The measurement of acoustic material characteristics using a standard impedance tube method is generally limited to the plane wave regime below the tube cut-on frequency. This implies that the size of the tube and, consequently, the size of the material specimen must remain smaller than a half of the wavelength. This paper presents a method that enables the extension of the frequency range beyond the plane wave regime by at least a factor of 3, so that the size of the material specimen can be much larger than the wavelength. The proposed method is based on measuring of the sound pressure at different axial locations and applying the spatial Fourier transform. A normal mode decomposition approach is used together with an optimization algorithm to minimize the discrepancy between the measured and predicted sound pressure spectra. This allows the frequency and angle dependent reflection and absorption coefficients of the material specimen to be calculated in an extended frequency range. The method has been tested successfully on samples of melamine foam and wood fiber. The measured data are in close agreement with the predictions by the equivalent fluid model for the acoustical properties of porous media.
[1]
Yvan Champoux,et al.
Dynamic tortuosity and bulk modulus in air‐saturated porous media
,
1991
.
[2]
J. Allard.
Propagation of Sound in Porous Media: Modelling Sound Absorbing Materials
,
1994
.
[3]
M. Tamura.
Spatial Fourier transform method of measuring reflection coefficients at oblique incidence. I: Theory and numerical examples
,
1990
.
[4]
Mohamed Akoum,et al.
MEASUREMENT OF THE REFLECTION MATRIX OF A DISCONTINUITY IN A DUCT
,
1998
.
[5]
Mark Sheplak,et al.
Modal decomposition method for acoustic impedance testing in square ducts.
,
2006,
The Journal of the Acoustical Society of America.
[6]
Denis Lafarge,et al.
Spatial Fourier‐transform method for measuring reflection coefficients at oblique incidence. II. Experimental results
,
1995
.