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Sound Quality (SQ) improvement is recently one of the important issues in diesel engine development. Therefore a new method was developed by the authors about ten years ago to establish objective criteria for SQ assessment. Considering the sensitivity of human hearing to impulsive sounds such as diesel noise, the human auditory mechanism was simulated by introducing temporal masking in the time domain. Furthermore, each of the human auditory organs was simulated by computer codes, providing reasonable analytical explanations of typical human hearing responses to diesel noise. This method finally provides us a good index of sound quality, i.e. an SQ index, of diesel noise that includes high-frequency intermittent offensive sounds caused by impacting excitations of combustion, piston slap, etc. This paper describes human audibility of sound structures of diesel noise and also human hearing models, which are implemented in SQ evaluation tools by utilizing recent physiological knowledge.