The Psychoacoustic Characteristics of Non-Linear Automotive Disk Brake Creep Groan: a Method Based on Accelerometer Data

The nature of friction within a vehicle’s disk brake system can cause a wide range of different noise phenomena. Especially high-frequency brake squeal was examined during the last decades. Numerous publications treat squeal phenomenology and its mitigation. Increasing shares of electrified powertrains, automatic driving functions such as park assists and further increasing quality demands have now shifted the research interest more and more towards low-frequency phenomena. One of these low-frequency phenomena is creep groan. Defined by its main frequency below 200 Hz, creep groan is characterized by a highly non-linear behavior: Global stick-slip transitions in the disk/pad contacts repeatedly excite the whole brake and axle system. Different bifurcations or even chaotic behavior occur. To ensure good creep groan behavior, defined assessment procedures and rating criteria are necessary. Currently, the German Association of the Automotive Industry recommends a combined rating via the subjective perception of trained test drivers and the objective, A-weighted sound pressure level. This practice could be improved with a more sophisticated objective rating: By considering the human perception, objective and subjective ratings would correlate even better. One possible approach towards an enhanced objective creep groan rating could therefore use psychoacoustic metrics. In 2009, this idea was formulated for the psychoacoustic loudness and the tonality of creep groan by Abdelhamid and Bray. The present work seizes this suggestion and provides additional psychoacoustic evaluations of full-vehicle creep groan signals. Based on measured accelerometer signals, a novel procedure for the psychoacoustic evaluation of structure-borne noise was applied: Optimized FIR filter transfer functions were used to compute equivalent sound pressure signals from the accelerometer data, with the equivalent signals resembling the measured signals but lacking unwanted noise. Both the measured and the simulated signal were then evaluated and compared regarding their psychoacoustic behavior. Results reveal the value of the equivalent sound pressure signal: Whereas loudness and sharpness were found very similar and tonality rather arbitrary for both measured and equivalent sound pressure signal, roughness and fluctuation strength showed strong differences between the signals: Here, only the accelerometer-based, equivalent sound pressure provided easily interpretable characteristics. The proposed method also compared psychoacoustic characteristics for different creep groan bifurcations. Possible applications comprise an enhanced objective rating of low-frequency noise phenomena, the detection and classification of creep groan bifurcations, or the possibility to estimate creep groan cabin noise based on simulative results during early development stages. Therefore, this study provides another step towards silent automotive brake technology.