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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 [1] 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: Impedance tube with transmission option
Standard: ASTM E2611, ASTM E1050, ISO10534-2
Method: Three or four microphone transmission tube and the transfer-function method [2, 3]
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): 200 to 6600 Hz
  • Medium impedance tube (44.44 mm diameter): 100 to 4300 Hz
  • Large impedance tube (100 mm diameter): 35 to 1900 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 is useful (here ω is the angular frequency):

specific impedance of air formula


                               
[1] R. Panneton, “Comments on the limp frame equivalent fluid model for porous media,” J. Acoust. Soc. Am. 122(6), EL217-EL222 (2007).
[2] O. Doutres, N. Atalla, Y. Salissou, and R. Panneton, “Evaluation of the acoustic and non-acoustic properties of sound absorbing materials using a three-microphone impedance tube,” Applied Acoustics 71, 506-509 (2010).
[3] Y. Salissou, O. Doutres, and R. Panneton, “Complement to standard method for measuring normal incidence sound transmission loss with three microphones,” J. Acoust. Soc. Am. 131(3), EL216-EL222.