The percussive excitation of a piano string by a hammer is expected to produce longitudinal string vibration components in addition to the desired transverse ones [e.g., Yanagisawa et al., J. Acoust. Soc. Jpn. 33, 412–416 (1977)]. However, so far little quantitative information exists in current acoustics literature on the contribution of such longitudinal components to the radiated sound of a piano. Our investigations revealed the presence of intial components of radiated sound generated by the longitudinal string vibration mechanism. These components, coined as the precursive sound, appear distinctly ahead of the main body of sound that is almost entirely due to transverse string vibration. Our experimental data on the longitudinal modes in piano strings indicate that the most apparent contribution to the radiated sound occurs in the attack transient of tones in the low bass register. For the lowest bass tones, the sound-pressure level (SPL) of the precursive sound may be only 10 to 20 dB below the overall peak SPL and the decay rate of the order of 100 dB/s. [Work supported by A.R.G.S. and C.T.E.C.]