Chuck,
I do not know what the exact FAA limits may be, but I expect they are similar to the UK CAA. The Medics here are interested in your hearing in the speech range, which is 250-4000Hz. Above that is not a reqirement for UK licence. A 15db loss is not too bad and is certainly way above the maximum loss permitted here.
We fall below the limits at a 35db loss, but that does not mean you lose your licence as the CAA move the goalposts if you are old and experienced. I have a 40db loss at 3000hz (deafness occurs at 50db loss), but as I am 54 and have 16,000 hours they say I can keep flying.
My advice to you is to buy some Active Noise reducing Headsets. They are amazing. I wish I had used them years ago, and then I would still have some hearing left.
As regards what the Db means, it is roughly equivalent to percentage loss of hearing at that frequency. So 15db is a 15% loss (about) at that frequency.