## 2 Methods  
The issue of noise within vehicles, concerning the source of origin, is divided into two perspectives: (1) as airborne noise and (2) as structural sound arising from within the vehicle structure. Predominantly, the majority of noise emissions originate notably from engine noise and reverberations induced by interactions with roadways [98; 99]. These external sonic disturbances penetrate the vehicle interior as sound waves through the path of least resistance in the vehicle body, such as door seals, welds or imperfections. Currently, noise mitigation strategies [100; 101; 102] are based on the judicious application of various insulating or absorption materials complemented by acoustic baffles, urethane-filled foams and rubber seals or gaskets. However, achieving substantial noise reduction in automotive environments requires meticulous noise management and measurements. In efforts to combat interior vehicle noise, two primary methodologies emerge as essential within passive noise reduction, on which this study is based: absorption and isolation, facilitated using materials with superior acoustic properties. The selection of materials for noise reduction in automobiles is a challenging process conditioned by factors such as the type of material, dimensions and frequency spectrum of the targeted sound. Other factors influencing the sound absorption capabilities of these materials include the fiber size, airflow resistance, porosity, tortuosity, thickness, density, compression and surface impedance. Starting in 2022, high-precision measurements were conducted in the acoustic laboratory using the acoustic interferometer method (Kundt's tube) [103], consolidating a significant database for the field of passive noise issues. The experimental measurements were conducted using a Kundt tube to evaluate the acoustic absorption properties of a variety of materials. The primary goal was to determine how well each material performed in terms of noise reduction across a wide range of frequencies. The frequency range for the tests spanned from 100 Hz to 6300 Hz, capturing both low-frequency and high-frequency noise typically encountered in automotive environments. This broad frequency spectrum was selected to assess the materials' performance in real-world conditions, as low-frequency noise (e.g., engine sounds) and high-frequency noise (e.g., wind or road noise) require different acoustic treatment solutions. Each material sample was tested at intervals of 100 Hz, providing a detailed understanding of its absorption behavior at critical frequency points. The absorption coefficient was measured at each interval, enabling a comparison of how well different materials performed across the entire spectrum. By testing across this wide frequency range, we were able to identify materials that exhibited strong absorption properties at specific frequencies, highlighting their potential for targeted applications, such as reducing low-frequency vibrations or high-frequency cabin noise. The results from these experiments offer valuable data that can inform future material selection for noise reduction in vehicles and provide a solid basis for ongoing research in acoustic absorption technologies. The measurements were carried out in accordance with applicable standards (ISO 10534-1) [104] using a setup for sample analysis consisting of an acoustic signal generator and acquisition system, an impedance tube and a signal amplifier. Extensive experimentation and empirical analysis, particularly focusing on parameters such as the acoustic absorption coefficient, as a specific concern defined within the doctoral thesis, have contributed to the accumulation of a comprehensive repository of tested materials. Based on the complexity of research on optimizing acoustic comfort through passive noise control and material selection strategies, a quantitative interpretation and analysis of the database is conducted.