These models account for wave propagations (and interactions) in both fluid and solid phases of an acoustical porous material. These models are the most comprehensive ones to describe the vibro-acoustics of porous media however they require more parameters to be used: a set of parameters for each of the two phases. Read more about the Biot's theory.

Under specific frequency, boundary conditions and/or excitation conditions, the solid phase (or skeleton) of the porous medium can be considered as motionless. In such cases, no wave propagates in the solid phase and the complete vibro-acoustical behavior of the material can be simplify compared to diphasic models. Read more about motionless skeleton models.

In a similar way, under different frequency, boundary conditions and/or excitation conditions, one can consider that no wave propagates in the fluid phase. Read more about uniform pressure models.

Choosing a suitable model

First, it is obvious that in case of mechanical excitation of the
medium, the motionless skeleton models can not be used. Depending of
the frequency range studied, a diphasic or a uniform pressure model
can be used.

At low frequencies, when the wavelength is very much larger
than the thickness of the material sample, a uniform pressure
approximation inside the sample can be considered. At higher
frequencies, a diphasic model must be used.

At very low frequencies when the wavelength \( \lambda \) is very much larger than the thickness \( h \) of a porous material sample, the pressure values on both sides of the sample: \( p_{2} \) and \( p_{1} \) can be considered as equivalent. A uniform pressure field is thus assumed in the sample and a "equivalent solid" model can be used to describe the acoustical behavior of the material.

In case of acoustical excitations, at low frequencies, waves can propagate in both phases and a diphasic model is required. Above a phase decoupling frequency [ZK49] a motion of the fluid phase does no more induce a motion of the solid phase. A Motionless skeleton model can thus be used to
describe the acoustical behavior of the material.