Hedgehog: Pushing the Range Limits of Ultrasonic Microphone Jammers

Abstract

Ultrasonic microphone jammers (UMJs) use ultrasonic waves to interfere with concealed microphone recorders, offering significant values in confidential meetings and secret talks. Existing UMJs, however, performs in a short range of typically 2 m, which remains a huge gap to practical applications. The key challenge lies in the fact that the ultrasonic signal will produce audible sounds during its transmission, caused by nonlinear distortion of power amplifiers and loudspeakers in the transmission chain of the UMJ. In this paper, we propose Hedgehog, a room-scale ultrasonic jammer design, which enhances the jamming range through two key methods. First, it corrects nonlinear distortion by modeling the transmission chain and applying digital pre-distortion. Furthermore, it redesigns the jamming signal to improve its effectiveness by not only reducing the signal-to-noise ratio (SNR) but also by suppressing the semantic information in human speech. The experimental results show that Hedgehog achieved a word error rate (WER) of over 95% at 8 meters and over 80% at 10 meters, which is a 4 times increase in jamming distance compared to existing solutions.