# User Visit Certification and Visit Trace System Using Inaudible Frequency
## 1 Introduction
However, this method requires continuous guidance for call check-in certification during user visits in front of the store, and if the store is not visited, a false authentication problem occurs if the user calls the designated number. In addition, visit times are inaccurate because the recorded time is the time the user calls. Recently, a visit certification service using Wi-Fi or a beacon has been released, but the signal has the disadvantage of having a wide range of reach. In other words, the problem lies in that it can recognize the store as having been visited even if the user has not visited the store and is standing in front of the door. Therefore, in this study, we propose a user visit certification and visit trace system using inaudible frequencies in the range of audible frequencies but not those audible to people. This proposed application is run to authenticate electronic access without having to line up at the entrance of the visit point. Most speakers can output 20-22 kHz, a range that people can hear (humans have an A-weighted dB hearing that changes drastically with age, and healthy children can hear the upper frequency band up to 30 kHz), and most smart devices can detect trigger signals by collecting ambient sounds through built-in microphones and analyzing collected sounds [10]. The proposed system uses an interval of 50 Hz between 18 kHz and 20 kHz for data communication studies using conventional high frequencies as a trigger signal. Sound in the range of 18 kHz to 22 kHz is almost inaudible to most people. Frequencies exceeding 20 kHz present the problem that the signal transmission distance gradually decreases. Therefore, we used the range of 18 kHz to 20 kHz, which has been used in many existing studies. The proposed application will perform user visit certification by sending signal values, user identification (ID), and location information to the server when the smart device detects the signal specified by the smart device located within the space where the signal is generated. The server checks the location information of the received data, stores the certification data in the database, and then sends the resulting values back to the smart device to complete authentication. The proposed application stores certificated information in a local database of smart devices, allowing users to view their visit histories at a glance through a list and maps by date. Further, certification information stored on the server can be classified by individual and store, and used to investigate users' dynamics and search for visitor information by store. To verify the performance of the proposed system, we developed a visit certification application based on a smart device and server system using inaudible frequencies, and conducted a visit certification experiment using 10 smart devices, resulting in an average of 99.6% certification accuracy. We conducted a comparative experiment using the proposed method and a Bluetooth low-energy (BLE) beacon and confirmed that the proposed method has better accuracy of visit certification than a beacon. We also conducted comparative experiments with visiting certification methods using QR codes on paper, authentication of QR codes on smart devices, and call check-in, and found that the proposed method could save more time for authentication. Thus, the proposed system is a technology that allows electronic access to multiple facilities using microphones of smart devices and inaudible frequencies that cannot be heard by people, and it will be a useful technology that can contribute greatly to the untact services required by society after COVID-19. The structure of this paper is as follows. The \"Previous Work\" section describes existing studies using inaudible high frequencies in the audible frequency domain used in the proposed method.