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Intrinsic acoustical properties
The intrinsic acoustical properties are dynamic physical properties of the material. They do not depend on the mounting configuration and thickness. They only depend on the frequency and porous material microstructure. Equivalent fluid models for rigid or limp frame porous materials define the porous medium in terms of a set of two intrinsic properties (typically, the dynamic density and the bulk compressibility). A set of two intrinsic properties can be used to define another set of two intrinsic properties.
Equipment: Transmission tube
Method: Three or four microphone transmission tube and the transfer-function method [1,2]
Property: Zc - characteristic impedance (Pa.s/m); k - complex wave number (1/m); γ - propagation constant (1/m); ρ_eq - dynamic density (kg/m³); β_eq - dynamic compressibility (1/Pa); K_eq - dynamic bulk modulus (Pa)
Tested frequency range
- Small impedance tube (29 mm diameter) : 500 to 6 000 Hz
- Medium impedance tube (44.5 mm diameter): 300 to 4 100 Hz
- Large impedance tube (100 mm diameter): 50 to 1 600 Hz.
Comment
The characteristic impedance can be normalized by the specific impedance of air (i.e. Zcn=Zc/Zo, where Zo=ρc, and ρ and c are the density and sound speed in air). The following set of relations are usefull (here ω is the angular frequency):
