Influence of installation and humidity conditions on the sound absorption coefficient of aeronautical glass wool: An experimental and machine-learning-based study

Porous materials such as glass wool are widely used in aircraft fuselage insulation systems for their sound absorption performance. In operational aeronautical environments, their acoustic performance may differ from that measured under nominal laboratory conditions due to several factors such as protective coverings, installation procedures and moisture variations occurring during flight operations and throughout the aircraft service life. Despite their practical relevance, the effects of these non-nominal conditions on sound absorption variability remain insufficiently characterized. This study investigates the frequency-dependent sound absorption coefficient of aeronautical glass wools under controlled non-nominal conditions by combining impedance tube measurements, machine-learning techniques and Shapley additive explanations (SHAP). The investigated factors include material type, protective layers, relative humidity, humidity cycling and both controlled and operator-dependent mounting configurations. Results indicate that material, layer, relative humidity and humidity cycling significantly influence the acoustic response, with moisture-related effects exhibiting a strongly frequency-dependent behavior concentrated within three distinct frequency bands. Installation-related effects are also found to produce identifiable spectral variations, with operator-dependent mounting conditions generally associated with higher absorption levels and shifts of the peak response toward lower frequencies. The results further indicate that more controlled and uniformly distributed contact conditions improve measurement repeatability and reduce installation-induced variability.