- Diphasic models
- 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 concerning the two
medium phases to be used.
- 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.
- 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.
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.
References
[ZK49] Zwikker C. and Kosten C. W., Sound absorbin materials, Elsevier, New-York, 1949.
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