# Peak-Frequency Histogram Similarity of Bowel Sounds for the Evaluation of Intestinal Conditions
## 1 Introduction  
Constipation is a disease that is frequently observed worldwide that causes difficulties in defecation, abdominal pain, and abdominal bloating and reduces quality of life (QOL), thus making treatment difficult. A 2011 meta-analysis estimated the prevalence of constipation to be approximately 14% and reported a higher prevalence in women than in men [1]. Constipation is common in the elderly, and its prevalence increases with age. Constipation is said to be caused by abnormal intestinal motility [2]. However, as there is no simple and noninvasive test to detect intestinal motility disruptions, in many cases in which a patient has not had a bowel movement for several days, he or she is diagnosed with colonic constipation based on an interview and is prescribed laxatives. Given that laxative abuse can lead to watery stools and to anal stenosis [3], it is hoped that constipation can be improved by adopting lifestyle improvements, including improved eating habits. Carbonated water has been shown to contribute to the improvement of constipation by improving intestinal motility [4; 5; 6]. Carbonated water is an effervescent beverage that introduces gas into the digestive system [7; 8]. Carbon dioxide contained in carbonated water stimulates the gastric fundus [5] and gastric distention, which are associated with gastric motility [4]. Furthermore,it has been shown that carbonated water intake improves movements of the entire gastrointestinal apparatus by acting on smooth muscles [5]. Therefore, it is thought that carbonated water intake affects intestinal conditions such as intestinal gas/liquid volume and intestinal distention. Previously, intestinal motility before and after carbonated water intake was evaluated using bowel sounds (BSs), which are measured noninvasively. In such evaluations, time-domain acoustic features extracted from BS signals (e.g., number of BS occurrences per minute, sound-to-sound interval (SSI), amplitude intensity, and BS length) have been used. Significant changes in these time-domain acoustic features were observed following carbonated water intake compared with their values before carbonated water intake [9; 10]. It has also been reported that intestinal motility evaluations can be conducted by using various time-domain BS acoustic features, such as the evaluation of the postoperative intestinal motility recovery state [11; 12] and estimation of the intestinal motility phase during interdigestive migrating motor contraction [13]. However, there are few reports on the acoustic features of BSs in the frequency domain compared with those in the time domain. Peak frequency (PF) [10; 14; 15], first formant (F1) [10; 15], spectral flatness (SF) [16], spectral centroid (SC) [10; 17], and spectral bandwidth (SBW) [17] have been used in such investigations. These BS features in the frequency domain has been used for the evaluation of intestinal obstruction in patients, as well as the evaluation of food/liquid ingestion. Despite the significant changes in time-domain features following the ingestion of carbonated water (compared with the state before ingestion), there are different opinions concerning the acoustic feature quantity in the frequency domain of BSs, and further investigation is required. According to the mathematical model of BS generation [16], the resonance frequency of the intestinal wall is thought to depend on intestinal conditions such as intestinal distention and contents. Therefore, it is thought that the peak frequency of power spectrum changes due to variations in intestinal distention and the amount of intestinal gas/liquid after carbonated water intake. Previous studies on the frequency domain of BSs used basic statistical estimates of basic features, but it is likely that the shape of the distribution of the feature data, which may be associated with the intestinal conditions, was not accurately represented.