# Diphasic models (Biot's theory)

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.

# Motionless skeleton models ("equivalent fluid" models)

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.

# Uniform pressure models ("equivalent solid" 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.