A method to determine the acoustic reflection and absorption coefficients of porous media by using modal dispersion in a waveguide

Abstract

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.

An application of normal mode decomposition to measure the acoustical properties of low growing plants in a broad frequency range

“…In what follows we briefly recall the measurement and analyse procedure presented in [9]. where K is the wavenumber.…”

“…The application of a new method [9] to measure the reflection and absorption of a rectangular impedance tube can be extended at least by a factor of 3. This allows to measure the acoustical properties of much larger material specimens well beyond the high frequency limit that is currently set in the commonly used ISO 10534-2 standard [1].…”

“…The walls of the tube were constructed from 38 mm thick fibreboard panels which were varnished to ensure that they are reflective. One end of the tube was terminated with a 30 mm thick metal lid and at the opposite end three loudspeakers were installed and extended this range to 1800 Hz by using a step-by-step 50-1800 Hz sine sweep and the method detailed in [9]. We recalled this method in Appendix A for the sake of completeness.…”

“…Therefore, it is of direct interest to be able to measure the acoustical properties of a plant specimen with representative dimensions. This paper attempts to address this issue through the application of an alternative impedance tube method [9] to measure the acoustic absorption of a representatively large specimen of a living plant in a relatively large impedance tube. In this way, the acoustical reflection and absorption coefficient of this plant can be measured in the frequency range well beyond the first cross-sectional resonance and a range of angles of incidence, as these depend on frequency for higher order modes.…”

Measurement of acoustic properties for passive-material samples using multichannel inverse filter

“…1 Many methods were proposed to measure the acoustic properties for either a small sample or a large sample. [2][3][4][5][6][7][8][9][10][11][12] There are many methods of measuring acoustic properties for a small sample in a sound tube. [2][3][4][5][6] However, measurements for small samples cannot fully reflect the overall acoustic performance for samples with complex structures.…”