## 2 Experimental Analysis
### Results and Discussions
It is clear that the two spectra are close when the region analyzed is inside the gray region, that is, when the critical state is reached. Although the \(b\)-value and NT analyses yield results mostly in agreement for both data sets, some significant discrepancies are noticeable in terms of (_i_) differences in total counts for each acquisition system, the (_ii_) accumulated AE energy, and (_iii_) signal duration time. As we discuss now, such discrepancies are most likely attributable to differences in the following factors for each acquisition system: threshold levels, sensors characteristics, and the sensibility of signal-conditioning circuits. The influence of the different threshold levels can be somewhat directly addressed here. As for the other factors, a deeper understanding can be reached only through further analyses in future work. Figure 7 represents the duration of the signals (in \(\upmu\)s) recorded by the two acquisition systems versus their corresponding number of oscillations (counts or threshold crossings), which calculates the signals' frequencies. Although the nominal operation ranges for the two systems are very similar (20 kHz-600 kHz for Mistras, 15 kHz-625 kHz for Lunitek), the frequency ranges of captured signals are very different: between 5 and 94 kHz for Mistras, and between 5 and 45 kHz for Lunitek. Furthermore, the accumulated counts with the first system are much larger than with the second one. Finally, both systems registered similar counts for large- and medium-amplitude signals (80-110 dB and 60-80 dB, respectively), whereas significant differences appeared for signals between 40 and 60 dB, which is precisely the range where the threshold differences occur: 40 dB for Mistras; 49 dB for Lunitek. Thus, although the \(b\)-value and NT parameters tend to be the same for both data sets, their calculation is affected by the number of available samples. Figure 8 presents a comparison between the Lunitek (grey background) and Mistras (purple background) data sets in terms of their statistical distribution of peak amplitudes (db), average frequency (\(AF\)) (kHz), and signal duration (\(\upmu\)s). The histograms' information is complemented in Table 1, containing their corresponding average values (\(\langle\,.\,\rangle\)), standard deviations (\(\sigma\)) and coefficients of variation (\(CV\)). Once again, the threshold difference clearly affects the shapes of all three distributions, especially the peak amplitude. It should also be noted that the higher signal duration in the case of the Lunitek results appears in the tail of the histogram presented in Figure 8c. The \(AF\) distribution for both acquisition systems is similar with a sharper peak for the Lunitek system and a flatter profile for the Mistras system. The different threshold levels are certainly an important cause for this effect, as illustrated in Figure 7. With a lower threshold, the Mistras system registers a broader spectrum of frequencies because smaller cracks tend generate AE signals with a lower amplitude and higher frequencies (or equivalently, lower duration times). As preliminary conjectures, one can also suggest other causes for the observed differences in the frequency distribution. For instance, the sensors from each system differ in mass and geometric dimensions, which affects their resonance characteristics. More specifically, since the Mistras sensor has a larger mass (105 g compared to 52 g for Lunitek), it is also expected to present a higher damping ratio, which makes it difficult to record different events that are close to each other because they would appear to be a single event in the sensor's response. Such challenges will be investigated in future work. Figure 5: \(b\)-value and NT analysis, data from the Lunitek system: (**a**) compressive-load and AE signal amplitudes, (**b**) average frequency (\(AF\)), (**c**) signal duration, (**d**) instantaneous and accumulated signal counts and normalized AE energy (\(E_{AE}\) ), (**e**) \(b\)-value evolution and (**f**) natural time analysis.