Researchers at the University of Illinois at Urbana-Champaign's department of Electrical and Computer Engineering have recently managed to turn a smartphone's vibration motor into a microphone. Associate professor Romit Roy Choudhury and Ph.D. candidate Nirupam Roy are the ones behind the "VibraPhone" research and plan to present their paper at MobiSys in Singapore later this month.
Although a speaker, a microphone, and a vibration motor have entirely different functions, in reality, these components rely on very similar basic principles. In layman's terms, a speaker works by changing the position of its magnetic surface through applied voltage; it vibrates and it creates sounds. On the other hand, a microphone works by reversing the process and changing the voltage in accordance with its own position. Technically speaking, a vibration motor is quite similar to a tiny speaker and borrows similar principles. In turn, because a speaker can be easily converted into a microphone (by simply reversing the polarity / wires), the researchers found that a smartphone's vibration motor can also act as a microphone. However, this is not something that can be achieved through software applications and requires tinkering with the vibration motor itself. Once the motor is reconfigured, it can turn vibrations into voltage variations, which can then be translated into waveforms. However, due to the nature of the vibration motor whose response is "considerably jagged", it can only capture low frequencies around 2 KHz. This will induce distortions, and the team revealed that as long as the recorded voice "does not contain enough low-frequency sound to capture, the performance of the system will degrade". In other words, while a vibration motor can be technically turned into a microphone, it's clearly not meant to replace it.
With that being said, are there any practical uses for turning a vibration motor into a microphone, and should this be another reason to worry about privacy? Not at the moment, and apparently not. Fortunately, the researchers don't want their findings to be used for spying or against user privacy agreements. Instead, the team wants to determine "if similar techniques can help us recover speech from the subtle vibration of vocal cords, facial bones, or skull", and the researchers are exploring the possibility of using a smartphone's vibration motor as "an assistive system for persons with speech impairment".