This study, for the first time, investigated the correlation between acoustic parameters and masticatory parameters to assess masticatory performance. The sound index, the mastication sound intensity (MI), showed a significant difference between genders in evaluating the masticatory performance, and it was significantly correlated with the critical masticatory parameters in two chewing studies. Moreover, compared to the masticatory parameters, the acoustic parameters showed more accuracy and comprehensiveness during data collection and more convenience in experimental process. This demonstrated that the further study of acoustic parameters evaluating mastication is meaningful.
Considering the sound sensitivity, the natural test food, raw peanut, was chosen to act as the masticatory sample, which is more fragile than many other test samples . Compared with CC and CT, which were demonstrated to be valid indicators that can be used as alternatives to the homologous indicators obtained by electromyography in early research , CF was more valuable than the other parameters [28,29,30,31,32]. Meanwhile, analysis of the indicators of the median particle size value (D50) of the food bolus granulometry images was a more reliable and effective method to assess the masticatory performance [9,10,11]. Concomitant evaluations of bolus granulometry and kinematic parameters in the same mastication process appeared to be good criteria for assessing masticatory performance . In summary, based on a previous study, the median particle size (D50) was a better indicator to evaluate the mastication in various research, and kinematic parameters provided valuable guidance [28, 33,34,35,36,37,38].
In the analysis of difference by sex, CC, CF, D50a and MI a showed significant statistical differences. In a previous study, on the condition that subjects were asked to chew the given food, the masticatory motion intensity of women was more strenuous than that of men, and the indicator of chewing frequency (CF) and the median particle size (D50) presented the significant differences between men and women [28, 31, 39, 40]. These results are in accordance with this study, and the variation in CC, CF and D50a in this study indicated that women showed a higher chewing degree while masticating the quantitative food [11, 26, 27, 35]. Meanwhile, in this study, the analysis of MIa, which represents the loudness and energy of the sound wave , displayed a significant difference by sex. As the sound wave created by mastication was a single tone and involved solid-bone transmission, the parameter of mastication sound intensity (MI) indicated the sound energy of the occlusion produced from masticatory movement [41, 42]. Hence, the variation in MIa by sex was in accordance with D50a, CF and CC. This result suggested that an indicator of mastication sound intensity (MI) would be valuable in evaluating mastication.
In the following analysis of correlation among acoustic and masticatory parameters, MIa revealed that a significant negative correlation existed with D50a (r = − 0.94), and a significant positive correlation existed with CF (r = 0.82). In the studies of the masticatory performance with fixed chewing strokes, the test food was in accordance with a previous study, and the number of chewing strokes was the average chewing cycle . As shown in the results, in all acoustic parameters, MIb was significantly negatively correlated with D50b (r = − 0.85). As a result, the more strenuous the mastication subjects made, the more energetic the soundwaves and the smaller food the bolus they produced. These results indicated that mastication sound intensity (MI) would be a meaningful indicator of masticatory performance.
Early studies mainly focused on the gnathosonic which is related to occlusal stability and interference [13,14,15,16]. To date, the related studies have mainly been based on the classification in occlusal sounds of Watt’s work [44, 45]. Due to limitations of adapterization and analysis methods, the subsequent doubt centralized the methodology, which lacked meaningful data analysis and complete sound capture . Hence, there were few reports about acoustic studies in the stomatology area. The problem was due to the narrow acoustic range of occlusal sound and the deficiency of noise filtering in conventional sound sensors. With the bone-conducted tech which established the independent skeleton path of sound transmission and provided a wider spectrum to annotate the sound data, sound capture could be accomplished without the disturbance of background noise [21, 22].
In the vast majority of studies about gnathosonic and chewing sounds, the indicator analysis was confined to sound frequency and was compared with electromyography. Although the sound frequency was significantly correlated with the value of electromyography, it could not be used to evaluate the mastication process [13,14,15,16,17,18,19, 41]. Moreover, previous studies of chewing sound focused on the behavior of chewing and swallowing and the measurement of food texture, rather than investigating masticatory performance [16,17,18, 42]. Essentially, the sound frequency is more relevant to the vibrational speed of soundwaves than to the vibrational energy . It is easy to understand that the mastication sound pitch (MP) and gnathosonic pitch (GP) were not correlated with the masticatory parameters. For gnathosonic intensity (GI), it was concluded that the physiological status of normal mastication varies significantly from that of occlusion without food. Furthermore, the gnathosonic intensity (GI) represents the energy of the direct contact of molars, which may be related to the texture of surface of molars and method of occlusion, instead of the status of mastication. In summary, the mastication sound intensity (MI) could be a meaningful indicator to evaluate the masticatory performance.
In addition, we found that, compared with conventional methods, the acoustic parameter data were acquired more accurately and conveniently by annotating the waveform in sound analysis software. The chewing cycle (CC) could be counted synchronously by the emergence of soundwave crests. Meanwhile the chewing time (CT) could be annotated distinctly with the variation of the chewing sound waveform. Furthermore, we could accomplish all of these experiments in a single quiet room without laboratory processing. In summary, the bone-conduction equipment and sound analysis has potential for masticatory studies.