Laser-induced incandescence at high-repetition rates can in principle be used to resolve the temporal evolution of soot processes. The intrusive character of this technique, however, requires due care of historical effects associated with multiple exposures of individual soot particles to laser light. On the other hand, repetitive heating and cooling opens up an independent, acoustic detection channel. We illustrate a photo-acoustic soot volume fraction measurement, and show that the comparison to simultaneously recorded laser-induced incandescence provides qualitative information on soot growth. Experiments are performed on a propane-fueled, co-flow stabilized diffusion flame, and signals are collected at varying heights above the burner deck. Results show a clear correlation between the laser-induced incandescence and photo-acoustic signals; small deviations are interpreted as a qualitative indicator for the particle size.