Abstract: In ultrasonic inspection,transducers with different center frequencies are used depen-ding on a number of factors such as thickness of the sample,resolution desired,and depth and orientation of the flaw.This poses a major problem when the data is analyzed using automated signal classification systems.Most pattern classification algorithms rely heavily on the shape of the signal,which can vary considerably with frequency.In order to recognize the same class of signals with different frequencies and classify them in such a manner that the system performance is independent of frequency,a frequency invariance technique using time scaling is described in this paper to transform different signal frequencies to a reference frequency.Two different databases containing weld inspection flaw signals were acquired,namely,the scaled database and the unscaled database,compared in quantity by sort separability criterion.Experimental results demonstrate effectiveness of the time scaling for invariance to the effect of transducer frequency.