The overall objective of this work was to demonstrate feasibility of adaptive nonlinear signal processing techniques applied to characterization of ultrasonic nondestructive testing (UNDT) waveforms for accurate inferences of flat -bottom hole sizes. The classified waveforms were ultrasonic pulse echoes obtained from two different sets of 7075-T6 aluminum area-amplitude test blocks and three different transducers. The eight flat- bottom hole defect sizes ranged from 1/64 to 8/64 inch in steps of l/64 inch.