Porous materials are those in which the solid part that constitutes them occupies only part of their volume, the remaining part being formed by small gaps, open to the exterior and which may (or may not) communicate with each other. Sound absorption is fundamentally processed by energy dissipation due to air friction during propagation along the pores that define it, and also by viscosity and internal friction in the vibration of the material structure itself.

The maximum efficiency in the operation of these systems is obtained by placing the material in the position where the vibration velocity of the air particles is greatest (see the following Figure), thus allowing the kinetic energy of the corresponding vibration to be reduced.

The speed of vibration of the particles is greater at a distance of ¼ of the wavelength, and hence the energy dissipating capacity of the material.

The energy dissipation processes that occur in this type of material derive much from the porosity they exhibit, and for this reason they are normally effective in acoustic corrections to be made in the high frequency domain (frequencies above 500 – 630 Hz).