The static thermal permeability is the low frequency limit of the dynamic thermal permeability. The thermal permeability problem is the thermal analogy of the viscous permeability problem. When the frame of a porous medium has a sufficient thermal capacity for the compressibility to reach its isothermal value at low frequencies, the excess acoustical temperature can be considered to vanish at the pore walls (this replaces the no-slip condition for viscous flow) and a static “thermal permeability” exists.
The open porosity (ϕ) is defined as the fraction of volume that is occupied by the fluid in the interconnected porous network. Non-interconnected voids trapped in the solid phase are not part of the open porosity: they are part of the closed porosity.
Viscoelastic materials are added to structures and plates in order to avoid large vibrations, espciallly around resonant frequencies. The properties of these materials are the Young’s modulus (E(ω)), the loss factor (η(ω)) and shear modulus (G(ω)). These parameters are frequency and temperature dependent.
For isotropic poroelastic materials, the elastic properties to be characterized are Young’s modulus (E), Poisson’s ratio (v) and loss factor (η). The method is based on the dynamic compression of two samples of different shape factors (s1 and s2). This enables the simultaneous characterization of the three elastic properties. The method is valid for frequencies from 20 to 100 Hz.
The viscous and thermal characteristic lengths are average macroscopic dimensions of the cells related to viscous and thermal losses, respectively. The former may be seen as an average radius of the smaller pores, and the later as the average radius of the larger pores.
The tortuosity, or structure factor, is a geometrical measurement of the deviation of the actual path followed by an acoustical wave in a porous material from a direct path in the air.
The static airflow resistivity (SAR), often represented by the Greek letter σ (sigma), is a very important parameter for acoustic material modeling. In fact, one of the first acoustic models presented by Delany & Bazley in 1970  was only relying on this parameter.