Absorbent materials, poroelastic in nature, have very complex acoustic wave propagation characteristics in terms of the physics and mathematics defining the phenomenon. Even for a single layer of absorbent, the mathematics governing the physics of the material under acoustic flow conditions is quite daunting. A critical review of the progress in the theories (and inherent assumptions) is being presented. Proper engineering judgment might allow a more simplified numerical simulation to get an idea of the complex acoustical behavior. More often it is the lack of availability of proper data which drives the numerical simulation to be done under certain inexact approximations. The present article tries to lay down the theories and semi-empirical relations developed over the past few decades, and assumptions allowing a simplified model, which at least help in the acoustic characterization of a complex problem. With growing emphasis on a ``better, safer, quieter'' acoustic environment and mandatory noise control measured in different countries, there would be an ever increasing demand for effective acoustic design and absorbent materials would play a key role in several applications. As would be clearly apparent from the current article, the available ``standard commercial'' data on absorbent materials gives an estimate of what is happening but does not contain enough information to accuratelypredict the physics of the problem in certain situations.