The sound field in a concert hall is, rigorously speaking, a time-variant system due to changes in temperature and air current. These time-variant factors cause sound propagation that results in the fluctuation and sound-pressure level. This paper shows that such a fluctuation may be identified by the autocorrelation function (ACF). As is widely known, effective duration of ACF ((tau)e) of a sound source, which is defined by the delay at which the envelope of the normalized ACF becomes 0.1, describes several important subjective attributes [Y. Ando, Concert Hall Acoustics (Springer-Verlag, Heidelberg, 1985)]. In this paper, values of (tau)e of a music signal with changing tempo, dynamics, and articulation produced by a computer-controlled piano were analyzed. Results indicate that the measured values of (tau)e in an existing hall are shorter than those of the source signal itself, while (tau)e values of reverberant sound field simulated as a time-invariant system are 1.2--1.6 times longer than that of the source signal. Therefore, it is considered that the reverberance perceived in the existing concert hall might be much shorter than that of the time-invariant simulated sound field.