Frequency separation of signals and interferences in noise-immune ultrasonic testing of articles manufactured from complexly structured materials

Methods for extracting ultrasonic echo signals from interferences (electroacoustic interference and structural noise) correlated with a probing signal are studied in detail. These methods are based on the frequency separation of the spectra of the informative signal and noise. Frequency-separation methods involving lock-in signal detection are considered. Various aspects of using a lock-in detector in flaw detection and tomography of complexly structured articles are analyzed. Examples of applying the frequency decorrelation of signals in ultrasonic testing of large old cast bronze bells of the Ivan the Great Bell Tower in the Moscow Kremlin and large bells of the Cathedral of Christ the Savior in Moscow are considered.