Black Hole of Sound: Constructing an Anti-eavesdropping Area Using Ultrasound

Abstract

With the miniature design and ubiquitous use of voice recording devices, the need for anti-eavesdropping has become essential, especially in confidential meetings. Previous studies prevent eavesdropping by injecting a noise into the recording devices using electromagnetic waves or ultrasounds, hoping that the noise can cover the voice recording in similar frequencies. In this paper, we propose a new anti-eavesdropping method that attenuates the voice recording instead of covering it. As the voice recording is suppressed to a low volume, it is difficult to recover sensitive information even using noise reduction techniques. The key idea is to inject an inaudible high-frequency noise that can saturate the preamplifier in the microphone circuit. In addition, to avoid unintentionally interfering with non-eavesdropping devices such as conference microphones, we construct an anti-eavesdropping area using an acoustic parametric array, which can transmit the high-frequency noise into a confined area of choice. Based on the above methods, we built a proof-of-concept anti-eavesdropping system and achieved good experimental results on 4 smartphones and 2 voice recorders.