Record sound through the phone's vibration motor

What happens if you connect the vibromotor with a wire to the audio input

Two engineers from the University of Illinois in Urbana-Champaign (USA) have developed an original method of hidden audio interception using a smartphone. The inventors have demonstrated that coils of the vibration motors of the phone are quite good for recording sound. Coils register oscillations at a frequency of up to 2 kHz. Scientists have developed a sound processing method ( pdf ) that amplifies the signal at higher frequencies so that the phone records not only simple vibrations, but also human speech.



The method showed a good result: in most cases (up to 80%) the recorded speech can be understood by ear, and some words are recognized even without post-processing, that is, on the original low-frequency material.



The signal is removed from the coil vibration motor. Due to external vibrations (for example, due to sound vibrations), the magnet moves inside the coil, changing the magnetic field around, so that, due to the action of the electromotive force, induction current occurs in the coil.

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As practice has shown, sending a signal to the audio input allows you to quite accurately record the vibrations of the vibration motor.



Here are the spectrograms for the sounds of “sa” and the full word “entertainment”, recorded by a microphone (left) and a vibration motor (right).







The recorded sound is then passed through filters to enhance the high frequencies. The illustration shows the original signal (a), the heat map of the calculated sound energy flow after processing the signal from the vibration motor (b) and, for comparison, the same sound recorded by a microphone with normal sensitivity (s).

Demonstration

Example 1

The word “Entertainment”



Vibration motor signal before processing: wav





The signal of the vibration motor after processing: wav





Microphone Signal: wav

Example 2

The word “Author”

Vibration motor signal before processing: wav

The signal of the vibration motor after processing: wav

Microphone Signal: wav

Example 3

The word “Black”

Vibration motor signal before processing: wav

The signal of the vibration motor after processing: wav

Microphone Signal: wav

Example 4

The word “Look”

Vibration motor signal before processing: wav

The signal of the vibration motor after processing: wav

Microphone Signal: wav

Example 5

Word "often"

Vibration motor signal before processing: wav





The signal of the vibration motor after processing: wav





Microphone Signal: wav





Scientific work is theoretical. In practice, this method is difficult to use, because you need to get physical access to the phone and modify it.



But this is an important study to understand what new ways of interception will theoretically become possible in the future. Paranoids may assume that such minor modifications will be made to phones certified for use in the market of a particular country, directly at the stage of production or certification.



In any case, if you suddenly notice that the vibromotor of your phone is connected with wire to the audio input - this is a reason to be alerted.



By the way, earlier at computer security conferences, it looked like work that theoretically allows you to record sound and recognize speech using:

• phone gyro: Gyrophone, pdf , conference USENIX Security Symposium 2014;
• phone accelerometer: AccelWord, pdf , MobiSys 2015 conference;
• physical vibrations of the subject: Ripple, pdf , NSDI 2015 conference; more efficient technique Ripple II, pdf , NSDI conference 2016.