my son had a device which was basically an FM transmitter, we can hook it into the aircraft intercom system and it broadcast directly to his hearing aids. Whilst this was running he did not hear any ambient noise through his hearing aids because it was a direct wireless link between the transmitter and his hearing aids. They use exactly the same system when teaching kids at school, the teacher wears the transmitter around their neck and everything they say and do get broadcast to the children with hearing aids. Made for some good fun when the teacher left it on and went to the toilet or lifted on in the staff room during lunch, as long as the kids were in the 30 to 50 m transmission region they can pick up everything including gosh/horror SWEARING in the staff room ! It's a big thing when you are only 10.