A computer-implemented method for processing an audio signal, the method comprising: receiving an encoded low-frequency range signal corresponding to the audio signal; decoding the encoded signal to produce a decoded signal having an energy spectrum of a shape including an energy depression; performing filter processing on the decoded signal, the filter processing separating the decoded signal into low-frequency range band signals; performing a smoothing process on the low-frequency range band signals, the smoothing process smoothing the energy depression of the low-frequency range band signals; performing a frequency shift on the smoothed low-frequency range band signals, the frequency shift generating high-frequency range band signals from the low-frequency range band signals; combining the low-frequency range band signals and the high-frequency range band signals to generate an output signal; and outputting the output signal, wherein performing the smoothing process on the low-frequency range band signals further comprises: computing an average energy of a plurality of low-frequency range band signals; computing a ratio for a selected one of the low-frequency range band signals by computing a ratio of the average energy of the plurality of low-frequency range band signals to an energy for the selected low-frequency range band signal; and multiplying the selected low-frequency range band signal by the computed ratio. receiving an encoded low-frequency range signal corresponding to the audio signal; decoding the encoded signal to produce a decoded signal having an energy spectrum of a shape including an energy depression; performing filter processing on the decoded signal, the filter processing separating the decoded signal into low-frequency range band signals; performing a smoothing process on the low-frequency range band signals, the smoothing process smoothing the energy depression of the low-frequency range band signals; performing a frequency shift on the smoothed low-frequency range band signals, the frequency shift generating high-frequency range band signals from the low-frequency range band signals; combining the low-frequency range band signals and the high-frequency range band signals to generate an output signal; and outputting the output signal, wherein performing the smoothing process on the low-frequency range band signals further comprises: computing an average energy of a plurality of low-frequency range band signals; computing a ratio for a selected one of the low-frequency range band signals by computing a ratio of the average energy of the plurality of low-frequency range band signals to an energy for the selected low-frequency range band signal; and multiplying the selected low-frequency range band signal by the computed ratio. computing an average energy of a plurality of low-frequency range band signals; computing a ratio for a selected one of the low-frequency range band signals by computing a ratio of the average energy of the plurality of low-frequency range band signals to an energy for the selected low-frequency range band signal; and multiplying the selected low-frequency range band signal by the computed ratio.