## 4 Strategies for Separation of AE Impulses from the Background Signals of Various Types
### Algorithm for Isolating the Useful Signal against the Background of Periodic Local Acoustic Pulses  
In the case of periodic local acoustic pulses of various intensities (Type 1 background signal), the processing strategy depends on the time at which the useful signal appears and on its maximum amplitude. If the acoustic emission pulse occurs in the intervals between the background acoustic pulses (Figure 12), then the processing strategy involves:  
* Isolation of a complete set of single pulses;
* Parametric filtering of single pulses, whose spectrum maximum frequency does not exceed 200 kHz, or multiparameter cluster analysis of the entire set of single pulses (the cluster associated with background pulses is excluded from further consideration). When the useful signal (AE pulse) is superimposed on the background acoustic signal with different degrees of overlapping, the strategy for isolating the useful signal (the ratio of the AE pulse amplitude to the background pulse amplitude increases) depends on the maximum AE pulse amplitude. We use the empirical mode decomposition (EMD) method to increase the ratio of the AE pulse amplitude to the background pulse amplitude. Decomposition of the signal composed of the superimposed AE pulse and the background acoustic pulse (Figure 13) into empirical modes is shown in Figure 14.  
Figure 12: Example of the signal with a sequential arrangement of an AE pulse and a background acoustic pulse. Figure 14: _Cont_. Figure 13: Example of the overlapping AE pulse and single background acoustic pulse (**a**) and the results of their overlapping (**b**). As one can see, the high-frequency signal component corresponding to the AE pulse is most fully represented in the first mode, where the value of the second maximum associated with the background pulse is significantly reduced. Thus, to increase the ratio of the AE pulse amplitude to the background pulse amplitude, it is necessary to pass from the initial signal to its first empirical mode. When the difference between the amplitudes of the AE pulse (the amplitude of the AE pulse is less) and the superimposed background pulse is \(-6.3\) dB in the first empirical mode, the local maxima associated with both signal components are equal. To determine the relative number of reliably detected AE pulses from all the pulses recorded as a detection threshold in laboratory experiments, we take the minimum amplitude of the pulses, the total number of which is more than 1%, minus 6.3 dB. For the  
Figure 14: Decomposition of a signal into empiric modes (the first six out of 15 modes are shown). maximum intensity of the background signal, the detection threshold was 63.7 dB, for the number 3 intensity was 61.7 dB. Figure 15 gives the relative number of AE pulses detected in each of the laboratory experiments (uniaxial tension of the woven laminate sample cut along the warp and weft directions and the unidirectional CFRP sample cut along the fiber direction) for different background signal intensities. It is seen that, in the case of periodic local acoustic pulses, the relative number of detected pulses is larger than that in the case of a continuous noise-like signal. Figure 16 presents the relative detection time of the AE pulses comprising of 5, 10, and 20% of all pulses for various background signal intensities. Figure 16: _Cont._  
Figure 15: Relative number of detected AE pulses in each of the laboratory experiments (uniaxial tension of the woven laminate sample cut along the warp and weft directions (**a**) and the unidirectional CFRP sample cut along the fiber direction (**b**) at different background signal intensities (**a**–0.9 mV, **b**–10 mV). The results demonstrate that, on average, the detection time for AE impulses comprising of 20% of the total number of impulses does not exceed 70% for all modes and types of the experiments, and the obtained values correlate with the results for the continuous background signal.