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A new, inexpensive permanent magnet material is now available which has a higher magnetic energy than any other ceramic magnet material previously known. It is a modified strontium ferrite. One of its most important applications is in moving coil loudspeakers. Sixteen widely used gap configurations, including the nine Electronic Industries Association standards, with gap energies between 0.1 and 20 mega-ergs are described for which strontium-ferrite rings are used. The leakage permeances, the flux densities inside the magnet, and the permeance of the return circuit are estimated. The process of successive approximations is used to calculate the dimensions of the strontium-ferrite rings. It is found that systems with strontium ferrite are smaller, simpler, and lighter in most cases than equivalent systems with Alnico-5 magnets. This is mainly due to the much lower permeability of strontium ferrite as compared to that of Alnico-5. The low permeability makes limb leakage negligibly small in compact strontium-ferrite structures. Strontium-ferrite speaker systems are particularly suitable for low-cost applications and whenever high gap energies and high gap densities are needed.