Noise Level Index in Transport Cabins: A Systematic Review of Measurement Methods, Passenger Impacts, and Mitigation Strategies

This systematic review summarizes a 30-year body of research (1996–2025) on in-cabin noise measurement and evaluation across transport modes, including road, rail, air, hybrid and electric vehicles. Guided by the PRISMA 2020 framework, 38 included records were identified and analyzed, comprising 33 empirical and 5 standards/guideline documents to address six research questions (RQ1–RQ6): the distribution of studies by mode and context, the methods and descriptors employed, comparative strengths and weaknesses of standard and psychoacoustic metrics, the identification of dominant noise sources and mitigation strategies, the development of a hybrid integrated noise index, and the existence of electric-vehicle-specific indicators. Results show that the equivalent continuous sound level (Leq) remains the most widely used descriptor due to regulatory acceptance. In contrast, psychoacoustic parameters such as loudness and sharpness show stronger correlations with perceived comfort and annoyance. Hybrid indices VACI, NCI, NAFI, and SQI offer enhanced predictive accuracy but are mainly limited to automotive and HVAC contexts. Primary interior noise sources include powertrain, tire–road or wheel–rail interactions, and HVAC systems, with effective mitigation achieved through acoustic insulation, aerodynamic refinement, and active noise control below 300 Hz. To unify these approaches, this review proposes a Hybrid Integrated Cabin Noise Index (HCNI) that combines physical SPL metrics with perceptual psychoacoustic descriptors, all measured under standardized protocols. The HCNI framework provides a pathway toward a cross-modal, perception-driven standard for evaluating acoustic comfort in next-generation transport cabins, including electric and autonomous vehicles.