Have you ever wondered what would happen with respect to noise, vibration and harshness analysis (NVH) if porous materials were compressed? This question often arises when designing sound packages and taking into account the architectural constraints.
This compression would indeed cause the porous micro-structure to be modified. For fibrous materials, the fibers will closely tighten, leaving less space for acoustic dissipation, causing the material to switch from absorbing sound waves to behaving more like a barrier to the noise and vibration. Compressing foam-like material will first bend and then rapidly buckle the skeleton ligaments. At this stage/under this state, the mechanical behavior of the foam is not elastic. By increasing the compression, densification of the matter takes place, turning the porous foam into a non-porous polymeric layer whose properties will be mainly driven by its damping loss factor and mass.
In turn, carefully designed solutions may not be appropriate once the compression has taken place. As seen the pictures below, the compression rate may be responsible for a decrease of 0.2 point of absorption (automotive fiberglass samples).
Several models have been proposed to predict the properties of porous materials, like the above example, resulting from a given compression rate. Some of them, empirically built, rely on hundreds of measured data points, while others are dependent on morphological laws to predict the macroscopic properties of fibrous foams. The Matelys team has long reviewed and tested these different works, leading to the proposition of a robust solution called AlphaCell. To predict acoustic and elastic properties, an AlphaCell user can simply input a given compression rate, or even rebuild the global response of the system with varying compression rates according to its related compressed surface area.