20 years ago, Anderson proposed that the thermoelastic properties of the dominant (mantle) phase within the Earth, MgSiO3 perovskite, could be described as a simple Debye solid. In the intervening years ab-initio calculations of the VDOS have become more sophisticated and challenged this view as too simplistic to be useful. HRPD has proved to be the most powerful tool for characterising the crystallographic aspects of structural phase transitions in perovskites, and a reassessment of older data challenges both viewpoints. Using the isochoric heat capacity, and low temperature thermal expansion data from HRPD, we have found a self-consistent modified Debye model that simultaneously that fits both types of experimental data and shows features in agreement with the calculated VDOS. We wish to study other perovskites to see whether this observation is indeed universal for these materials.