Not the most reliable test I suspect, as it's self-administered, but useful nonetheless.
Can't analyse it scientifically, but it looks like ten years of wearing decent moulded earplugs has served me well. That said, I only started wearing them once I had permanent tinnitus, as evidenced by the total loss of top end (although I believe that isn't unusal for an adult anyway).
Good thread; kudos to the OP.
Gareth