A mobile device that includes a motion sensor, multiple vibrators, and a motor speed determinator module. The motion sensor senses and records vibrations of the mobile device while the multiple vibrators are simultaneously running. The sensed vibrations are transmitted to the motor speed determinator module, which transfers the sensed vibrations into the frequency domain. The vibrations caused by each of the multiple vibrators are represented by peaks in the frequency representation. In one embodiment, the motor speed determinator module drives each vibrator such that the rotational speed and phase of each vibrator are equal or essentially equal. This is represented in the spectral representation of the frequency domain as the overlapping of each peak into a single peak.