An objective of the field strength requirements of standards for AFILs is to make the perceived loudness of sound from the loop the same as from the microphone in the hearing aid. This is the basis of the average field strength of 100mA/m used to generate today's performance standards around the world.
IEC 60118-4 (formerly Britain's BS6083 part 4, also known as EN 60118-4) is now the main specification for international use. This is based on the principle that the long term average of the field strength at a typical listening location must be 100mA/m, +- 3dB. To determine this long term average requires a measurement over 60 seconds or more. The standard therefore sets a more pragmatic requirement for determining the short term peaks of the signal. Short term peaks need to be 12dB (x4) higher than the long term average, based on the fact that peaks of speech are approximately 12dB higher than the long term average level of speech. Therefore an induction loop system or AFILS must be capable of delivering field strength peaks of 400mA/m +- 3dB (280 to 560mA/m). Peaks must be measured using fast RMS measurement (125ms averaging time).
In addition the IEC 60118-4 standard sets limits on acceptable background noise, and requires that the system delivers frequency response of +- 3dB from 100 Hz to 5 kHz relative to the field strength at 1 kHz. All measurements must be made with a coil that picks up only the vertical component of the magnetic field, the component that is picked up by the telecoil of a hearing aid.
BS7594 (published by the BSI and widely used in Britain) is a non-mandatory guideline for the design and installation of induction loops. It has a comprehensive guide to theory, as well as guidance for those considering the installation of AFILs in buildings for which they may be responsible. It also contains some valuable guidance relating to other equipment within the loop area. The calibration of field strength measuring devices is also included.
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