In this paper, an objective distortion estimator called auditory-oriented spectral distortion(ASD) is proposed. It is confirmed that the ASD can accurately predict the auditory perceptual distortions represented by the mean opinion score(MOS). The ASD is used as a criterion to optimize the noise reduction algorithm for instruments that need to reduce noises appealing to the ear, for example hearing aids. Experimental results say that a suitable value of the parameter should be different for the purpose to use as between improving the auditory impressions and a front-end of speech recognition systems.