Measurement and enhancement of acoustical properties of porous materials

Bryan Heukjin Song, Purdue University

Abstract

In this work, a method for evaluating the characteristic impedance and wave number of homogeneous and isotropic porous materials is described. To implement the procedure, a two-microphone standing wave tube was modified and an existing transfer matrix method was made more efficient. Good agreement was found between the estimated acoustical properties of glass fiber and theoretical predictions. It has been noted that the absorption coefficient of a porous sample placed in a standing wave tube is affected at low frequencies by the sample's edge-constraint. Measurements made using the new transfer matrix procedure suggest that the edge-constraint results in a shearing resonance of the solid phase of the sample, at which frequency the transmission loss is a minimum: below that frequency, the transmission loss increases with decreasing frequency to a finite, low frequency limit. The transmission loss in this frequency range is typically larger than that predicted on the basis of the mass law. It was found that this constraint effect could be modeled accurately by using a finite element model based on the Biot poroelastic theory. Laser vibrometer measurements were then used to verify the effect of the edge-constraint on circular glass fiber samples and to visualize the samples' mode shapes. The laser measurements confirmed the existence of a shearing resonance at the transmission loss minimum. The measured spatial mode shapes were found to be in good agreement with corresponding FE predictions. The effects of placing axial and radial constraints within a fibrous sample were then considered. By appropriate choice of internal constraints, it was found possible to control and extend the frequency range over which the low frequency transmission loss enhancement occurred. Finally, it was demonstrated both experimentally and by using FE models that the low frequency transmission loss enhancement created by using internal constraints also occurs in three-dimensional geometries. Thus, it is possible to create systems that simultaneously display effective barrier and absorptive properties. For example, it has been shown that a segmented liner is more effective at reducing interior noise levels resulting from structure-borne noise than is a conventional homogeneous lining.

Degree

Ph.D.

Advisors

Bolton, Purdue University.

Subject Area

Mechanical engineering|Materials science|Acoustics

COinS