## 5 Conclusions  
In this paper, a user-aware audio marker has been proposed using low frequency ultrasonic signal processing techniques such as LFU-based object detection and LFU communication. The proposed audio marker system consisted of a commercial off-the-shelf (COTS) loudspeaker and microphone as a transmitter and a receiver, respectively, and the frequency response of the LFU-based audio marker ranged from 18 to 20 kHz. Therefore, various devices equipped with audio input and output could easily act as audio markers for AR applications. Figure 10: Comparison of the power spectral densities of the LFU signal and bus-stop noise with a noise level of (**a**) 90 dBSPL and (**b**) 100 dBSPL. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**AttributeSystems** & **GNSS-based Dense** & **Bluetooth Beacon [25]** & **Proposed Audio Marker** \\ \hline Range & \textless{}20 km & \textless{}70 m & \textless{}4 m \\ Resolution & \textless{}10 cm & \textless{}5 cm & \textgreater{}30 cm \\ Sensor type & Antenna station & Bluetooth beacon, modern & COTS speaker, microphone \\ Application & Outdoor positioning & Indoor positioning & Location-specific broadcasting \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of the attributes of the proposed audio marker system with other positioning systems. GNSS: global navigation satellite system. The proposed audio marker system could detect the existence of a user located around the marker without any additional sensors. In other words, the object detection was performed by applying an image background subtraction method to the LFU images. Afterwards, the marker's information was broadcast near the audio marker through DPSK modulation. In particular, the proposed LFU communication method controlled the LFU transmission power so that the transmitted signal could be confined within the maximum transmission zone that corresponded to the region in which the user was located. As a result, the information on an audio marker could only be sent to the user in front of the audio marker. This was done by incorporating the LFU-based object detection gain into the LFU communication. The performance of the proposed audio marker system was evaluated in two ways: the LFU-based object detection performance and LFU communication performance. First, the performance of the LFU-based object detection was measured in terms of the true positive rate (TPR) and false positive rate (FPR). The experimental results showed that the proposed LFU-based object detection method achieved a much lower FPR than the conventional PIR sensor-based one when a person was walking at a distance of 4 m from the transmitter. Consequently, it was also shown that the proposed LFU-based object detection method had a 0.05 higher AUC than the PIR sensor-based one. Second, the performance of the LFU communication was measured in terms of the successful transmission rate (STR). It was shown from the performance evaluation that the proposed LFU communication method could transmit the audio markers to the receiver with an average STR of 100%, 97.98%, and 90.64% at distance of 1 m, 2 m, and 4 m, respectively, which was better than the conventional LFU communication. This was because the LFU communication of the proposed audio marker system was able to transmit audio markers to a specific user only by controlling the emission energy according to the LFU-based object detection. In addition, it was revealed that the proposed audio marker system could robustly operate with an STR of above 80% under an ambient noise level of 90 dBSPL. As a concluding remark, the proposed audio marker system can be utilized as a means of providing AR contents that are specific to each place by overcoming the disadvantage of a conventional audio marker, which only broadcasts marker information to an unspecified group of users. However, the proposed system still has a drawback when multiple users are grouped in front of the audio marker.