A theory was developed to estimate the change in the apparent sonic velocity in porous ceramics by solving Lavrov's integral equation of the cylindrical inclusion problem. It was shown that the change in apparent sonic velocity was caused by phase shift of the sonic wave. To extend the theory to the case of multiple inclusions, the superposition principle was applied. The theory was verified by analyzing the apparent sonic velocity during a compression test of polycrystalline graphite, and the number of cracks that formed during loading was estimated to be 700, when the dimensions of crack length and width were assumed to be 1mm and 10μm, respectively. © 2009 IOP Publishing Ltd.