**5. Conclusions**

It has been indicated that there is a possibility of predicting the occurrence of fcr based on the analysis of sound spectra 3D, the occurring groups of micro-events that precede the end of the load-deflection proportionality area.

The control of micro destruction process before the first crack fcr with the use of acoustic emission (AE) by means of recorded micro-events (increase sumAE) and spectrograms can be used. These analyses should be expanded by adding an interpretation of individual sound spectra (and not the groups of them) and correlated with the load-deflection curve.

The 3D spectra provide a better image of the whole destruction process (particularly with respect to micro-events). The identification of individual events should be correlated with the corresponding individual spectra including the analysis of spectrum in various frequency ranges.

It has been confirmed that the use of spectra with respect to grouped events (destruction processes), in particular in the case of 2D spectra, results in incorrect conclusions if it refers to different events.

**Author Contributions:** D.L. analyzed the test results and performed editing, prepared the specimens, and analyzed the test results, K.S. conducted the correction of results and applications. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflicts of interest.
