# Development of a New Sound Quality Parameter for Road Noise Perception Inside Vehicle Cabinet
## 1 Introduction  
Acoustic events are generally described in terms of sound pressure level (SPL) with a unit of dB. However, the human ear not only focuses on the level but also on the frequency content of the sound waves. In fact, the variety in the spectrum results in completely different perceptions even under the same levels of sound pressure [1, 2]. Studies on the perception of sound waves were gathered under the discipline of psycho-acoustics, and the spectral events were matched with sound quality metrics. These metrics are obtained by an extensive study on the perception of a huge number of people. Loudness, sharpness, roughness, fluctuation strength, and pitch constitute the fundamentals of sound quality [1]. Evaluating the sound quality (SQ) of vehicles, especially regarding road noise, necessitates a comprehensive assessment beyond just sound level. A multitude of metrics paint a vivid picture of the aural experience. Loudness, measured in A-weighted decibels (dBA), in sons, or in phons, reflects the perceived intensity. Sharpness, quantified by metrics like Loudness Spectrum Term (LST), gauges the high-frequency content influencing satisfaction. Roughness, assessed by the Roughness Perception Index (RPI), captures the unpleasantness caused by rapid sound pressure fluctuations [1, 2, 3, 4, 5, 6]. The Articulation Index (AI) indicates how clearly speech can be understood over the noise, crucial for in-cabin environments. Fluctuation Strength (FS) reflects the variation in sound level over time, influencing how distracting the noise might be. Tonality Index (TI) measures the presence of prominent tonal components, often from engine noise [7]. Thetire-road noise, which can be a dominant sound source at some velocities depending on the engine type, can be better characterized using psychoacoustical parameters, such as loudness and sharpness, rather than sound pressure level [8]. By analyzing these metrics, alongside others, researchers and engineers gain a deeper understanding of how road noise impacts human perception and well-being. This knowledge empowers them to tailor noise reduction strategies that address the specific aspects deemed most unpleasant by individuals, ultimately refining the vehicle's acoustic environment. The historical advances in automotive NVH reduced noises of internal combustion engines and the powertrain systems to very low levels. Low noise levels do not necessarily mean good noise. The demand of customers for good sound are still increasing. Therefore, studies to improve the sound quality are essential for developing a car that sounds good [2]. Since the engine masking is reduced, the other systems are becoming more critical for their perception. In fact, the contribution of road noise on the vehicles has increased a lot in recent years, while the contribution of the engine noise is reducing [3,4]. Electrification is a dominant trend in the automotive industry, and it has benefits, like its silent soundscape, but it also has major NVH impacts due to lack of the masking effect of the engine. The NVH concern on electric or hybrid vehicles are concentrated in three major points: low-frequency road noise, high-frequency wind noise, and tonal components of motors and compressors. This situation results in a necessity to take countermeasures for emerging road noise perception [9,10]. In accordance with the demanding electrification trend, several aspects were studied for both interior and exterior noise impacts. [PERSON] et al. investigated the sound quality of small motors, developing a sound quality metric related to the interior noise of vehicles [11]. [PERSON] examined the psychoacoustic properties of electrified vehicles, proposing a new sound quality metric that addresses their environmental impact during pass-by events [8]. Additionally, [PERSON] et al. studied engine noise to create a sound quality metric that captures the sportiness of interior sound perception in vehicles [2].