Acoustic properties which model the vibro-acoustic behaviour of systems are described in this slide. These characteristics take into account the behaviour of fluids placed in the solid phase structure.
Flow resistivity is the resistance to fluid flow through the thickness of the porous material.
Porosity is the volumetric relation between fluid and solid phase of the material.
Viscous characteristic length is the average macroscopic dimension of the cells related to viscous losses or the average radius of the smaller pores of the material. It is only possible to determine this by indirect measurements.
Thermal characteristic length is the average macroscopic dimension of the cells related to thermal losses or the average radius of the larger pores of the material. Again, it is only possible to determine this by indirect measurements. Thermal characteristic length is also twice the ratio between volume and surface of cellular structure. It is related to specific surface of a porous material.
Tortuosity is the geometric average of the deviation of the acoustic wave path from the direct path (thickness). It is equivalent to structural factor (ks). It is related to the electric resistance of a porous non conducting material saturated with an electrolyte. It is possible to measure it with ultrasound or electrically, but it is very difficult.
The next slide show a helpful diagram to illustrate theses parameters.