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Magnetic Induction Loops with Geometrically Restricted Fields for Communication in Schools for the Hard-of-Hearing

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In the education of the hard-of-hearing the quality of communication is of prime importance. One method to achieve this is through the use of a magnetic loop induction system. In schools with adjacent classrooms, each fitted with its own loop system, the crosstalk between adjacent classrooms will be quite objectionable unless special measures are taken to discriminate against spill over. Discrimination between classrooms can be effected either by frequency-selectivity (carrier systems) or by restriction of the range of each classroom field. It is possible to design magnetic induction loops generating fields which are geometrically confined within a prescribed area. An earlier system (1965, de Boer, Rosman, Joosten) not only realized low cosstalk between adjacent classrooms, but also afforded freedom of movement of the pupils. It depends for its operation on two spatially orthogonal fields which are excited by two loops fed by speech currents being 90° out of phase over the entire frequency range. In this paper a different approach is described, with which comparable results have been obtained with only one induction loop.

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