## 3 Testing Results
### Frequency Characteristics  
Figure 5 shows the dominant frequency distribution of AE waveforms corresponding to the normalized applied stress (\(\sigma/\sigma_{\mathrm{t}}\)) throughout the whole process of the test. It can be easily observed that dominant frequencies of AE waveforms are mainly concentrated in two frequency bands. The AE waveforms with low band dominant frequencies (1-117 kHz) were represented by the red dots; meanwhile, the AE waveforms with high band dominant frequencies (214-491 kHz) were represented by the blue dots. The figure also shows the AE that occurred during the whole process of the test. This means the samples' global failure is accumulation of microcosmic cracks. For dried samples DM1 to DM4, the AE increased when the normalized applied stress reached 85%. On the contrary, for saturated samples SM1 to SM4, the AE increased when the normalized applied stress reached 80%. \begin{table}
\begin{tabular}{c c} \hline \hline
**Specimen No.** & **UCS (MPa)** \\ \hline DM1 & 67.06 \\ DM2 & 68.86 \\ DM4 & 74.57 \\ SM1 & 61.72 \\ SM2 & 52.90 \\ SM3 & 69.03 \\ SM4 & 61.33 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The UCS (uniaxial compressive strength) of dried and saturated rock specimens. Figure 4: Macroscopic Failure of rock samples (red: shear failure, blue: tensile failure). (**a**) Dried sample; (**b**) saturated sample  
Figure 3: Stress-strain curves of typical dried and saturated specimens. AE waveforms in rocks are mostly lower than 450 kHz, the band range from 0 to 450 kHz was divided equally into the first 45 bands, and the No. 46 band contains the rest. Figure 4 clearly demonstrates that there are two concentrations of dominant frequency bands in both dried and saturated rocks. Following the findings of [PERSON] et al. (2019) and [PERSON] (2019), the feature of double dominant frequency bands is intrinsic for rocks is further confirmed by this observation, regardless of the water content in the rock specimen. Figure 6: Proportions of the dominant frequency bands of AE signals in specimens (**a–c**) DM1–DM4; (**d–g**) SM1–SM4  
As shown in Table 2, the range of dried specimens' percentage of high frequency AE is from 53.41% to 77.03% compared with 17.43% to 42.91% for the low frequency AE. On the contrary, the percentage of saturated specimens of high frequency AE ranges from 12.41% to 43.88% compared with 68.25% to 86.29% for the low frequency AE. The rock specimens' average percentage of low frequency AE was raised from 28.80% to 75.40% after saturation. The range and central frequency band of both low and high frequency bands are shown in Figure 7. For dried specimens, the range of their low frequency band is from No. 2 to No. 11 dominant frequency bands, and the central frequency band is around No. 6 dominant frequency band. Besides, the range of their high frequency band is from No. 22 to No. 28 dominant frequency band, and the central frequency band is around No. 25 dominant frequency band. On the contrary, the range of saturated specimens in both low and high frequency bands varies considerably. Their low frequency band is between No. 2 and No. 16 dominant frequency bands with No. 6 dominant frequency band as the central frequency. The high frequency band is between No. 19 and No. 32 dominant frequency band with No. 25 dominant frequency band as central frequency. The test result indicates that pore water has little effect on marble specimens' central frequency of both low and high frequency bands. This is due to the marble consisting of a single mineral and relatively homogeneous. Besides, the pore water broadens the saturated specimens' range of both low and high frequency bands. This phenomenon reflects that pore water in rocks results in a more complex failure.