Some articles on afils:

Audio Induction Loop - Technical Standards
... 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 ... Therefore an induction loop system or AFILS must be capable of delivering field strength peaks of 400mA/m +- 3dB (280 to 560mA/m) ... guide to theory, as well as guidance for those considering the installation of AFILs in buildings for which they may be responsible ...