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The relationship between the spectrum structures of passenger compartment noise and the results of subjective evaluations of sound quality-has been studied on a fron-wheel-drive car with a four cylinder engine. As a result of an analysis using a car interior noise simulator, which is a kind of digital sound shynthesizer, most of the sound quality indicies such as the crank rumble noise, the roughness or unstable characteristics, and the muddiness were found to be related to the structure of engine revolution harmonics and to the strength of fourmants. Further, the physical mechanisms which characterize these spectrum structures were identified through both engine running tests and shaker tests. As a results, the dominant factors governing sound quality problems were found to be the crankshaft bending or torsional vibration coupled with the total power plant vibration shystem.