I started to reply to Bonnie's questions about my audiogram in a comment, then realized it was stretching out to novel-length, so here goes.

The axis labels are frequency in hertz (horizontal) and hearing level in decibels (vertical) - so the horizontal axis is how high or low the sound is, and the vertical one is how loud it has to be for me to be able to hear it when I know it's coming, and am straining to perceive it in a sound-insulated testing room. Mileage varies for less ideal conditions.

I gave some comparisons for scale on the pitchwise axis (where frequencies fall on the piano keyboard), but for the db axis, here are some back-of-the-envelopes: 0 db is the threshold of normal-people hearing, 60db is normal conversational speech, 90db is standing in front of a blender, 130db is a jet engine. That means you'd have to be playing the note 3 octaves above middle C at the volume of a jet engine for me to begin to know it's there at all. This happens pretty often, actually; a small electronic device will let out a piercing squeal, making everyone else in the room cringe. I look up. "There's some high-pitched noise in here, isn't there?" Someone will hand me the offending device, and everyone in the room will flinch in (phantom?) empathy as I press it to my ear and hear a faint, faint whine (if anything at all).

Keep in mind that decibels are logarithmic too, so increasing something by 3db means doubling the loudness (so a jet engine is 130-90 = 40, 40/3 = 13.3, 2^13.3 over 8,000 times as loud in the absolute amplitude sense). They describe the amount we perceive volume increases in. It's the mathematical version of saying "when something's really soft, making it a tiny bit louder is a really obvious change in volume, but if it's ridiculously loud already, a tiny volume increase will usually be imperceptible to people who are already hitting the ceiling with a broomstick trying to get you to turn it down." Other senses (sight, etc.) work the same way.

Hearing aids make certain frequencies louder, boosting them to the decibel levels where I can perceive them (or perceive them better, anyway). For instance, flutes sound pretty faint to me, and disappear against any orchestra they're playing in, but my hearing aids pick up their (very high) notes and make them louder in comparison to everything else, and I'll perk up and go "...wait, there's a melody in this part? Wicked!" Imagine playing with the equalizer on your stereo, but really fine-grained. Not just bass and treble, but a couple dozen sliders, maybe more; I'm not sure how many there actually are in a hearing aid, and may be off by orders of magnitude. Now imagine your equalizer could make frequencies really loud or soft instead of just making low notes a tiny bit more emphasized.

I'm not exactly sure what the frequency curve of mine is (but it would be fun to find out - maybe I should visit Olin for a "let's play with equipment" visit. ;) It usually looks vaguely like the inverse of your hearing curve. Anyway. Now pack that processing, in real-time, into a package the size of your pinky finger that runs on a battery the size of a pencil eraser and has to survive being thrown, covered with sweat, across the room by an exasperated 9-year-old whose ears are getting itchy after running a mile with little hard plastic plugs filling them. This is why hearing aids are so expensive.

It does not work perfectly. Imagine that all the sounds you ever heard were played to you as a lossily compressed mp3. It's why I always said no to the idea of a cochlear implant (so far, anyway) and usually don't wear my hearing aids. I'd rather have a few things sound good than a lot of things sound crappy. Also, I'm used to what things sound like without them at this point, and struggle to interpret the morass of high sounds when I put my hearing aids on - it's like adding more noise. I'll usually put them on when I specifically want to be aware of something in that frequency range - most often listening to music.

One of the things I have been musing about is taking some classes on audio engineering - as in music recording, mixing, mastering, that kind of thing, and learning how to listen to sounds and a vocabulary (and a world to talk with in non-clinical terms) to describe them to others. Not sure if this would be setting myself up for a lot of undue pain. But then again, that's what masochists are good at.