Effect of solid phase properties on the acoustical performance of poroelastic materials

Ryan A Schultz, Purdue University

Abstract

Efficient design of noise control treatments can have significant positive impacts not only on the acoustical performance of the treatment, but also in terms of optimizing cost and weight. A key component in the efficient design of a treatment is a clear understanding of the acoustical performance of the treatment components: i.e., the acoustic impedance of not only the absorbent material, but also of any facings applied and the mounting method employed. As melamine foam, and plastic foams in general, are commonly used in a range of noise control applications, from building treatments to aircraft fuselage liners, a detailed understanding of the acoustical performance of these poroelastic materials is important to the design of noise control treatments. Models based on the Biot theory have been widely applied to predict the acoustical properties of poroelastic materials: i.e., materials consisting of an elastic matrix (the frame) forming pores that are filled with an interstitial fluid. Over the years, many authors have provided improvements to terms in the Biot theory which allow for more accurate prediction of acoustical properties through improved representation of the oscillating fluid flow through microstructures of complicated geometry. However, though it is widely known that plastic foams exhibit viscoelastic behavior when subject to static and dynamic loading, the solid phase properties, namely the shear modulus, typically employed in Biot-based poroelastic models are assumed to be linear elastic with the damping represented by a hysteretic loss factor which has no explicit frequency dependence. In this work, the influence of a foam's solid phase properties on the acoustical performance is explored through model predictions using both linear elastic and viscoelastic definitions for the shear modulus. These predictions are then compared to standing wave tube measurements of melamine foam samples subject to a range of boundary conditions which represent different facing and mounting conditions for the foam that are typically used in noise control treatments. It was found that the boundary condition configurations serve to change the relative influence of the solid and fluid phases in terms of intensity distribution. For boundary condition configurations where the majority of the intensity is carried by the fluid phase, model predictions tended to match fairly well with measurements. However, for boundary condition configurations where the intensity is carried largely by the frame, not only is prediction accuracy diminished, but measurements also become much more sensitive to factors such as sample fit in the impedance tube. The modification of the stiffness term from a linear elastic to a viscoelastic form does have an effect on the predicted absorption coefficients, but only in boundary condition configurations where the bulk of the wave motion is carried by the solid phase rather than the fluid phase.

Degree

M.S.M.E.

Advisors

Bolton, Purdue University.

Subject Area

Acoustics

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