The frequencies waves can be going through the air. I've heard of stories that some deaf students from my HS were able to hear the police station from down the street on their FM because they used the same freq as the school was using for their FM system. Same with the PDA and security system at the school, the students with FM boots were able to hear what was being said.
With advancement technology of t-coil, loops, cell phone assistance, ect, many things can be interfering.
Of course the school solved the issue by setting up a Freq chart, labeling freq to their use, and each single FM system in the school has their own Freq channel that no one else in the school uses.
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I want to say that I do understand the benefits for FM in schools. It gives a very clear, easy-to-listen voice to the student. Nothing else to say about that.
But just having it for home use or as someone said in this thread "able to talk in the car easy", is pretty wrong imo. Its like saying, "son, you can't hear shit, so I'm gonna hook you up with a FM and I'm gonna have a mic on me all times." That's pretty sad and kinda hurts the kid. Why it hurts the kid? Because then the kid will feel like he doesn't fit in right, he will feel like because of his deafness, he failed you as a son. He's going to wish he was hearing so he doesn't have a crap load of technology helping him out to "fit in". This is where the benefits of ASL comes in. But thats a whole different topic.
Many kids at the end of the day love to just take their hearing aids, implants out because its so tiring to listen all day, usually when they're alone. They feel (even me) that if they have their implant's off when the parent is around, there are communication barriers, and stress.
Idk, I just feel like putting a FM on a kid 24/7 is plain dumb. It will make him less confident in real world situations, will make him more tired, will make him feel like he needs a lot of technology to have the need of feeling, "fitting in".