# User Visit Certification and Visit Trace System Using Inaudible Frequency
## 4 Experiments and Evaluations
The third experiment was a visit certification using call check-in, and we have not developed an application. The certification time was measured by turning on the smart device, dialing the call check-in number, calling the number, and tracking the time until the first beep. As a fourth experiment, visiting certification using the proposed method was conducted in the same way as in the environment used in the performance verification experiment. The certification time was measured by turning on the smart device, running the proposed application, recognizing inaudible frequency trigger signals, and completing the visit certification. The experimental method measured the time it took for 10 participants to enter each laboratory to certify their visits. In the event of an error in certification failure, the total time required for certification was measured by reattempting the certification, and the visit certification test for each space was conducted 10 times for each participant. Figure 9 shows the results of the four experiments. In Figure 9, the time spent on each laboratory's certification shows the average time required for 10 participants to attempt certification 10 times per space. If a QR code on paper was used, it took 3.7 s, 5.1 s, 2.6 s, 5.8 s, and 4.7 s for the first through the fifth spaces, respectively, and the average time for visit certification was 4.38 s. It took more than 5 s in the second and fourth spaces, probably due to the light or light effect of QR code recognition, or the camera was not properly positioned for QR code paper recognition. When using the certificate QR code, it took 8.6 s, 6.3 s, 10.3 s, 6.6 s, and 12.6 s for the first through the fifth spaces, respectively, and the average time for visit certification was 8.88 s. The average time for visit certification was higher than in the other experiments, probably due to the reflection of liquid crystal surfaces or light ref  
Figure 9: The result of the time measurement experiment for user visit certification. with certificate QR codes. Further, we speculated that the user holding the smart device expected that the camera position of the smart device would recognize the certificate QR code based on the screen position and tried to recognize it. When using call check-in, certification took 5.2 s, 5.5 s, 6.2 s, 4.9 s, and 5.8 s for the first through the fifth spaces, respectively, and the average time for visit certification was 5.52 s. Call check-in requires the user to dial the phone number for certification. Further, there was no error except when the number was pressed incorrectly; thus, the visit certification time was uniform within 5 to 6 s in all five spaces. When using the proposed method of inaudible frequency triggering signals, it took 1.8 s, 2.2 s, 3.1 s, 2.9 s, and 2.6 s for the first through the fifth spaces, respectively, and the average time for visit certification was 2.52 s. The proposed method successfully performed certification within approximately 3 s in all spaces, and the average time for visit certification was faster than that of the existing methods. Lastly, we conducted an experiment on the visiting certification performance of the proposed application when using the same trigger signal in three other spaces more than 50 m apart and not in the same building. This was to ensure that even if the same trigger signal was used, accurate visit certification would be provided based on the user's location information and the location of the store visited. The location of the speakers was under the same conditions as in the first experiment, and the experimental method was conducted by one participant 10 times, 20 times, 50 times, and 100 times in each space. The results are shown in Table 2. In Table 2, each space is named A, B, and C, with GPS values for A (37.379297, 126.927540), B (37.380938, 126.926215), and C (37.380499, 126.928816). We observed 100% certification in all spaces during 10 and 20 certification attempts.