I will describe a speci_c project which involved the understanding of the basic physics of acoustics in porous and permeable uid saturated media. The end product is a commercially available measurement of the uid-ow resistance of porous rock in a real oil-_eld borehole which we deduce from an analysis of the measured attenuation and dispersion of guided acoustic waves in the borehole. One key ingredient of the understanding was obtained by laboratory measurements of the acoustic properties of a porous sample saturated with superuid He4. Another key ingredient is the theoretical understanding of the properties of the frequency dependent uid-ow resistance, considered as a causal response function, extended to complex-valued frequencies. I will conclude with a discussion of our new results on nonlinear three-wave mixing experiments on guided acoustic waves.