Listening through a Vibration Motor

This paper demonstrates the feasibility of using the vibration motor in mobile devices as a sound sensor, almost like a microphone. We show that the vibrating mass inside the motor -- designed to oscillate to changing magnetic fields -- also responds to air vibrations from nearby sounds. With appropriate processing, the responses become intelligible, to the extent that humans can understand the vibra-motor recorded words with greater than 80% average accuracy. Even off-the-shelf speech recognition softwares are able to decode at 60% accuracy, without any training or machine learning. We present our overall techniques and results through a system called VibraPhone, and discuss implications to both sensing and security.