Monitoring the alternations of the frequency spectrum of transmitted and received signal is essential in the use of ultrasonic spectroscopy for NDT. The comparison of the spectra of the both signals provides much more information than time-amplitude analysis of the signal reflected off a defect. Additionally, in the traditional method, assessment of e.g. the size of defect in case of heterogeneous media gives ambiguous results. This is caused by the differences in attenuation of frequency components of a signal. This work presents the results of measurements of the effect the angle at which a defect is located in relation to ultrasonic beam axis has on spectrum characteristics of signal. Tests were performed for different reflectors located at several angles in relation to the incident wave. The next stage of measurements involved utilizing a different method of Fourier transform, which was modified by using Hann window. The use of window function results in accentuation of the “useful” signal and dampening of the “useless” one (noise). This method causes clear increase of spectrum signal amplitude and more significant shift of the main spectral line in the direction of lower frequencies. The paper also presents examples of using ultrasonic spectroscopy in testing glued joints and of the effect binding material discontinuity has on signal spectrum characteristics.