1. 1) M. Ghosh, ``On the Hertzian impact of an elastic hammer on a damped pianoforte string,'' Philos. Mag., 7, 521-544 (1934).
2. 2) T. Yanagisawa and K. Nakamura, ``Dynamic compression characteristics of piano felt,'' J. Acoust. Soc. Jpn. (J), 40, 725-729 (1984) (in Japanese).
3. 3) H. Suzuki, ``Model analysis of a force-deformation characteristic of a piano hammer,'' CBS Technology Center Report (1985).
4. 4) H. Hertz, Hertz's Miscellaneous Papers, translated by D. E. Jones and G. A Schott (Macmillan, New York, 1896).