1. A method of classifying an audio signal into a fast signal or a slow signal for audio coding, wherein the audio signal has a plurality of frames, the method comprising: determining a parameter of each of the plurality of frames of the audio signal; and comparing the parameter with a pre-defined threshold to determine whether each of the plurality of frames should be classified into the fast signal or the slow signal, wherein the parameter is or is a function of spectral sharpness, and the spectral sharpness (Spec_Sharp) is defined by a ratio between a largest coefficient and an average coefficient in one frequency subband as follows: Spec_Sharp = Max ⁢ {  MDCT i ⁡ ( k )  , k - 0 , 1 , 2 , … ⁢ ⁢ N i - 1 } 1 N i · ∑ k ⁢ ⁢  MDCT i ⁡ ( k )  where MDCT i (k), k=0, 1,. .. , N i −1, are frequency coefficients in the i-th frequency subband, N i is the number of the frequency coefficients of the i-th subband, and the Spec_Sharp is expressed in a Linear domain or a Log domain.