US Patent 7151840 Magnet system of a sound transducer

A miniaturized electro-dynamic sound transducer has a housing made of deep drawn sheet metal. A magnet system with a magnet is arranged in the housing. A diaphragm provided with a coil is arranged in the housing, and wires extend from the coil to contacts. The housing has an outer diameter of maximally 20 mm. The thin sheet metal of the housing has an average thickness of maximally 0.2 mm. The magnet system has a lower pole piece and an upper pole piece, wherein the pole upper and lower pieces are made of sheet steel having a thickness of at least 1.5 times an average thickness of the bottom of the housing.