Patent ID: 7551746

Claim:
A compound electro-acoustic drive unit comprising: a first drive unit and a second drive unit, wherein each said drive unit comprises at least one permanent magnet assembly and at least an inner pole piece and an outer pole piece forming a pair of pole pieces, said permanent magnet assembly together with the pair of pole pieces forming a magnetic circuit with a pole gap for exciting a voice coil assembly, wherein, said pole gap is arranged to provide a magnetic field directed radially with respect to a center axis of the loudspeaker, and said permanent magnet assembly comprises at least two permanent magnetic bars, said magnetic bars having a radially extending magnetization direction with respect to said center axis of the loudspeaker, wherein, the space between the pair of pole pieces accommodating the magnetic bars, as seen in a plane perpendicular to said axial direction, is substantially circular or elliptical; and said magnetic bars have a substantially arched cross section in a plane perpendicular to said axial direction.