Experimental contribution analysis of external noise components to interior noise of an automobile1)
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The contribution analysis of various noise sources in the automobile interior noise is important for a well-designed vehicle with a low interior noise level. The proposed modified Cholesky Decomposition (CD) is able to decompose the interior noise spectra into multiple spectra, each physically representing the contribution of a specific noise source to the interior noise of the automobile. During an experiment with two speakers driven by two independent white noise signals, it is shown that the measured noise spectrum can be successfully decomposed into two contributions, each associated with noise radiated from speakers. Then, a simplified, scaled automobile model (with one side mirror, one front and two side windows, and flat top, back, and bottom panels) was tested in a wind tunnel at the airflow speeds of 15 m/s (54 km/h) and 24 m/s (86.4 km/h). In this experiment, 14 external and 1 interior microphones were implemented to measure the external aeroacoustic source signals and the interior noise signal respectively. The results obtained by processing the microphone signals reveal the contributions of the external aeroacoustic noise sources to the interior noise. In addition, an automobile was tested on a road at the speeds of 104.6 km/h (65 mph, or 29.1 m/s) and 128.7 km/h (80 mph, or 35.8 m/s). In this experiment, 64 exterior and 4 interior microphones measured the external noise source signals and interior noise signals, respectively. The results obtained by processing the measured microphone signals indicated the highest contributor of the external aeroacoustic noises are the windows and the hood. It was also detected that the contribution of the windows increased as the speed increased and the highest contribution of the interior noise occurred to the seat closer to the window. The proposed CD-based procedure requires installing microphones flush-mounted on the exterior surface of the automobile to measure the noise source signals. However, the surface-flush-mounted microphone installation can be labor-intensive and time-consuming, making it difficult to evaluate a large number of aeroacoustic design cases. Thus, an innovative, CD-based contribution analysis system integrated with an exterior microphone array is proposed. In this integrated system, the surface-flush-mounted microphones are replaced with the exterior microphone arrays. This array-measurement-based contribution analysis procedure was validated by conducting an experiment with two speakers and an 8 by 8 array of microphones.
