# User Visit Certification and Visit Trace System Using Inaudible Frequency
## 4 Experiments and Evaluations  
This section presents the proposed application for user visit certification and the visit trace system, and shows the experiments and analysis of results for performance verification using the proposed application and certification server. The screen configuration of the proposed application is the same as in Figure 6, and is well developed for the iOS-based iPhone 11. Figure 6: The screen for the user visit certification of the proposed application: (**a**) detecting view of the trigger signal; (**b**) visit certification view. Figure 6a is the main view when the user first runs the application for visit authentication, and it displays the time in seconds spent on authentication in the red circle position. On the right side of the screen, the \"Visit list\" button is placed to view the visit history of the places the user visited recently. After running the application, the proposed application performs the certification procedure in the order shown in Figure 1 if a trigger signal corresponding to the proposed method exists. The application switches to the view in Figure 6b when the visit certification in Figure 2 is approved. This is a successful visit authentication, in which the application receives information about the visited store from the server and displays the name of the certified multi-facility, store ID, and store address. This screen allows the user to reenter the store visit certification by touching the \"Certification\" button located on the right, and the user can touch the \"Visit list\" button to see the places they have visited recently. Further, if the visit certification is not approved by the server, the application takes more time to approve without changing the view on the screen shown in Figure 6a. We set the trigger signal to \(\mathbf{P_{df}=50}\) Hz (18-20 kHz) for visit certification of the proposed application, and output the signals to the speaker using five pieces of 40-bit data, including the CRC data, as shown in Table 1.  
The frequency range was set from 18 kHz to 20 kHz, because the response rate of microphones built into smart devices is inconsistent and depends on the type of smart device when high frequencies above 20 kHz are used [19; 20]. For example, in [PERSON]'s experiments, speaker volume typically decreased from 22.05 kHz to 24 kHz, depending on speaker performance and frequency sample rate, and [PERSON]'s experiments showed a 99.88% transmission accuracy at a sampling rate of 32,768 for FFT analysis. In Table 1, the 40-bit data consist of 32-bit information data and 8-bit CRC data, with CRC data attached to the back of the 32-bit data. The CRC data generated CRC values for the 32-bit data using the generator polynomial \(\mathbf{\alpha\varphi B}\) (\(\mathbf{x^{8}+x^{7}+x^{4}+x^{3}+x+1}\)) used in WCDMA wireless communication [21]. Next, the space for the experiment was determined as the location of five 7 m-by-4 m laboratories located on the same floor of the same building, as shown in Figure 7. The trigger signal for each space was set in turn from **first** to **fifth** in Table 1 to generate a signal from the speaker, which was placed within 1 m of the entrance of each laboratory, as shown in Figure 7.  
There were 10 participants, and the participants were required to visit each space 10 times in turn to run the proposed application and certify their visit. The server system for authorizing the visit certification and storing the information used Apache 2.2.14, PHP 5.2.12, MySQL 5.1.39, and Intel(r) Core(tm) i5 CPU 750, with 8 G RAM as the hardware. The speakers for trigger signal generation and output in each laboratory used a 2019 13-inch MacBook Pro and built-in speakers, while the signal generation used JavaScript-based web applications. The smart device used in the experiment used 10 iPhone 11 devices. The comparative experiment was conducted under the same conditions as the BLE beacon, considering how the RSSI signal of the beacon was used as the trigger signal.