Sonic Infrared (SIR) NDE is a novel nondestructive testing technique that uses a short pulse of ultrasonic excitation along with infrared imaging to expose hidden defects in the structures being inspected. In [17], we presented a model that describes circular subsurface heat sources. This model can be used to describe heat diffusion from impact damage in composite structures during SIR inspection. The model uses certain aspects of the temperature-time curve for defect depth profiling, namely, half-maximum power point, the peak slope point, and the second derivative peak point. In this study, we investigate the effect of defect size on defect’s detectability and the effect of defect size on the quantitative estimation of defect’s depthspecifically, half-maximum power time.