Knowledge of the physical and chemical state of the Earth s inner silicate mantle is central to our understanding of plate tectonics, mantle convection, magma generation and the composition of the Earth as a whole. The key to this knowledge is the ability to interpret studies of seismic wave velocities through the deep Earth using laboratory measurements of mineral sound velocities at high pressures and temperatures. Scientists have for many years measured these properties as a function of pressure but due to the experimental difficulties the majority of studies have been performed only at room temperature. Large extrapolations of these data to mantle temperatures are required to link seismic velocity observations with physical and chemical properties of mantle rocks, resulting in large uncertainties that obscure firm conclusions. An additional uncertainty arises because there is currently no primary scale for accurately measuring pressure at high temperatures. In this study mineral sound velocities and densities will be measured in the diamond anvil cell at simultaneous high pressures and high temperatures to at least 50 GPa and 1300K. This will be possible by a pioneering combination of Brillouin scattering spectroscopy, to measure sound velocities, and single crystal X-ray diffraction determinations of density. By making both types of measurements on the same sample while it is maintained at a constant pressure and temperature, the pressure can be independently measured. These absolute pressure determinations will be used to derive a new universal pressure scale for use at high temperatures. Sound velocities of the major mantle minerals will be determined at high temperatures and absolute pressures thereby drastically decreasing the uncertainties in velocity calculations for rock assemblages at deep mantle conditions. The resulting data will be employed to finally interpret a host of seismic observations made at both global and local scales.