Apple notes that portable devices such as the iPhone have become more and more compact. As the form factor shrinks, system enclosures become smaller and there is less space available for speaker integration.
In the case of an audio speaker having a voicecoil suspended below a diaphragm within a gap of a magnetic return structure, precious space is occupied by the magnetic return structure that is required to direct the magnetic field produced by the magnet around the voicecoil. Since the voicecoil and the magnetic return structure extend along the axis of sound emission, they take up z-height and limit the degree to which the speaker thickness can be reduced.
Apple proposes eliminating the magnetic return structure and helical voicecoil which allows for the vertical thickness of the speaker to be reduced.
The voicecoil may be integrated along a surface of the diaphragm and configured to interact with a magnetic field produced by a magnetic array such that the voicecoil operates within the fringe flux of the magnetic field and the thickness of the speaker is limited only by the magnetic array thickness and the excursion clearance of the diaphragm.
Here's the details of one embodiment:
An electromagnetic transducer for sound generation includes a diaphragm configured to move along a central axis. The diaphragm may include a dielectric surface orthogonal to the central axis, and a voicecoil may be coupled with the dielectric surface. The voicecoil may have a conductive winding on the diaphragm, e.g., with one or more conductive paths running along the dielectric surface. Furthermore, the electromagnetic transducer may include a magnetic Halbach array having at least three magnetized portions arranged side-by-side. Each magnetized portion may extend along a respective longitudinal axis and produce respective magnetic field lines perpendicular to the respective longitudinal axis. Thus, the magnetic Halbach array may direct the magnetic field lines toward the voicecoil such that the magnetic field lines intersect the voicecoil to cause a Lorentz force to move the diaphragm along the central axis. The magnetic field lines that intersect the voicecoil may run parallel to the dielectric surface and perpendicular to the conductive winding.
The very technical patent can be read at the link below...
Read More [via Patently Apple]