# Bone Conduction Auditory Navigation Device for Blind People
## 3 Sound Localization Performance of Panned Binaural Sound
### Procedure of the Sound Localization Test  
The present study attempt to achieve high-accuracy sound-image-localization using HRTF measured with a head and torso simulator (HATS). However, the results of the experiments performed in our previous study [22] showed that misjudgments often occurred in both air-borne and bone-conducted sound reproduction, as shown in Figure 3, due to various factors, one of which was that each of the individual HRTFs of the subject was not used in this experiment.  
Figure 2: (**a**) Actuator used for the bone-conducted sound reproduction, (**b**) hairband-type fixing device of the actuator, (**c**) contact position of the actuator, (**d**,**e**) measurement settings of the excitation characteristics of the adopted actuator with the fixing device, and (**f**) measured frequency characteristics of the vibration acceleration exited by the actuator.  
To guide a visually impaired person using binaural reproduction, the accuracy of sound localization performance should be as high as possible to guarantee the spatial resolution of every direction of the clock positions used by visually impaired people for direction recognition (every 30\({}^{\circ}\), twelve divisions of 360\({}^{\circ}\) in the horizontal plane). In the present study, the improvement of the accuracy of the sound localization in bone-conducted sound reproduction was investigated using the following panning method.  
The panning method moves the reproduced sound image to the left and right to improve the human perception of sound localization. Figure 4 shows an overview of the panning method. The angle F from the front to the direction of O is the main angle, and the angle \(\theta\) from the direction of position A to that of position B is the panning angle. The main angle \(\Phi\) is fixed at every 30\({}^{\circ}\) in the horizontal plane, following the method of clock position. The sound for the direction of O was played back 3 dB louder than that for the directions of A and B, so that the subjects could distinguish the panning sound. The switching time between the signals was set to 0.5 s. In the present study, three panning angles, 10\({}^{\circ}\), 20\({}^{\circ}\), and 30\({}^{\circ}\), were evaluated. The reproduced sound is a flashing sound that was generated by convolving the dry source of the flashing sound and the HRTFs measured with the HATS by KEMAR, provided by the Massachusetts Institute of Technology (MIT) [23]. Note that the flashing sound will be used in the main navigation system to guide the subjects in the target direction.  
In the sound localization tests, the subjects were surrounded by twelve virtual sound sources placed at 30\({}^{\circ}\) intervals in the horizontal plane (main angle: \(\Phi\)), and subjects reported the perceived incoming direction of the sound reproduced at random. As the subjects, ten males in their 20 s were adopted.  
Figure 4: Overview of the sound reproduction method. (**a**) Procedure for generating panned sounds and (**b**) example of reproduction sound.  
Figure 3: Sound localization performance for the cases in which the test signals were presented by (**a**) air-conducted sound and (**b**) bone-conducted sound [22].