The combustion noise of diesel engine during run up and idle condition makes unpleasant interior noise in passenger car whose market share is increasing nowadays. There were few studies to quantify the unpleasantness of combustion noise. We tried to introduce an objective metrics which matches well with subjective ratings of combustion noise. To get the objective metrics, the combustion noise signal was analyzed by many different methods based on the human hearing sense. In the end, we found out that the modulation amplitudes of each frequency band match well with the unpleasant characteristics of combustion noise. Then, an objective index was developed from the combination of modulation amplitudes on the basis of the human hearing sense. The index was shown to be in good correlations with subjective ratings in general.

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