# Peak-Frequency Histogram Similarity of Bowel Sounds for the Evaluation of Intestinal Conditions
## 4 Discussion  
This study confirmed, based on the cold water/carbonated water intake test, that PFHS before and after carbonated water intake is significantly lower than before and after cold water intake. Further, PFH, which expresses the peak-frequency distribution of BSs, changed to a larger extent after carbonated water intake. It has been reported that it takes approximately 15 min for carbon dioxide supplied to the intestine to disappear [19,20]. In the present experiment, recordings were conducted for 10 min after ingestion. Accordingly, it was thus considered that carbon dioxide gas remained in the intestinal tract of the subject during the recording period after the ingestion of carbonated water. Therefore, it was suggested that the proportion of gas in the intestinal contents remained at an elevated level compared with its level before carbonated water intake. When tablets are ingested in a fasting state, carbonated water promotes gastrointestinal motility, resulting in faster drug absorption from the intestinal tract than achieved with tap water [8]. Considering the mathematical model of BS generation [16], the resonance frequency of the intestinal wall is thought to be related to intestinal distention and intestinal contents. Additionally, it is \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Frequency Domain Features** & **BSI-ASI** & **BWI-AWI** & \(p\) **Value** \\ \hline Conventional features & & & \\ \hline SC & \(0.98\pm 0.06\) & \(0.97\pm 0.02\) & 0.682 \\ SF & \(0.98\pm 0.11\) & \(0.93\pm 0.08\) & 0.151 \\ SBW & \(0.99\pm 0.25\) & \(0.99\pm 0.11\) & 0.383 \\ PF & \(0.98\pm 0.11\) & \(0.99\pm 0.05\) & 0.505 \\ \hline Proposed features & & & \\ \hline PFHS (Case 1) & \(0.84\pm 0.14\) & \(0.95\pm 0.09\) & 0.008 ** \\ PFHS (Case 2) & \(0.75\pm 0.16\) & \(0.88\pm 0.11\) & 0.021 * \\ \hline \hline \end{tabular} \({}^{*}\) Significant difference at \(p\) \(<\) 0.05. ** Significant difference at \(p\) \(<\) 0.01.  
\end{table} 
Table 1: Characteristics of the frequency domain features of BSs. confirmed that intraluminal gas is bubble-shaped and a smaller intraluminal gas bubble radius results in a higher BS dominant frequency [21]. Therefore, compared with water intake, carbonated water intake is thought to exhibit more changes in intestinal motility and intestinal conditions wherein the intestinal distinction and amount of intraluminal gas may result in additional PFH changes. It was confirmed in the present experiment that the PFHS tended to stabilize as the number of BS episodes increased when the PFH was obtained, and we were unexpectedly able to confirm this tendency from a relatively small number of BS episodes. This suggests that changes in intestinal conditions can be identified based on relatively short data recordings upon the creation of PFHs. Regarding the conventional BS frequency-domain feature quantities, reports have indicating that there are no differences in the dominant and peak frequencies of BSs between patients with and without intestinal obstructions [15; 22]. However, there are also reports that have indicated that BSs with high-frequency components could be confirmed when intestinal obstruction occurs [14]. Additionally, it has been reported that SC and SBW changed depending on dietary intake [17]. Meanwhile, another report showed that SC, F1, and PF did not change, even after the intake of carbonated water [10]. These results provide evidence for the ambivalent opinions on the feature quantities of this system in the frequency domain. In this study, we calculated the rate of change of the frequency-domain feature values of conventional BSs before and after liquid intake. Then, we compared the results between water and carbonated water intake experiments; however, no significant differences were found. A previous report also indicated the possibility of capturing changes in postprandial intestinal motility using frequency-domain features by setting the fasting time to a sufficiently long period [17].