The finite difference time domain (FDTD) method is commonly used as a numerically accurate way of propagating sound. However, it requires extensive computation. We present a simple method for accelerating FDTD. Specifically, we modify the FDTD update loop to prioritize computation where it is needed most in order to faithfully propagate waves through the simulated space. We estimate for each potential cell update its importance to the simulation output and only update the N most important cells, where N is dependent on the time available for computation. In this paper, we explain the algorithm and discuss how it can bring enhanced accuracy and dynamism to real-time audio propagation.