# Dental Condition as A Factor Modifying the Transmission of the Sound Vibration in the Skull Bones
## 4 Discussion  
Currently, a direct translation of bone cochlear vibrations levels into auditory impressions is impossible. However, to approximate the level of excitation, we can measure the amplitude of vibration around the moving spatially cochlea [24; 25; 26]. Even when measurements are taken at one point and one direction, the loss of information is insignificant [1; 27; 28; 29]. The human skull is a system with a large number of resonant frequencies that oscillate in the acoustic field [24; 30; 31]. At the lowest frequencies, the skull moves as a whole [27; 32]. From 300-400 Hz to above 1 kHz, the skull vibrates as a mass-spring system and its large parts, e.g., temporal bone, mandible, maxilla, or even sets of teeth move in phase [33; 34]. Next, between 1 and 2 kHz, the dominant system of vibrating elements becomes to be affected, and finally, only above 6 kHz dominated by wave transmission [2]. Characteristics of the human skull vibrations and efficiency of HR stimulation depends on the area that the acoustic wave reaches [2; 27]. We have not yet found any study in which vibrations would be applied to the most pronounced massif of the face, i.e., the jaw [24; 28]. The study evaluates the morphology of speed vibrations records around the cochlea after applying vibrations to the alveolar process in different states of dentition. Rs and ARs frequencies, which constitute individual features of the human skull, were taken into account. Each skull showed high individual variability of the recorded vibration velocities. The differences between the frequencies of their r and l side were statistically significant. Simultaneously, we observed several regularities, e.g., similar recordings in TJ, CD, and FD, with RD being distinguishable, which may be the first sign suggesting that patients' teeth are essential in the process of hearing. In the ranges of readable records, we determined unilaterally 10-21 Rs and 9-21 ARs, respectively. Our results are similar to those reported by [PERSON] et al. [24]. Frequencies of the Rs and ARs in TJ, CD, and FD only slightly (\(+/-5\%\)) differed. In about 30% of cases, however, they were each time inconsistent with the \"physiological\" state-RD. Changes in the recording of vibration velocity depending on the state of dentition were most pronounced in the so-called \"range of differences\". For instance, in TJ, the number of Rs and ARs increased there by about 25%. Moreover, the number of frequency incompatibilities Rs and ARs (pathological vs. physiological status) was very high--each time over 70%. Dentures significantly reduced differences. Thus, TJ could be considered as the most different, and FD and CD as more similar to the physiological RD. Up to about 2 kHz, skull segments vibrate separately with teeth, both natural and reconstructed, being one of them. The statistical analysis showed that at such frequencies, the state of dentition most noticeably impacts the transmission of vibrational energy from the jaw to the HR. The lowest amount of energy is transmitted in skulls with RD, the highest in skulls with TJ. Thanks to dentures, the conditions of vibration transmission become similar to the physiological state. The entire frequency spectrum of speech lies between 100 Hz-9 kHz, the 500 Hz-2 kHz band is the most important for speech understanding. The highest percentage of comprehensible words lies subsequently in the range of 1.3-1.6 kHz [35; 36]. Our work has two main limitations: a relatively small number of collected measurements and conducting tests on dry skulls, not on cadaver heads, or on heads of living persons. The small number of samples tested (six skulls, 12 bilateral measurements) is, in our opinion, a relative limitation of our study, because the reason is ethical difficulties in obtaining material, and the similar scientific papers reports are based on an equal number of samples [33; 37; 38]. It has been proven that sound conduction through the skin and soft tissues is weaker by about 15 dB. In our experiments, we used only dry sculls.