A use of the cepstrum analysis to compute data of an envelope as described above enables a power spectrum to be smoothed through simple computation. Thus, it is possible to reduce the amount of calculation. The envelope computation unit 214 may use a method other than the cepstrum analysis. For example, the envelope computation unit 214 may compute an envelope by applying a general smoothing method to log-transformed amplitude values. As the smoothing method, a simple moving average, a Savitzky-Golay filter, a smoothing spline, or the like may be used.

The scale conversion unit 215 converts a scale of envelope data in such a way that, on the logarithmic axis, non-equally spaced spectral data are equally spaced. The envelope data that are computed by the envelope computation unit 214 are equally spaced in terms of frequency. In other words, since the envelope data are equally spaced on the linear frequency axis, the envelope data are not equally spaced on the logarithmic frequency axis. Thus, the scale conversion unit 215 performs interpolation processing on envelope data in such a way that, on the logarithmic frequency axis, the envelope data are equally spaced.