# Omnidirectional Haptic Guidance for the Hearing Impaired to Track Sound Sources
## 4 Results  
By combining the two ways of driving the vibrators, as described above, it is possible to convey information to the user about the direction of the sound source and the distance to the source through vibration. The vibration intensity ratio of the left and right vibrators indicates the direction of the sound source, and the vibration intensity itself indicates the distance to the source. In principle, if only the intensity of the vibrations is increased without changing the intensity ratio of the vibrators, the user should be able to see the source approaching from any direction. However, the directional dependence (directionality) of the mean value of the standard deviation introduces uncertainty into the distance estimate, and thus, it is considered more effective for estimating relative distance rather than absolute distance. In other words, it may only tell us whether the sound source is approaching or moving away from its current position. In order to simplify the evaluation of the system, the subject tracked a moving sound source. When a loudspeaker fixed on a linearly moving platform crossed a point 2 m away from the subject, we examined whether the subject, wearing an eye mask, could track the moving loudspeaker. The subject, a 21-year-old male student with no hearing impairment, was instructed to turn his face in the direction where the vibration intensity of the left and right vibrators was equal. Four seconds after the start of the loudspeaker motion, the loudspeaker continuously generated the sound of a motorcycle engine. A schematic diagram of the experiment is shown in Figure 14.  
Figure 13: Flowchart of hearing assistance system including subject’s movement (colored). The measurement results are shown in Figure 15; Figure 15a shows the time variation of the time difference between the two ears, Figure 15b shows the time variation of the mean value of the standard deviation, and Figure 15c shows the output voltage to the left and right vibrators. Immediately after the onset of sound, the value of the interaural time difference was as low as \(-60\) us because the speaker was located on the subject's right side (Figure 15a). At that time, the output voltage of the vibrator was higher on the right (Figure 15c), so the subject turned his face to the right. After that, the time difference between the two ears was gradually corrected to around zero. This is because the subject continued to face the direction of the moving loudspeaker according to the vibration of the vibrator. The average value of the standard deviation was 70\({}^{\circ}\) immediately after the onset of the sound but decreased to around 50\({}^{\circ}\) as the loudspeaker approached. The output voltage to the left and right vibrators increased accordingly. As the loudspeaker approached, the vibration of the vibrators gradually intensified, so the subject could see how the loudspeaker was approaching. For an indoor sound source that is moving a relatively short distance, it is possible to accurately notify the subject of its movement. As shown in the flowchart in Figure 13, since the subject's motion is included in the control loop, individual differences in tracking motion are likely to occur. The action of pointing the head in the direction where the vibrations of both vibrators are equal to the movement of the sound source is not always easy, especially when the movement is fast. If the subject does not accurately point in the direction of the sound source, the vibration intensity does not correspond to the distance to the source. By training the subject, the accuracy of the motion of pointing the head in the direction of the sound source could be improved to some extent and the effect of individual differences can be controlled. Figure 14: Schematic diagram of experiment. We examined whether the subject, wearing an eye mask, could track a loudspeaker fixed on a linearly moving platform and crossing a point 2 m away.