Questions and answers about assistive listening devices for hearing impaired people:
A. Because there are echoes and noises in an auditorium. A person with normal hearing can mentally zoom in on the speaker and filter out echoes and noises – even though the listener is not aware of all the auditory processing and filtering that their brain is doing.
Q. Why does an assistive listening device help comprehension more than a hearing aid by itself?
A. Because you are hearing the desired sound directly from a microphone in front of the person who is speaking. There is no distracting noise or echo.
Q. My hearing aid restores my hearing to almost normal, so shouldn't I be able to understand normally?
A. Many hearing aid users struggle to understand what is said in an auditorium. A slight reduction in hearing can have a large effect on the ability of your brain to filter out extraneous sounds which in turn reduces your ability to understand speech. This inability to understand speech in a noisy or echo prone environment might also be partly due to reduced brain function resulting from stroke, a head injury, dementia, or other causes.
Q. Why can't I understand the words when people are singing?
A. Because many hearing impaired people have trouble separating voices and instruments. Captions, overhead screens, and a special mix of mainly vocals for assistive listening devices can help.
Q. Why not just place some extra speakers in front of those having trouble hearing?
A. It sounds like a good idea, but it does not work -- it only makes the echoes worse. Sound travels relatively slowly. Hearing sound from widely separated sources is much worse than hearing sound from a single source. In order for the sound from these extra speakers to drown out the delayed sound from the main PA system, they would have to be quite loud. Then people farther away would still hear sound from these speakers as an echo. Echo and reverberation are major factors in preventing people from understanding speech. It is already bad enough that sound from the monitor speakers bounces off the walls and ceilings to create more echo.
Q. Why don't more public places have assistive listening devices?
A. Probably a combination of factors including:
A. In large cities there may not be any clear channels available. No matter where you are, the FCC only allows very low power unlicensed transmitters in the FM band which results in very limited coverage. The more expensive Assistive Listening Devices with decent coverage are probably not much more sophisticated than FM radios, but they are specialty items that do not enjoy the price advantage of a mass market.