# Omnidirectional Haptic Guidance for the Hearing Impaired to Track Sound Sources
## 5 Discussion  
It is more effective to apply tactile stimuli directly to the skin, which is extremely sensitive to perception, in order to encourage the behavior of hearing-impaired people. However, the application of electrical or vibrating stimuli to the skin is not appropriate for long-term use because it tends to cause rashes and abrasions due to friction. In this study, two small loudspeaker drivers attached to a west were used as vibrators so that the vessel could be worn outdoors in layers (Figure 9). The vibration amplitude is up to 1.5 mm, which means that a hearing-impaired person could wear this west over his or her clothes when going out, even when wearing layers. The placement of the vibrators, shown in Figure 9, is an important factor that affects the perceptibility of the vibrations. When we tested the placement of the vibrators on the subject's shoulders and back, we found that the protruding area around the scapula was more sensitive to vibration than the area above the muscles. The shape and size of the scapula varies from person to person, and there may also be individual differences in the sensitivity to vibration and the location where it is easily perceived. The best position to place the vibrator needs to be adjusted individually. The degree of adhesion of the vibrator is also important for perception. This can be solved by choosing the best size that is optimal for the hearing-impaired person. The subject who participated in this experiment was not inconvenienced by wearing the hearing assist system. In this study, we tracked a moving sound source in a room to verify whether the hearing aid system could transmit information to the subject about the direction of the sound source and the distance to the sound source through vibration. The vibration intensity ratio of the left and right vibrators indicates the direction of the sound source, and  
Figure 15: Experimental results: (**a**) time variation of time difference between the two ears, (**b**) time variation of mean value of standard deviation, and (**c**) output voltage to left and right vibrators. the vibration intensity itself indicates the distance to the source. In principle, if the vibration intensity is increased without changing the intensity ratio, the subject should be able to identify the source approaching from any direction. However, as mentioned in the previous section, the directional dependence of the mean value of the standard deviation (directivity) is expected to cause uncertainty in the distance estimate. Therefore, we examined whether the subject, wearing an eye mask, could track a loudspeaker when it was fixed on a linearly moving platform and crossed a point 2 m away from the subject indoors. As mentioned in Section 1, the purpose of this study is to provide hearing assistance to the hearing impaired by detecting the location of a vehicle traveling on a road outdoors. Further improvements are needed in the future. [PERSON] et al. (1979) reported that the location of a sound source placed at a predetermined discrete direction and distance could be estimated by subjects through learning. In our system, the sound was detected and processed in real time, and the position of the sound source was tracked continuously by the vibration of the vibrator alone, without the subject having to learn. The subject wore an eye mask and did not use visual information to track the loudspeaker. The time variation of the interaural time difference (Figure 15a) shows that immediately after the onset of the sound, the value of the interaural time difference was \(-60\)\(\upmu\)s. This indicates that the subject's face was initially oriented about \(5^{\circ}\) to the loudspeaker. As the subject continued to face the direction of the moving loudspeaker according to the vibration of the vibrator, the interaural time difference was gradually corrected to near zero. Since the interaural time difference remained below about \(60\)\(\upmu\)s, the direction of the sound source was below about \(5^{\circ}\).