## 3 Analysis of Test Results
### Acoustic Emission Characteristics
#### 3.4.3 Main Frequency Characteristics  
Peak frequency serves as a crucial parameter for signal spectrum analysis [38]. It provides insights into the nature of the cracks that are formed [39], with shear cracks representing low-frequency signals and tensile cracks representing high-frequency signals. Figure 11a-e illustrate the scatter plots of the main frequency for sandstone at different soaking times and the trend lines of peak frequency. It can be observed that for soaking times lower than 30 days, the main frequency scatter points are mostly concentrated at 10-80 kHz, 100-120 kHz, and 250-300 kHz. When the soaking time exceeds 60 days, the scatter points at 100-120 kHz gradually disappear, and with a further increase in the soaking time, the proportion of scatter points at 250-300 kHz increases, while the proportion at 250-300 kHz decreases. At 90 days, the scatter points at 250-300 kHz are almost negligible, while a significant portion of the scatter points is concentrated at 10-80 kHz.  
The variational trend curve of the main frequency shows that the values for samples soaked for 0, 7, 14, 30, 60, and 90 days are 220, 200, 160, 150, 140, and 120 kHz, respectively. This indicates that low-frequency acoustic emission signals become more apparent in sandstone samples at higher soaking times. Therefore, with increasing soaking time, the crack patterns in sandstone samples gradually shift from tensile cracks to shear cracks during uniaxial compression failure.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Soaking** & **Peak Impact** & **Decline** & **Cumulative Number** & **Decline** \\
**Time/d** & **Rate** & **Rate/\%** & **of Impacts** & **Rate/\%** \\ \hline
0 & 316 & 0 & 41,964 & 0 \\
7 & 281 & 11.08 & 38,754 & 7.65 \\
14 & 242 & 23.42 & 33,328 & 20.58 \\
30 & 226 & 28.48 & 29,722 & 29.17 \\
60 & 202 & 36.08 & 26,256 & 37.43 \\
90 & 154 & 51.27 & 18,405 & 56.14 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Peak impact rate and cumulative impact count of sandstone at different soaking times.  
Figure 10: Variations in maximum impact rate and cumulative impact count of sandstone with different soaking times.  
Figure 11: Scatter plots of main frequency versus time for sandstone at different soaking times: (**a**) dry state; (**b**) submerged for 7 days; (**c**) submerged for 14 days; (**d**) submerged for 30 days; (**e**) submerged for 60 days; (**f**) submerged for 90 days; (**g**) changes in number and proportion of three frequency bands of sandstone over soaking time.  
To better visualize the changes in proportion of each frequency band, the data from the above bar chart were compiled into the line chart in Figure 11g. It can be observed that the proportion of the 10-80 kHz band gradually increases with soaking time. The proportions at 0, 7, 14, 30, 60, and 90 days of soaking are 21.94%, 29.57%, 31.93%, 45.79%, 81.57%, and 91.38%, respectively. The proportion of the 250-300 kHz band gradually decreases, and the corresponding proportions are 66.71%, 58.17%, 51.14%, 35.73%, 17.65%, and 8.25%. This confirms the pattern identified previously in the qualitative analysis.