Electromagnetic interference form a TDMA cellular telephone with a hearing aid

Cellular telephones such as the global system for mobile communication (GSM) and personal communication services (PCS) are well known to cause electromagnetic interference (EMI) with hearing aids. The magnitude of the acoustic interference depends on several parameters of the telephone and aid. For a given hearing aid and telephone, the EMI depends on the electric and magnetic fields in the location of the hearing aid. The magnitude and direction of electromagnetic fields from a cellular telephone are very different in the ear canal compared to those in free space. In this contribution we present the results of numerical modeling of one telephone and one hearing aid. This hearing aid was extensively investigated by the US Food and Drug Administration laboratory. They evaluated the ratios of acoustic signals at 217 Hz in the ear canal and free space from the same telephone. The telephone was for SGM, and operated at about 900 MHz with TDMA. Electric and magnetic fields in free space in very close proximity to the telephone were measured in its manufacturer's research laboratory.