This paper investigates the accuracy of the typical experimental practices that estimate the unsteady pressure in the flame region of an unstable combustor from pressure measurements along the combustor walls. This measurement, along with that of the unsteady heat release, is often used to determine information about the flame's driving characteristics. These wall pressure measurements are only meaningful, however, if the unsteady pressure is nearly one-dimensional in the combustor; that is, if the transverse gradient of the pressure in the flame region is small. This paper presents computational results of the interior acoustic field in a combuslor undergoing longitudinal oscillations in order to quantify the difference between the acoustic pressures at the wall (where they are typically measured) and the flame. These results show that the duct pressure at the flame and the wall typically differ in magnitude by five to twenty five percent, and in phase by ten to twenty degrees. It is concluded from these results that wall pressure measurements provide qualitative information about the fluctuating pressure at the flame and can, in some cases, be used in conjunction with heat release measurements to characterize the flame driving.