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In order to efficiently enhance engine sound quality under acceleration, the authors have developed an evaluation method for primary judgment of the sound quality of engine combustion noise at the stage of advanced engine development before the prototype vehicle is built. This method is an application of an existing method for evaluating the sound quality of engine combustion noise in vehicle interiors to the evaluation of noise and vibration at an engine acoustic test bench. In this method, it is necessary to consider the air-borne and the structure-borne components separately. The analysis procedure for the air-borne component is as follows. First, the sound pressure at a point 1 m away from the engine and the in-cylinder pressure of each cylinder are measured simultaneously in a semi-anechoic engine dynamometer test chamber. Next, the signal correlated with engine combustion is extracted from the measured sound pressure using the time domain combustion noise separation method.

To increase the efficiency of measures targeting combustion noise in complete vehicle development, the authors developed a new sound quality evaluation method by combining two known methods: the time domain combustion noise separation method (T-CNSM) and a psychoacoustic metric, fluctuation intensity. The T-CNSM was applied to the vehicle interior noise, allowing for precise extraction of the combustion contribution in the time domain. Furthermore, the T-CNSM has enabled a sound quality check of combustion noise using a headphone-playback system. The procedure of this method is as follows. Firstly, simultaneous measurement of vehicle interior noise and in-cylinder pressure of each cylinder is carried out under acceleration. Afterward, the application of digital signal processing using Fourier transform and multiple regression analysis to the measured data extracts the contribution of combustion noise from the vehicle interior noise in the time domain.