Enhancement of the barrier performance of porous linings by using internal constraints

In previous work it was shown that a circular sample of fibrous material placed in a standing wave tube is stiffened at low frequencies by the frictional constraint of the sample around its circumference. That constraint makes possible a shearing resonance of the sample that is associated with a transmission loss minimum. Below that frequency the transmission loss increases to a low frequency limit that is determined by the flow resistance of the sample: that limit can be well above the value predicted on the basis of the mass law. Here it has been shown both experimentally and by finite element simulation that the frequency range of the transmission loss enhancement can be extended by placing axial and radial constraints within the sample: i.e., by the use of internal constraints. The materials considered in the present work were aviation-grade glass fibers, and the measurements were conducted using a four-microphone standing wave tube. It has also been found that the mass of the internal constraints must be above a certain threshold for the transmission loss enhancement to be realized. Design guidelines presented here may make it possible to create internally-constrained fibrous linings that enhance the transmission loss of barrier systems at low frequencies.