Fig. 6: Time evolution of one AE signal (RM2) annotated based on the X-ray movies (on the left) and its spectrogram representing the signal power spectral density (on the right).

The orange signal on the left corresponds to the ground truth, which has a value of 1 when the keyhole regime is occurring and 0 when the process regime is conduction. The vertical yellow (red) dotted lines on the right plot represent the start (end) of the keyhole regime. As can be noticed, most of the signal energy content is located in the frequency range 35–105 kHz when the corresponding regime is keyhole (region highlighted by the horizontal dotted white lines). To obtain the spectrogram, the AE signal is split into segments of 1024 data points each, with 512 overlapping data points between consecutive segments. The Hanning windowing function window is then applied to each segment, and the spectrum of each section is computed.