Investigation of inverse acoustical characterization of porous materials used in aircraft noise control application
Sound propagation through porous media such as foams and fibers is governed by five parameters that describe the geometry of the porous frame: porosity, tortuosity, flow resistivity, viscous characteristic length, and thermal characteristic length. The conventional laboratory methods for measuring these geometric properties are prone to errors and can be highly cumbersome. In this work, an alternative method of determining the geometric properties of porous materials, based on an inverse acoustical technique, was investigated for materials used in aircraft noise-control applications. This technique is incorporated in commercial software codes, such as FOAM-X (ESI Group) and Comet TrimTM (Comet Acoustics), which require the absorption coefficient and/or transmission loss (TL) to be measured in Brüel and Kjær (or equivalent) standing wave tubes as inputs. The estimated geometric properties are required to define the porous material for complex vibroacoustic analysis in commercial code such as AutoSEA2 (ESI Group). One of the goals of this work was to evaluate the accuracy of the estimated geometric properties. A closed-loop validation technique was previously developed where the absorption coefficient and transmission loss were predicted using AutoSEA2 and compared with the standing wave tube measurements. Good agreement between the measured and predicted absorption coefficient was observed for both foams and fibers. However, in the case of transmission loss, good agreement was observed for fibers but not for foams. In order to eliminate inconsistencies, the existing validation loop was modified by incorporating Comet TrimTM inverse characterization software that took both the normal incidence absorption coefficient and transmission loss as sequential inputs to estimate the geometric properties. To complete the modified loop, sound absorption and transmission loss of porous materials was predicted using the performance analysis module in Comet TrimTM and compared with the test results. In general, the absorption coefficient of most of the foams and fibers, prediction using both validation loops was in good correlation with the measured data. On the other hand, the correlation in normal incidence transmission loss was better using the modified loop. In the process of investigating the repeatability of estimating the physical properties, previously measured porous material samples were re-measured for their absorption coefficient and transmission loss. A possible effect of sample aging was discovered and reported. As an alternate method to the forward TL calculation, a finite element model of the standing wave tube was also developed. This could be used to study the effect of boundary conditions on acoustic properties. Finally, individually validated samples were combined to develop optimized multilayer aircraft noise-control treatments and were experimentally demonstrated to produce excellent acoustical performance.
Thesis (M.S)-- Wichita State University, College of Engineering, Dept. of Mechanical Engineering