Sound dissipation in porous media

0.1 . The particular field that has been the concern of this work is that of Building Science. The porous media of interest are consequently those commonly used as absorbents in Architectural Acoustics The object of the work has been to formulate quantitatively a theory of the dissipation of sound in such materials, so that a basis can be laid for optimising and predicting their coefficients of absorption. The theory has aimed at avoiding the inclusion of empirical constants. 0.2 A review of literature is made involving a somewhat wider range of porous media, including those of interest in the fields of Geophysics and Engineering Geology. Porous fluids, a term employed by A.B. Wood (in "A textbook on Sound" Ch.3 ) , as they occur, for instance, in Underwater Acoustics are also considered. Further, the literature concerned with sound propagation in more general inhomogeneous and composite fluids and solids, is examined, where the theoretical techniques are relevant to our study. 0.3 It is found that the literature specifically related to sound absorbing materials and also to unconsolidated or consolidated granular media:- (a) develops theories which are essentially macroscopic and do not allow adequately for the microstructure of non-isotropic flexible framed media i.e. fibrous media. (b) provides little realistic description of the dissipation in closed pore viscoelastic absorbers e .g. cellular rubber. 0.4 A theoretical technique, previously reserved for problems in under- water acoustics and sound propagation in suspensions is applied, as an alternative, to cases of fibrous and viscoelastic foam media. The predictions of absorbtion thus obtained for fibrous media are found to correlate reasonably with experimental data on glass fibre block samples. Further, an explanation of the physics of sound absorption in cellular viscoelastic media is suggested and conclusions and observations of previous literature are corroborated. The literature is examined in the wider context previously mentioned. As might be expected 9 the field of interest has determined the particular type of porous medium considered; the model assumed; and often the theoretical technique. The foldout diagram shows these links, together with those discussed in the thesis and provides a classificaion for the review. Models numbered in the chart are now discussed