Nearly all acoustic systems absorb sound using porous materials, such as foam, fiberglass, cotton or felt. These materials absorbs sound energy in two methods— viscous friction between air molecules as they flow throughout the open spaces inside the material, and mechanical damping from microscopic vibrations caused by air molecules impacting the porous material’s inner structure.
Due to the physical limitations of mechanical damping, porous materials absorb sound energy more effectively at low SPLs than at higher levels. Although increasing a porous material’s thickness improves its overall absorption capacity, it disproportionately improves the material’s low SPL absorption.
This bias is problematic, since the amount of porous material needed to sufficiently address higher SPLs results in the environment sounding “dead” and unnatural at lower SPL levels, due to the excessive sound absorption.