Although seismic wave dispersion and attenuation have been found to occur in sedimentary rocks, it remains challenging to experimentally observe these effects. A new experimental setup has been developed to measure the Young’s modulus and Poisson’s ratio of rocks over a wide range in pressure () and frequency (). Calibration with standard samples determined the following: (1) no dependence of the apparatus to pressure and frequency and (2) a good fit between published data and the measured and inferred elastic properties. The measured Young’s modulus dispersion and attenuation of Plexiglas were also consistent with the published data. The Young’s modulus and the attenuation of Fontainebleau sandstone samples saturated by water and glycerin were then measured. Although small variations were observed for one sample, the second one exhibited strong pressure- and frequency-dependent variations of Young’s modulus and attenuation. A frequency-dependent fluid flow was simultaneously measured. The characteristic frequency for these variations was highly fluid dependent. Accounting for the in situ fluids’ viscosity using an apparent frequency parameter, we determined the Young’s modulus and attenuation of a fluid-saturated Fontainebleau sandstone over an apparent frequency band of . The measurements under water and glycerin saturation compared favorably, and two frequency-dependent phenomena were observed that were interpreted as the drained/undrained and undrained/unrelaxed transitions. The undrained/unrelaxed transition occurred in a large frequency range, which was attributed to a distribution in aspect ratio of the rock’s microcracks.