RF Interference in Hearing Aids from Cellphones Part 1: Near-field cellphone emissions measurements and the effects of hands.

Cellular telephones (cellphones) are currently categorized for hearing aid compatibility based on a calculated value (metric) obtained from the measurement of near-field, radio-frequency emissions according to a procedure described in ANSI Standard C63.19 "Measurement of Compatibility between Wireless Communications Devices and Hearing Aids". There has been a lack of documentation, however, that relates this metric to a cellphone's potential for interference in actual use, that is, when it is held at the ear in a normal-use position by a hearing aid wearer. In Part 1 of this two-part series, we compare the ANSI C63.19 metric to simpler metrics, still based on the near-field test procedure of the standard, and to near-field measurements made when the cellphones are hand-held. The results justify employing a simpler no-hand metric than the exclusion area procedure presently specified by the standard, but not the addition of a test hand to the procedure. The further effect of the head and interaction with the hearing aid is examined in Part 2 of the series.

RF Interference in Hearing Aids from Cellphones Part 2: Comparing in-use RF coupling to predictions.

Cellphones and hearing aids are presently tested for their near-field RF emissions and RF immunity, respectively, to predict their mutual compatibility when used together. In the concluding part of this two-part series, we examine the relationship between these independent device measurements and the resultant in-use coupled RF interference, which may be heard as audio frequency noises by the hearing aid wearer. The established standards are seen to be generally reasonable in meeting the compatibility goals (i.e., ensuring a low level of perceived audio interference), but the combined effects of the relative device positioning, the hand, and especially the head add a high degree of uncertainty to the relationship between the actual in-use RF interference coupling and predictions based on individual emissions and immunity measurements.

Radio Frequency Immunity Testing of Hearing Aids.

For many Equipment Under Test (EUT), such as the hearing aids examined in this study, the desired RF immunity measurement result is that which would be measured in the most sensitive EUT orientation relative to an applied RF field. This is generally approximated from measurements at a number of predetermined orientations within a GTEM cell. This paper presents new 6 and 12-orientation "maximal sum" methods of small EUT immunity measurement, which may be considered extensions to present sorted three-input vector summation techniques. Experimental results for the new methods approached the established reference goal more consistently than did other approaches examined employing a comparable number of contributing measurements.