A sound processing apparatus comprising: a preprocessor adapted to: divide a source signal into a high-frequency signal and a low-frequency signal, divide the low-frequency signal into a plurality of sub-bands, obtain a tonality measure indicating a degree of spectral flatness and an envelope information for each of the sub-bands of the low-frequency signal; and an adaptive harmonic signal generator adapted to: generate for each sub-band a normalized signal by adjusting an amplitude for each of the sub-bands of the low-frequency signal based on the envelope information, generate a tonality-weighted signal for each of the sub-bands by multiplying the normalized signal with the tonality measure, and generate for each sub-band a higher harmonic signal in substitution of a portion of the low-frequency signal by applying a non-linear operation to the tonality-weighted signal; and a gain controller adapted to: adjust for each sub-band a mixing ratio of the low-frequency signal and the higher harmonic signal adaptively so that; in each sub-band a first gain for the higher harmonic signal is proportional to the tonality measure and a second gain for the low-frequency signal is in inverse-proportion to the tonality measure, and generate for each sub-band a gain-adjusted low-frequency signal applying the first gain, and sum each sub-band of a gain-adjusted higher harmonic signal adjusted with the second gain and the high-frequency signal; a postprocessor adapted to: synthesize the gain-adjusted low-frequency signal with the summed high-frequency signal. a preprocessor adapted to: divide a source signal into a high-frequency signal and a low-frequency signal, divide the low-frequency signal into a plurality of sub-bands, obtain a tonality measure indicating a degree of spectral flatness and an envelope information for each of the sub-bands of the low-frequency signal; and divide a source signal into a high-frequency signal and a low-frequency signal, divide the low-frequency signal into a plurality of sub-bands, obtain a tonality measure indicating a degree of spectral flatness and an envelope information for each of the sub-bands of the low-frequency signal; and an adaptive harmonic signal generator adapted to: generate for each sub-band a normalized signal by adjusting an amplitude for each of the sub-bands of the low-frequency signal based on the envelope information, generate a tonality-weighted signal for each of the sub-bands by multiplying the normalized signal with the tonality measure, and generate for each sub-band a higher harmonic signal in substitution of a portion of the low-frequency signal by applying a non-linear operation to the tonality-weighted signal; and generate for each sub-band a normalized signal by adjusting an amplitude for each of the sub-bands of the low-frequency signal based on the envelope information, generate a tonality-weighted signal for each of the sub-bands by multiplying the normalized signal with the tonality measure, and generate for each sub-band a higher harmonic signal in substitution of a portion of the low-frequency signal by applying a non-linear operation to the tonality-weighted signal; and a gain controller adapted to: adjust for each sub-band a mixing ratio of the low-frequency signal and the higher harmonic signal adaptively so that; in each sub-band a first gain for the higher harmonic signal is proportional to the tonality measure and a second gain for the low-frequency signal is in inverse-proportion to the tonality measure, and generate for each sub-band a gain-adjusted low-frequency signal applying the first gain, and sum each sub-band of a gain-adjusted higher harmonic signal adjusted with the second gain and the high-frequency signal; a postprocessor adapted to: synthesize the gain-adjusted low-frequency signal with the summed high-frequency signal. adjust for each sub-band a mixing ratio of the low-frequency signal and the higher harmonic signal adaptively so that; in each sub-band a first gain for the higher harmonic signal is proportional to the tonality measure and a second gain for the low-frequency signal is in inverse-proportion to the tonality measure, and generate for each sub-band a gain-adjusted low-frequency signal applying the first gain, and sum each sub-band of a gain-adjusted higher harmonic signal adjusted with the second gain and the high-frequency signal; a postprocessor adapted to: synthesize the gain-adjusted low-frequency signal with the summed high-frequency signal.