A computational method for spectral analysis of the electrophysical test results based on sequential mathematical algorithm transformations using a discrete linear response function has been developed. The procedure for constructing spectral functions has a certain order and is aimed at obtaining adequate results of the experimental sample approximation in the frequency domain. It is shown that use of the low-frequency FIR filter function as part of the convolution, together with the fast Fourier transform, gives accurate results for structural inhomogeneities localization in welded joints.