A new approach to a space-confined magnetic loop induction system

An investigation of possible communication systems for use in schools for hard-of-hearing children is described. The requirements are set forth and the relative merits of various systems are briefly discussed. A new concept, "The Orthogonal-Fields System," is shown to have definite advantages in those cases where several adjoining rooms have to be addressed independently. In this system the magnetic field, which is the coupling medium between the teacher's microphone amplifier and the pupils' hearing aids, is confined to a limited space. Within this "hearing space" the field strength of the vertical component remains virtually constant, yet it decreases rapidly outside the space. An analytical design for the required loop configuration is given in the companion paper. Measurements on an actual system have shown that the horizontal discrimination is excellent and closely follows theoretical prediction. The loop configurations considered, however, have poor vertical discrimination due to an unfulfilled requirement for vertical asymmetry. Although it is feared that this requirement will be difficult to meet in practice, the system may still be the best available and can be quite useful, especially in new schools where the dimensions of the classroom can be chosen to overcome this limitation.