A method for evaluating the acoustical properties of homogeneous and isotropic porous materials that may be modeled as fluids having complex properties is described here. To implement the procedure, a conventional, two-microphone standing wave tube was modified to include: a new sample holder; a section downstream of the sample holder that accommodated a second pair of microphone holders and an approximately anechoic termination. Sound-pressure measurements at two upstream and two downstream locations were then used to estimate the two-by-two transfer matrix of porous material samples. The experimental transfer matrix method has been most widely used in the past to measure the acoustical properties of silencer system components. That procedure was made more efficient here by taking advantage of the reciprocal nature of sound transmission through homogeneous and isotropic porous layers. The transfer matrix of a homogeneous and isotropic, rigid or limp porous layer can easily be used to identify the material’s characteristic impedance and wave number, from which other acoustical quantities of interest can be calculated. The procedure has been used to estimate the acoustical properties of a glass fiber material: good agreement was found between the estimated acoustical properties and those predicted by using the formulas of Delany and Bazley.

A transfer-matrix approach for estimating the characteristic impedance and wave numbers of limp and rigid porous materials

In this paper, the question of the acoustical determination of macroscopic thermal parameters used to describe heat exchanges in rigid open-cell porous media subjected to acoustical excitations is addressed. The proposed method is based on the measurement of the dynamic bulk modulus of the material, and analytical inverse solutions derived from different semiphenomenological models governing the thermal dissipation of acoustic waves in the material. Three models are considered: (1) Champoux–Allard model [J. Appl. Phys. 20, 1975–1979 (1991)] requiring knowledge of the porosity and thermal characteristic length, (2) Lafarge et al. model [J. Acoust. Soc. Am. 102, 1995–2006 (1997)] using the same parameters and the thermal permeability, and (3) Wilson model [J. Acoust. Soc. Am. 94, 1136–1145 (1993)] that requires two adjusted parameters. Except for the porosity that is obtained from direct measurement, all the other thermal parameters are derived from the analytical inversion of the models. The method is appl...

Acoustical determination of the parameters governing viscous dissipation in porous media.

Metal foams, especially close-celled foams, are generally regarded as poor sound absorbers. This paper studies the sound absorption behaviour of the open-celled Al foams manufactured by the infiltration process, and the mechanisms involved. The foams show a significant improvement in sound absorption compared with close-celled Al foams, because of their high flow resistance. The absorption performance can be further enhanced, especially at low frequencies, if the foam panel is backed by an appropriate air gap. Increasing the air-gap depth usually increases both the height and the width of the absorption peak and shifts the peak towards lower frequencies. The foam samples with the smallest pore size exhibit the best absorption capacities when there is no air gap, whereas those with medium pore sizes have the best overall performance when there is an air gap. The typical maximum absorption coefficient, noise reduction coefficient and half-width of the absorption peak are 0.96–0.99, 0.44–0.62 and 1500–3500 Hz, respectively. The sound dissipation mechanisms in the open-celled foams are principally viscous and thermal losses when there is no air-gap backing and predominantly Helmholtz resonant absorption when there is an air-gap backing.

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Acoustic absorption behaviour of an open-celled aluminium foam

Calculation of sound absorption characteristics of porous sintered fiber metal

In this letter, the low and high frequency limits of the effective density characterizing a limp frame porous medium are investigated. These theoretical limits are compared to the ones found for a classical rigid frame porous medium, and to experimental measurements. While the high frequency asymptotic behaviors of both limp and rigid effective densities are usually only slightly different, their low frequency behaviors are significantly different. Compared to experimental measurements performed on a limp frame fibrous layer, only the limp frame effective density yields good correlations over the whole frequency range.

Comments on the limp frame equivalent fluid model for porous media

A method for measuring the characteristic impedance and propagation constant of porous materials is described in this paper. Measurements were performed based on a surface impedance method that required a set of distinct acoustic impedances derived at the material surface. This requirement is satisfied by arbitrarily changing the air space depth behind the material, and then a new formulation is derived so that a recently developed method of determination, called the transfer function method, can be applied. An appropriate set of air space depths is also discussed. Glass wool and porous aluminum were used to assess the usefulness of the present method. The normal acoustic impedance and normal absorption coefficient of the test materials with arbitrary thicknesses or with an arbitrary air space depth behind them were calculated from the obtained characteristic impedance and propagation constant and were compared with the measured values that were obtained directly by using the transfer function method. The good agreement achieved suggests that the present method is reliable and effective enough to measure the characteristic impedance and propagation constant over a broadband frequency range.

http://pcfarina.eng.unipr.it/Public/Standing-Wave/Bulk%20Properties_1.pdf

Transfer function method for measuring characteristic impedance and propagation constant of porous materials

A porous sound absorbing structure (10) capable of suppressing the lowering of sound absorbing performance in a wide frequency band, comprising an external plate (1) and a closing plate (2). Also, an internal plate (3) is disposed between the external plate (1) and the closing plate (2) through air layers (4) and (5), and the external plate (1), the internal plate (3), and the closing plate (2) are disposed oppositely to each other. A large number of through holes (1a) and (3a) are formed in the external plate (1) and the internal plate (2). The internal plate (3) is set so that a plate thickness t2 and the diameter D2 and opening rate β2 of the through holes (3a) can have viscous effect on the air flowing in the through holes (3a). The opening rate β1 of the through holes (1a) in the external plate (1) is set to more than 3% to 50%.

Porous sound absorbing structure

A three dimensional cavity was studied using the boundary element method. The sound absorbing material was considered as bulk reacting. Two cavity models were utilized to assess the accuracy of calculation: one was a cavity with a bulk of sound absorbing material within the interior, and the other was a larger cavity divided into two smaller cavities by a panel of the sound absorbing material.

Prediction of sound fields in cavities with sound absorbing materials

A Method for the Analytical Prediction of Insertion Loss of a Two-Dimensional Muffler Model Based on the Transfer Matrix Derived from the Boundary Element Method

PURPOSE:To provide a radiant panel for the ceiling type cooling/heating which is easy to install at the jobsite, has good heat conductivity between the panel and a temperature medium flow pipe, and is free from possibility of marring the appearance of the interior. CONSTITUTION:A panel body 1 is formed by extrusion of an aluminum alloy and provided on the upper side with pipe receivers 2 each of which is U-shaped in cross section so as to allow a temperature medium flow pipe to be fitted in and placed continuously in the longitudinal direction of the panel body 1. At one end in the direction of the breadth of the panel body 1 a vertical plate 8 is formed with a fastening part 4 extending continuously and horizontally therefrom and a fixing part 3 connected to the fastening part 4 as a part to be screwed to the lower material of the ceiling. At the other end in the breadth direction of the panel bode 1, a vertical plate 7 is formed with a fastening piece 5 extending horizontally therefrom as a part to be locked with a fastening means of the adjoining panel body.

Tanaka Toshimitsu

Papers

Transfer function method for measuring characteristic impedance and propagation constant of porous materials

Porous sound absorbing structure

Prediction of sound fields in cavities with sound absorbing materials

A Method for the Analytical Prediction of Insertion Loss of a Two-Dimensional Muffler Model Based on the Transfer Matrix Derived from the Boundary Element Method

Radiant panel for ceiling type cooling/heating