Electromagnetic compatibility of hearing aids with digital mobile phones

Abstract

When the intensity of emissions from a radio transmitter varies at an audible rate, it causes interference on sensitive electronic equipment. Hearing aid user complaints caused by this kind of interference were increased by the introduction of digital mobile phones in recent years. Interference reveals itself as a disturbing "buzzing" sound in the hearing aid output. The electromagnetic compatibility of 16 different hearing aid types was measured for high-frequency electromagnetic fields, particularly for those emitted by digital mobile phones. An automated test setup was developed for testing. Setup consisted of a GTEM cell where high frequency fields were generated. Draft version of IEC 118-13 standard was utilized as a basis for the test methodology. All of the hearing aids showed susceptibility to some degree. Interference levels up to 140 dB SPL was measured in 800-960 MHz range of the carrier frequency. Six of the hearing aids were found to produce levels above the level (55 dB SPL Input Related Interference Level) proposed by the standard. Increasing levels of the electric field strength resulted in a quadratic increase in sound pressure levels produced by the hearing aid. This 1:2 dB ratio proves to be a helpful tool in interpolating for any field strength level that was not tested. Setup developed, methodology followed and results obtained in this study can be used to evaluate the electromagnetic compatibility of hearing aids.