Sustainable Raw Earth–Straw Fiber Composites: Estimated Sound Insulation Potential and Building-Relevant Properties

This study presents a comprehensive multiphysics characterization of unstabilized raw earth and earth–wheat straw composites to address the scarcity of data regarding their acoustic performance. Four soils from the Champagne-Ardenne region were initially analyzed to evaluate the impact of mineralogical composition, specifically calcium carbonate (CaCO3), on sound absorption. Results indicate that despite compositional variability, sound absorption remains low (αw = 0.12–0.19), while no clear influence of mineralogical variability was identified within the investigated soil set. While raw earth demonstrates limited experimentally measured sound absorption, its high surface mass suggests a potentially favorable sound-insulation performance, as indicated by theoretical mass-law estimates. Subsequently, wheat straw (0–6 wt%) was incorporated into a selected soil to develop lightweight composites. Increasing fiber content reduced bulk density by up to 35% and markedly enhanced sound absorption (αw up to 0.34, a 162% increase) due to increased porosity and fiber–matrix interactions. Furthermore, thermal conductivity decreased, improving insulation. Although these improvements coincided with a transition from brittle to ductile mechanical behavior and reduced stiffness—limiting their use to infill or self-supporting structures—the composites maintained good-to-excellent moisture buffering and satisfactory short-term resistance under the investigated water-exposure conditions. These findings support the potential of earth–straw composites for low-impact, non-structural building applications.