Linear resonance vibration actuators are great alternative to Eccentric rotating mass (ERM) motors, these are both used to vibrate handheld devices such as cellphones, driving LRAs is not the same as driving ERMs for such that LRA needs a specific resonant frequency. More on these on Precision Microdrives’ link here.
Linear Resonance Actuators (LRA) use a movable mass, permanent magnet, voice coil and spring to generate vibrations. The voice coil produces a magnetic field which interacts with the permanent magnet, causing it to move and by doing so, compresses or stretches the spring to which it is attached. The drive signal needs to alternate the direction of current (i.e. it’s an AC signal) and hence the magnetic field to make the permanent magnet oscillate back and forth with the spring. The moving mass is attached to the magnet, and it is the moving of the mass back and forth that generates the vibration.