Hearing aid electromagnetic interference from digital cellular telephones

Several in-the-ear (ITE) and behind-the-ear (BTE) hearing aids were tested for audible interference at various distances from five types of digital cellular telephones. The interference, which takes the form of a buzzing and a static sound, was quantified using a calibrated system including a frequency analyzer and a pressure field microphone. The output of the each hearing aid was coupled to the microphone via Tygon tubing and a standard 2 cc coupler. The highest interference induced sound pressure level (SPL), 122.5 dB, was measured from a BTE hearing aid placed within 2 cm of a transmitting GSM phone. In this case, interference was detected up to a separation distance of almost 3 meters. While all phones tested produced a similar interference level within 2 cm of this hearing aid, interference SPL from the CDMA based system decreased more rapidly with distance than the TDMA based phones tested. Subjective testing was also conducted using a listening stethoscope in order to further characterize the interference.