## 4 Strategies for Separation of AE Impulses from the Background Signals of Various Types
### Algorithm for Isolating the Useful Signal from the Continuous Noise-Like Background  
Local acoustic emission pulses against the background of a continuous signal can be reliably recorded when their maximum amplitude exceeds the maximum background amplitude by at least 5 dB. The ratio of the useful signal maximum amplitude to the background maximum amplitude can be increased by the additional initial data processing. To do this, we assume that the main frequency characteristics of the continuous signal (Type 3 background signal) do not exceed 200 kHz; under laboratory conditions this value is higher. We use the frequency filtering, namely the sixth power Butterworth filter with a cutoff frequency of 200 kHz, to \"suppress\" the background noise. Figure 9 presents the results of changes in the maximum amplitude of the third background signal of different intensity after partial filtering. Figure 8: Characteristic background acoustic signals: the single arrival of Type 1 pulses with random intervals (**a**), the signal corresponding to the arrival of Type 2 signals with random intervals (**b**), the combination of both signals (**c**). After frequency filtering, the maximum acoustic signal amplitude decreases varying between 13 dB (1 level of signal intensity) and 8 dB (3 level of signal intensity). Therefore, after the frequency filtering with a high-frequency filter, even if the maximum amplitude of the useful signal remains unchanged, the ratio of the useful signal amplitude to the background amplitude increases, by 10 dB on average. To determine the number of AE pulses that can be reliably recorded at a certain intensity of the background noise, for each type of the laboratory experiments, we have calculated the percentage of AE pulses, the maximum amplitude of which exceeds the maximum amplitude of the Type 3 acoustic signal after frequency filtering by 5 dB. The obtained intensity values are shown in Figure 10.  
Figure 10: Relative number of detected AE pulses for each of the laboratory uniaxial tests (uniaxial tension of the woven laminate sample cut along the warp and welt directions and the unidirectional CFRP sample cut along the fiber direction) at different background noise intensity levels: 0.13 mV (**a**), 0.9 mV (**b**), 4.5 mV (**c**), 10 mV (**d**). Figure 9: Changes in the maximum amplitude of the Type 3 acoustic signal at different levels of its intensity. Analysis of the obtained data has indicated that, at minimum intensity, the background signal level is below the discrimination threshold used for recording AE under laboratory conditions. At increased intensity of the background noise, the situation changes dramatically. A relative number of detected AE pulses decreases, depending on the type of the tested sample, from 2 to 1.15% for the woven laminate sample cut along the warp direction. Regardless of the background noise intensity, the minimum number of detected AE pulses is observed for unidirectional CFRP. An important indicator of the predictive power of the AE method applied to monitoring real parts is the useful signal detection time versus the time of the \"possible\" fracture of the part itself. Calculations gave the detection time of the AE pulses comprising 5%, 10% and 20% of the total number of reliably recorded pulses for the sample of a certain type and specified background noise intensity (Figure 11). Figure 11: Relative detection time of AE pulses comprising of 5%, 10%, and 20% of the total number of reliably detected pulses at different intensities of the background noise for each of the laboratory samples (uniaxial tension of the woven laminate sample cut along the warp (**a**) and weft (**b**) directions, and the unidirectional CFRP sample (**c**) cut along the fiber direction). On average, for each type of the laboratory experiments, the relative detection time of the AE pulses comprising of 5%, 10%, and 20% of all pulses varies slightly, regardless of the background noise intensity.