Effect of circumferential edge constraint on the acoustical properties of glass fiber materials

Abstract

It has been noted that the absorption coefficient of a porous material sample placed in a standing wave tube is affected at low frequencies by the nature of the sample’s edge constraint. The edge constraint has the effect of inhibiting the motion of the solid phase of the material. The latter can be strongly coupled to the material’s fluid phase, and hence the incident sound field, by viscous means at low frequencies. Here the absorption effect noted earlier was demonstrated experimentally. The main focus of the work, however, was on a corresponding transmission loss effect. The material considered was aviation grade glass fiber in two densities. It was found that the edge constraint results in a shearing resonance 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 proportional to the sample’s flow resistance. It was found that the constraint effect could be modeled by using a poroelastic finite element model. It was also found that the transmission loss of the edge-constrained samples approximated that of unconstrained samples at frequencies above approximately 100 Hz when measured in a 10-cm-diam tube.