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2. GOST (State Standard) 23702-90: Nondestructive Testing. Ultrasonic Transducers. Test Methods, 1990, p. 57.
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5. Vainshtein, L.A. and Vakman, D.E., Razdelenie chastot v teorii kolebanii i voln (Frequency Separation in the Wave and Vibration Theory), Moscow: Nauka, 1983.