I don't get what prescription is required for a hearing aid? Are there different types? Like with equalizers or volume controls? Why can't a person, who is ha

I don't get what prescription is required for a hearing aid? Are there different types? Like with equalizers or volume controls? Why can't a person, who is hard of hearing, go to a store, pick one and start using it? Granted, let FDA approve them and mandate using only FDA approved ones.