Parametric amplification mechanism of sound is described by using computer simulation including absorption due to medium. Secondary wave with difference frequency is parametrically generated by distortion of envelop caused by finite amplitude effect of carrier which is composed of two different frequencies. In this case, shock formation at larger amplitude periods in amplitude modulated carrier is prevented by the absorption. Therefore, the difference-frequency generation is continued by desired distortion of envelop, even if propagation distance is over the shock formation distance. It is concluded that conversion efficiency from carrier energy to energy of the secondary wave with the difference frequency is improved by absorption and undesired distortion of the secondary wave is suppressed at the same time.