Modeling with the FDTD of cellphone — Hearing aid interaction

The new cellular telephones and other personal communication services (PCS), as well as global system for mobile communication (GSM), pose new problems and challenges in interactions with the human body and medical devices. One type of device affected are hearing aids. Electromagnetic interference (EMI) with hearing aids is associated with the frame rate in the time division multiple access (TDMA) systems resulting in the pulse modulation of the RF signal in the audible range. The frequency of the pulses depends on the communication system, and typically is 50 Hz, or 217 Hz. While the EMI, in this case, does not generally pose a health risk, it constitutes a considerable annoyance, that may prevent the hearing aid bearers from using some of the new devices. Also, the proximity to cellular telephones used by others or the base station may result in sufficiently strong signals to interfere with proper perception of sounds. Evaluation of EMI has been done experimentally and protocols for testing are being developed, e.g. in the USA in the Center for Devices and Radiological Health, FDA [1], [2]. The experimental test procedures can be greatly simplified, if the levels of the electric and magnetic fields around and in the ear at various location where hearing aids are typically located are known for actual complex exposure situations. The complexity results from the perturbation by the human head of the electric and magnetic fields from cellular telephones or their equivalent representation. To evaluate the actual potential for EMI, the assessment of the fields has to be made for actual locations of the hearing aid, which is typically in the ear canal. Simplified testing of actual hearing aids performed in free space using a resonant dipole representing a cellular telephone [1], [2] can then be related to the practical complex fields.