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Digital cellphones and interference with hearing aid users

The study, conducted on behalf of the Ministry of Health, with the cooperation of Telecom and BellSouth, measured interference effects of two varieties of TDMA (time division multiple access) digital phone systems, DAMPS (digital advanced mobile phone service)and GSM (global system for mobile communications) together with analog phones on 29 regular hearing aid wearers.

The report rated the two types of digital phones equally in terms of interference, although the noise each type induced differed. No subjects reported any interference from the analog phone.

Handset of different power (2W, 3W and 8W) were tested. The 8W phones caused significantly more interference than the standard cellphones.

Hearing aid wearers had six brands of hearing aids among them, and styles of hearing aid varied from BTEs (one with an additional microphone on a lead), through ITEs to ITCs. Ages ranged from 24 to 83 years. The average 4-frequency hearing loss for ears tested was 64 dB HL (range 29-108 dB).

The interference threshold varied considerably. Typically it was less than 1 meter. In a few cases there was no interference detected, and in 2 cases interference from the 8W phone was detected at a distance of 5m.

In 20% of trials, interference occurred at a distance exceeding 1m. When the 8W phone was excluded, this fell to 11.5%.

Hearing aid type was significantly related to interference detectability - but this effect was dominated by the impact of the one subject who had a BTE with a microphone on a lead (which would have acted like a radio aerial).

In a few cases interference noise was audible to researchers well before it was responded to by the hearing aid wearers.