Locally and Nonlocally Reacting Flexible Porous Layers; A Comparison of Acoustical Properties

Abstract

The acoustical properties of a porous layer backed by a rigid wall are studied with particular attention to the difference between locally and nonlocally reacting layers and the role of flexibility of the material. Propagation constant, impedance, reflection, and absorption coefficients are discussed as functions of frequency and the angle of incidence. Characteristics frequencies and scaling laws are considered and the conditions for maximizing the absorption coefficient are determined in terms of the normalized layer thickness D/λ, where λ is the wavelength. The possibility of using the flexibility of the material for the purpose of creating a low frequency resonance with high absorption without significant loss at high frequencies is explored.