, such as its vibrations with a constant driving force, and the sound-pressure directivity of soundboards, have been investigated.
