# Omnidirectional Haptic Guidance for the Hearing Impaired to Track Sound Sources
## 5 Discussion
As shown in Figure 3, the subject was almost facing the front of the loudspeaker, and the increase in the mean value of the standard deviation estimated from the directional characteristics of the mean was slight (about \(2\)-\(3^{\circ}\); Figure 6). Therefore, the time variation of the mean of the standard deviation (Figure 15b) can be obtained without having to depend on the source direction. As the distance between the subject and the loudspeaker reached about \(2.69\) to \(1.40\) m, the mean value of the standard deviation gradually decreased from about \(70\) to \(50^{\circ}\). The time difference between the two ears was near zero, indicating that the subject was directly facing the direction of the loudspeaker. Even though the direction of the loudspeaker from the subject's point of view changed totally by about \(24^{\circ}\) before and after the movement, the output voltage to the left and right vibrators (Figure 15c) remained almost the same. This indicates that the subject kept changing the direction of his face so that the vibration intensity of the vibrators on both shoulders became equal. The movement of the loudspeaker was set to a relatively slow speed of \(3.8\) cm/s. The speed at which the sound source direction changed from the subject's perspective was about \(0.6^{\circ}\)/s. When tracking the sound of a vehicle on a road, it is necessary to consider the validity of the moving speed. The farther away the vehicle, the lower the speed should be at which the direction of the sound source changes. In real scenarios, hearing-impaired people would benefit most if it were possible for them to perceive sound sources from directions that are not included in their visual field. Although experimental data are not shown, the proposed system is capable of detecting the direction of a sound source not only in front but also to the side and rear of the subject's field of vision and of providing notification. The authors reported a method of detecting the direction of lateral and backward sound sources and of providing notification with vibrating vibrators attached to the subject's shoulders [39]. The method of driving the vibrators is simpler than that shown in Figure 11. If the sign of the time difference between the two ears is positive, the left vibrator is driven by constant voltage, and if it is negative, the right vibrator is driven by constant voltage. The subject was instructed to turn his face to the vibrating side. Both vibrators vibrated only when the subject faced the direction of the sound source. The subject could continue to change the direction of his face by relying on the vibrations to indicate the direction of the sound source, not only in front but also to the side or behind. In this system, the right and left vibrators are driven by constant voltage when the time difference between the two ears is less than \(-300\)\(\upmu\)m and more than \(+300\)\(\upmu\)m, respectively, as shown in Figure 11, so it should be able to present the direction of lateral and backward sound sources as well as the previous system. Therefore, even if the hearing-impaired person is looking to the right in order to see a vehicle when crossing the road, this system can still provide notification of a vehicle approaching from the left or rear. The evaluation was performed by conducting measurements of a sound source with the sound of a motorcycle engine. For real applications, common sound sources (e.g., sound of a car motor) need to be considered. In this system, both the direction of the sound source and the distance to the source were obtained from the interaural phase difference of the sound pressure. This is performed to take advantage of the fact that phase is independent of amplitude. The phase difference spectrum can be obtained independently of the sound quality. The authors reported that the spectra of the phase difference detected by two spatially separated microphones are almost equal when noise and voice are generated from loudspeakers [42].