A system and method of performing acoustic thermography in which invalid data is filtered from data used to detect defects on a structure. An ultrasonic sound input signal is provided to a structure to produce a thermal image output. A sensor senses an input energy corresponding to the sound input signal and produces an input energy signal. The input energy signal is transformed to a test spectrum and is compared to a reference spectrum. The comparison of the test spectrum to the reference spectrum is used to determine whether to include the thermal image output in an analysis for detecting defects in the structure.