I. Introduction
Now we are in an era of the rapid development of network information, the importance of information is self-evident. There is so many information attracts criminals who have various ways to steal information. Common methods include eavesdropping on recordings, spyware on the PC, keyloggers, and advertisers’ records of browsing information. In recent years, more and more researchers have paid attention to information leakage caused by voice-based smart devices. Some researchers have realized that some smart devices can monitor and save voice information, such as smartphones, smart watches and smart speakers. These smart devices can execute commands by recognizing vocal keywords. In order to respond to voice commands at any time, they will record and analyze in real time. In this process, the user's voice data may be leaked, then the attacker can analyze it and get the user's privacy information [1]-[3]. Some researchers found that ultrasound can affect the microphone of smart devices. Because ultrasonic waves are inaudible, this phenomenon can be used covertly on smart devices, and has high research value and application value. An attack method is proposed [4],[5]. By sending a special ultrasonic wave to the microphone of the smart voice device, the voice command to the smart voice device can be realized. Some experiment indicates that this method can activate and control the smart device at a distance of 2-3 meters. This attack method has been analyzed from the feasibility to prove its effectiveness [6]. Other researchers try to realize precisely positioned the attack by cleverly arranging ultrasonic speakers [7]. Inspired by this phenomenon, some researchers want to use a more powerful ultrasound to cover the human voice in the microphone recording [8]. They designed a bracelet covered with ultrasonic speakers and wanted to use it to block the recording of the microphone. However, the effective interference distance of this scheme is too close, and the equipment is too large. There is still a long distance for practical use.