# Bone Conduction Auditory Navigation Device for Blind People
## 1 Introduction
The bone-conducted navigation sounds [11;14] are used to increase the ability to hear environmental sounds. Although, the applicability of the presented bonephones to efficient audio navigation was confirmed, the importance of spatialization algorithms using such a database of HRTFs (head-related transfer functions) was also mentioned [14]. Head-related transfer functions are generally used to investigate sound localization by air-borne sound transmission, and the detailed mechanisms of the HRTFs are investigated [15]. This concept may also be applied to bone-conducted sound transmission. [PERSON] et al. experimentally investigated the bone-conduction transfer functions inside human heads using otoacoustic emissions [16]. In contrast, [PERSON] et al. numerically developed a whole-head human finite-element model to simulate the transmission of the bone-conducted sound [17]. [PERSON] et al. developed head sensitivity mapping to investigate the bone conduction reception by a human head [18]. Although the abovementioned studies have been conducted, binaural reproduction technology in the bone-conducted sound field has not been established, as compared to that in air-conducted sound. Moreover, this technology cannot be applied to localization in the direction of clock positions every 30\({}^{\circ}\) in the horizontal plane, which is generally used by visually impaired people when they recognize the direction. Therefore, a navigation system that uses binaural bone-conducted sound reproduction has not yet been put into practice. If the accuracy of the binaural reproduction technique using bone-conducted sounds can be improved to the level of that using air-conducted sounds, then this improvement in accuracy will be greatly beneficial to both healthy and visually impaired people. In the present study, we develop an auditory navigation system based on binaural bone-conducted sound and obstacle-detection techniques. First, the system reproduces the bone-conducted sound of a voice announcement, i.e., in which direction to move at each corner point. A binaural flashing sound that indicates the direction of the next movement near the corner point is reproduced as the bone-conducted sound, in addition to the voice announcement. The appearance of objects such as other walking people is then announced. Such objects can be detected by the you only look once detection algorithm. Here, a binaural warning sound indicating the direction of the object can be reproduced by bone-conducted sound in addition to the announcement of the appearance of objects. To investigate this navigation system, first, a panning scheme is adopted for the binaural bone-conducted sound to improve the sound localization performance, and the misjudgment of the perceived direction is effectively decreased, especially under a panning angle of 20\({}^{\circ}\). Second, an efficient warning sound is designed through a subjective evaluation investigation in three phases. Through a basic investigation of Phase 1, the risk factor, which is most important as a warning sound, is the highest as a result of the subjective evaluation experiment by the semantic differential method, for the case in which the sound is reproduced as a square wave at a single frequency of 1.2 kHz. Then, in Phases 2 and 3, the appropriate setting of the on/off timing of the square wave reproduction is investigated so that both of the risk and comfort factors have a relatively higher score. Finally, a navigation experiment is conducted using the proposed system, including the obtained panned bone-conducted binaural sound and the efficient warning sound, and the effectiveness of the proposed system was investigated.