Patent ID: 7873180

Claim:
An actuator for moving a voice coil in response to an input signal, the actuator comprising: a magnetic assembly, substantially symmetrically disposed about a central axis, that comprises a back plate, magnetic material, a front plate having a central opening, and a central pole that extends at least partially through the central opening, the magnetic material producing a primary magnetic field that extends through a magnetic gap separating the central pole and the central opening, wherein the central opening comprises a first surface groove abutting the magnetic gap, and the central pole comprises a circumferential second surface groove abutting the magnetic gap opposite the first surface groove, wherein the central opening and the central pole configure the primary magnetic field in the magnetic gap into regions of greater and lesser magnetic intensity along the central axis, at least one region of lesser magnetic intensity being axially disposed between at least two regions of greater magnetic intensity; a voice coil that is movably positioned within the magnetic gap and that, in response to the input signal, produces a voice coil magnetic field in the magnetic gap; and a counter coil that circumferentially abuts the magnetic gap and is axially aligned with the region of lesser magnetic intensity, the counter coil producing, in response to the input signal, a counter magnetic field in the magnetic gap.