Noise Metrics and Subjective Assessment of Automotive HVAC Systems

Abstract

HVAC systems are of critical importance in ensuring passengers' thermal comfort inside the car cabin as well as safety requirements for defogging functions. These systems involve various components and subcomponents such as blowers, thermal exchangers or actuators, with a wide range of well-known technologies and also new ones on recently introduced innovative products. Currently, within established electrification trends worldwide, the HVAC system is becoming the most important embedded system that can induce major contribution of noise and vibration. These NVH issues can emerge through different transfer paths inside the car cabin possibly causing significant discomfort to passengers. During developments, the NVH issues are mastered and contained by both suppliers according to internal requirements and OEMs according to specifications. However, OEM specifications are mainly defined by overall noise levels and improvements over the years are generally consisting of reducing these specified levels. Furthermore, some HVAC NVH issues may not be well detected when using regular NVH metrics. This raises concerns about the limitations of the regularly used metrics in ensuring specification compliance and, above all, in depicting a subjective assessment at component level. Throughout a statistical analysis of HVAC systems population, this paper first focuses on the discrepancies between the data provided by traditional NVH metrics and subjective evaluations. Then, a deeper analysis involving psychoacoustic metrics provides a relevance overview of the applied metrics depending on the encountered issues and compared to a subjective assessment. Finally, from a responsibility perspective, these findings raise questions about the relevance of regular metrics used by OEMs and the right way to handle HVAC NVH topics nowadays.