my question was not professional. i should have said

送交者: cornbug 于 2007-06-19, 10:18:49:

回答: Fourier transform 由 sl 于 2007-06-19, 07:57:25:

that the if the dominant frequency changes over time, or just the magnitude of the sine wave attenuates?

Sure, Fourier transform is perfect to evaluate this.