A system may include a data analysis device that is configured to receive from an ultrasonic waveform detector ultrasonic waveform data representative of an ultrasonic waveform that propagated through a sample and resonated within a defect within the sample. The data analysis device may be further configured to select a portion of the ultrasonic waveform data, apply a Fast Fourier Transform to the portion of the ultrasonic waveform data to transform the portion from a time domain to a frequency domain, identify a characteristic frequency of the portion in the frequency domain, and determine a characteristic of the defect based on the characteristic frequency of the portion. In some examples, the characteristic of the defect may be at least one of an approximate size or an approximate shape of the defect.