An apparatus includes a surface arranged to be mechanically displaced, a first permanent magnet coupled with the surface, at least one supporting member for supporting the surface, a base, a second permanent magnet, a coil coupled with the second magnet, and a signal port electrically coupled with the coil. An electrical signal is configured to travel between the signal port and the coil. The electrical signal in the coil is proportional to mechanic displacement of the surface when a force equilibrium state of the surface is broken by the electrical signal in the coil or the mechanic displacement of the surface from a position of the force equilibrium state The coil is arranged at least partially between the first magnet and the second magnet or around one of the first magnet, the second magnet. Either the same or different polarities of the first and second magnets are arranged to directly face each other at a distance.