Telecoil-mode hearing aid compatibility performance requirements for wireless and cordless handsets: magnetic signal...

In the development of the requirements for telecoil-compatible magnetic signal sources for wireless and cordless telephones to be specified in the American National Standards Institute (ANSI) C63.19 and ANSI/Telecommunications Industry Association-1083 compatibility standards, it became evident that additional data concerning in-the-field telecoil use and subjective preferences were needed.

Primarily, the magnetic signal levels and, secondarily, the field orientations required for effective and comfortable telecoil use with wireless and cordless handsets needed further characterization. (A companion article addresses user signal-to-noise needs and preferences.)

Test subjects used their own hearing aids, which were addressed with both a controlled acoustic speech source and a controlled magnetic speech source. Each subject's hearing aid was first measured to find the telecoil's magnetic field orientation for maximum response, and an appropriate large magnetic head-worn coil was selected to apply the magnetic signal. Subjects could control the strength of the magnetic signal, first to match the loudness of a reference acoustic signal and then to find their Most Comfortable Level (MCL). The subjective judgments were compared against objective in-ear probe tube level measurements.

The 57 test subjects covered an age range of 22 to 79 yr, with a self-reported hearing loss duration of 12 to 72 yr. All had telecoils that they used for at least some telecommunications needs. The self-reported degree of hearing loss ranged from moderate to profound. A total of 69 hearing aids were surveyed for their telecoil orientation.

A guided intake questionnaire yielded general background information for each subject. A custom-built test jig enabled hearing aid telecoil orientation within the aid to be determined. By comparing this observation with the in-use hearing aid position, the in-use orientation for each telecoil was determined. A custom-built test control box fed by prepared speech recordings from computer files enabled the tester to switch between acoustic and magnetic speech signals and to read and record the subject's selected magnetic level settings.

The overwhelming majority of behind-the-ear aids tested exhibited in-use telecoil orientations that were substantially vertical. An insufficient number of participants used in-the-ear aids to be able to draw general conclusions concerning the telecoil orientations of this style aid. The subjects showed a generally consistent preference for telecoil speech levels that subjectively matched the level that they heard from 65 dB SPL acoustic speech. The magnetic level needed to achieve their MCL, however, varied over a 30 dB range.

Producing the necessary magnetic field strengths from a wireless or cordless telephone's handset in an in-use vertical orientation is vital for compatibility with the vast majority of behind-the-ear aids. Due to the very wide range of preferred magnetic signal levels shown, only indirect conclusions can be drawn concerning required signal levels. The strong preference for a 65 dB SPL equivalent level can be combined with established standards addressing hearing aid performance to derive reasonable source level requirements. Greater consistency between in-the-field hearing aid telecoil and microphone sensitivity adjustments could yield improved results for some users.