UltraJam: Ultrasonic adaptive jammer based on nonlinearity effect of microphone circuits

The widely used devices (e.g. smartphones, recorders) equipped with microphones have posed a severe threat to confidential conversations. In this paper, we design an inaudible anti-eavesdropping method: UltraJam, to reduce the risk of unwanted and secret recordings. UltraJam uses the ultrasonic signal to mask conversation. By leveraging the nonlinear effect of microphone circuits, the adaptive ultrasonic signal can be recorded and demodulated into low-frequency which can effectively squash the sound. Based on the characteristics of the attenuation coefficient and frequency response, we construct a number of jamming signals with different bandwidths and designed a wideband signal injection array, meanwhile adaptively adjust the power at each bandwidth signal to cover more frequency bands and increase usage scenarios. To verify the security of the microphone jamming system, we also utilize several audio recovery methods to recover the raw signal from jamming noise. The experimental results show that less than 1% of the words are recognized in the jamming recording, and even with the audio recovery method, 99% of the words still cannot be recognized in the recovered jamming recording.