## 1 Introduction
However, this study primarily addressed the estimation of the position of a silent intruder. Figure 1: Conceptual illustration of an active localization system based on acoustic sensors in home appliances. The process of a home security system can be divided into sensing, assessing, and responding. Sensing is very important because it functions as a trigger to operate the security system. Thus, the sensors must be interoperable with each other [5], with a combination of various individual sensors [5; 7], or with the information measured by one sensor module [19]. Therefore, through this study, we expected to increase the utilization of microphones used in home security systems. This is because the data measured by a conventional linear microphone array provide only angular information. However, the proposed method also provides the distance, which increases the number of scenarios that can be combined with the information of other sensors. We present two examples of complementary sensing. In the first one, passive infrared (PIR) sensors function as triggers to awake the security system and record the intrusion using a camera [20]. However, PIR sensors have the disadvantage of being unable to detect an intruder who does not move, moves slowly, or uses heat-insulating clothes. IR sensors have limitations that often cause errors because of their nonlinear sensitivity and the effects of nearby objects [21]. Therefore, if the acoustic-based intrusion detection in [16; 17; 18] is applied to the security system to compensate for the weakness in IR sensors, the two sensing systems can complement each other to increase the robustness of the intrusion detection. In the second example, when a microphone array detects the direction of an event, a pan-tilt-zoom (PTZ) camera is rotated and focused on the region of interest [8]. However, because the camera has misrecognitions owing to poor resolutions, distant targets, changes in illuminations, or occlusions [22], the PTZ camera can be operated robustly by providing a distance and an angle of the intruder based on the proposed method. Therefore, to overcome the shortcomings of conventional acoustic-based intrusion detection systems and achieve the complementary intrusion detection system proposed in [5], this paper describes our proposed active localization method that estimates the position (distance and angle) of a silent intruder using a generated reflection. The main concept is that a loudspeaker generates a signal in the security space. The microphone array extracts the changed signals owing to the intruder, and then the distance and direction are estimated using the changed signals (sound field variation). Echolocation is a technology that detects a location through an echo which is emitted from a sound source and then returns, and it has been primarily implemented using an ultrasonic sensor. In [23], a biomimetic sonar system was mounted on a robot arm to recognize an object through the vector of the echo envelope. A biomimetic study was conducted to estimate the distance and angle [24]. The distance was estimated using the time delay between the maximum activity owing to the call and the activity owing to the echo, and the angle was predicted by comparing the directivity pattern of the sensor using the notch pattern in the frequency range. Ultrasonic sensors are acoustic sensors used in conventional home security systems. Ultrasonic sensors are active sensors that send signals in a straight line; therefore, the source and receiver can be placed face-to-face [21] or in the same direction to physically detect the intruder [25]. However, owing to the straightness of the signal, they have the disadvantages of utilizing several sensors to increase the detection rate [26] and being unable to detect a person that passes behind an obstacle. The proposed active localization in the audible frequency utilizes the phenomenon of scattering rather than straightness.