With the considerations above, the sound absorption coefficient of a sound-absorbing specimen in the Kundt tube can be predicted when the characteristic acoustic properties of each porous layer are known. The thickness of the medium is a straightforward quantity, the question then arises regarding how to determine the characteristic impedance and the complex wave number of a porous layer.

These parameters cannot directly be obtained through numerical simulation. Instead, different models exist for various types of porous materials that derive these parameters from other more measurable or computable quantities.

For highly porous absorbers, the Delany–Bazley model can be used and for stiff absorbers, the Johnson–Champoux–Allard model is appropriate.

Delany–Bazley Model

The model of Delany and Bazley is applicable for highly porous materials with porosity close to the maximum of 1. The model expressions for the complex wave number and the characteristic impedance are functions of frequency [Allard and Atalla, Eq 2.28 and 2.29]:

for , where the dimensionless parameter is given by

Here, is the air density, is the sound speed in air, is the frequency in Hertz, is the angular frequency, is the static air flow resistivity of the porous medium, and denotes the complex unity. Thus, the only unknown material parameter in this model is the static air flow resistivity .

The Delany–Bazley command of AcoustoDict therefore computes the static air flow resistivity  of porous materials—the only parameter needed to describe the acoustic behavior of highly porous materials. The results predicted by Delany–Bazley are in best agreement with experimental results, even for low frequencies. To achieve continuous dependence of the absorption on the frequency, the correction of Mechel and Miki are used. This modifies the Delany–Bazley formulas to

Schladitz et al apply this approach for the acoustic design of nonwoven materials.

Johnson–Champoux–Allard Model

The Johnson–Champoux–Allard Model (cf. Allard and Champoux, Champoux and Allard) is applicable to porous materials with a rigid frame and arbitrary pore shapes. The model expressions for the complex wave number and the characteristic impedance are

where is the angular frequency, the complex effective density, and the complex dynamic bulk modulus.

The effective density is expressed by (cf. [Allard and Champoux, Eq. 13], [Soltani et al, Eq. 18], [Xu and Lin, Eq. 1])

and the dynamic bulk modulus by

In these equations, the constants (viscosity), (density), (specific heat ratio),  (Prandtl number) describe the properties of the ambient air and are given as defined in Table 1.

The parameters  (air flow resistivity),  (tortuosity),  (porosity), and  (viscous characteristic length) describe the properties of the porous material.