Few of the "acoustical" parameters used to describe
the visco-inertial and thermal behviors of acoustical porous materials
are directly measurable.
The open porosity, the static air flow resistivity and the high frequency limit of the dynamic tortuosity can be directly measured. The devices used in these cases are borrowed from science fields related to other porous materials (such as rocks) or are adapted from these other porous materials to acoustical ones (which have usually very large permeability and porosity compared to rocks).
The major part of the "Acoustical" parameters are estimated via indirect measurements, numerical computations or curve fiting techniques.