## 3 Experimental Results  
### Waveform Characteristics  
The streamed AE signals are selected from the NavaIR gearbox test in the early stage of testing, as no crack is expected to occur and within the acoustically stable regions of testing right after the shaft rotation reaches to its full speed. The AE signals are highly influenced by the initiation and ending of gearbox operation until it reaches the stable rotational speed. The selected streamed AE signal is shown in Figure 4a,b in time domain and frequency domain, respectively. The duration of the streamed AE signal is 150 ms with the amplitude of 50 mV for the selected streamed signal. A digital filter of 20 kHz to 400 kHz is applied to the streamed waveform in order to have the same frequency range as the UIC fatigue test. The sampling frequency of the NavaIR gearbox test is 2 MHz, while the sampling frequency of the UIC fatigue test is 3 MHz. The waveforms of the NavaIR gearbox test is resampled in order to bring both test conditions identical.  
Figure 4c,e shows the time history signals of the UIC fatigue test due to the crack growth, and the pencil lead break (PLB), respectively. The crack signal causes less energy release than the PLB signal. The peak amplitude of the crack growth is 1.292 V for the selected AE event, while it is 4.052 V due to PLB. The amplitude of the crack growth depends on the energy release caused by the crack jump. The histograms of AE amplitudes obtained from the first day of NavaIR gearbox test and the clustered events near the notch tip of UIC fatigue test are shown in Figure 4g,h, respectively. The selected waveforms represent examples of the streamed AE signals and the fatigue crack signal. There are also slight differences in the frequency spectra of two AE sources as shown in Figure 4d,f due to the differences in source functions. The crack growth causes dipole source generated parallel to the planar direction of the specimen, while the PLB source causes monopole source that is generated at about 30\({}^{\circ}\) angled with respect to the planar direction of the specimen.  
On the other hand, there are some differences in the frequency spectra of the streamed AE signal and the crack AE signal. The durations of AE signals are 150 ms and 1 ms for the NavaIR gearbox test, and the UIC fatigue test, respectively. For better understanding the characteristics of the streamed signal, a time window of 1 ms (similar to the UIC fatigue test) is defined to calculate AE power, peak frequency, and frequency centroid, as shown in Figure 5. Additionally, a windowed analysis is applied to the pre-trigger region of the crack signal in order to determine the operational noise of the UIC fatigue test, as shown in Figure 6. The environmental noise characteristics of the NavaIR gearbox test and the UIC fatigue test have some differences, as expected; however, the peak frequencies are similar. Figure 7 shows the AE features in each time window and their distributions. In general, they have similar features and normal distribution.  
The summary of the mean AE features obtained from the entire streamed signal, the windowed streamed signal in comparison to the crack signal, the noise signal of UIC fatigue test, and the PLB signal are shown in Table 2. The entire and windowed stream signals have similar characteristics. The AE power and frequency information of the crack growth signal and the streamed signal are significantly different due to the difference in the energy releases of the operational noise of the NNAVAIR gearbox test and the crack signal of the UIC fatigue test.  
Figure 4: Examples of AE signals and frequency spectra (**a,b**) streamed AE signal obtained from the NNAVAIR gearbox test, (**c,d**) signals due to crack growth obtained from the UIC fatigue test, (**e,f**) signals due to the pencil lead break (PLB) testing obtained from the UIC fatigue test, and (**g,h**) the histograms of AE amplitudes from the NNAVAIR gearbox test and the UIC fatigue test. FFT: fast Fourier transform.