I asked her to come back, and I turned off each monitor one at a time - she could "hear" both of them, but the Apple II running by itself didn't bother her.
Strictly speaking you could wire a speaker into your Apple II that would reproduce the frequency; the 15.7khz sound is the frequency of the horizontal sync signal, give me five minutes with the schematic and I can tell you where to tap it and feed it into a preamp, but obviously there's not a whole lot of point of doing that unless you *really* want to annoy her. The reason the CRTs make this frequency audible is they have big coils of wire inside of them (IE, the flyback transformer) which oscillate in a sawtooth pattern at this frequency to swing the electron beam back and forth across the screen. Unless the flyback is *extremely* solidly mounted and in perfect shape this action is going to make it vibrate, just like a speaker does. (Well, not *exactly*, the physical vibration is because of magnetostriction of the coil itself or its support structure rather than the field interacting with a permanent magnet or whatever that's *intended* to move, but you get the idea.) This is why the whole problem goes away if you use an LCD instead; you'll have the same Hsync frequency instructing a scaler circuit to "do things", but it's all happening on a practically quantum scale inside a piece of silicon. The only way you could possibly hear anything is, well, see below.
I just wonder if other folks are bothered by frequencies of other electronics - a kind of "your color red isn't the same as my color red" kind of effect -- or is it really mostly just that band around 15-16 khz?
It's not really related, but to this very day it's not that uncommon to find that your computer or whatever else that has a speaker or headphone jack is badly shielded/isolated between the digital and analog portions, resulting in random digital noise in the background. My Tandy 1000s are pretty atrocious in that respect; if you turn the volume up you can hear all sorts of little chirps and whirrs. Even on modern computers there are things like USB bus polling that happen at intervals that translate to audible frequencies. I suppose in theory if you had an absolutely catastrophically poorly designed LCD monitor that ran a sync line close enough to an audio amplifier line you could get crosstalk sufficient to reproduce a CRT frequency buzz. So far as I *know* nobody has ever screwed up that badly designing a monitor, but never underestimate any given industry's capacity for incompetence, I guess.
Other infamous sources of electronic noise (starting all the way down at the other end of the spectrum) are the buzzing you can hear from some kinds of light fixtures. Older folks can testify to the oppressive 120hz buzz old fluorescent light fixtures produced; the root cause is exactly the same as the noise from flyback transformers. Newer industrial fluorescent lights with electronic ballasts oscillate much faster, usually quoted at frequencies in the 20-40khz range, so the vast majority of adults can't hear them, but the low end of that range is still within the audible range of some lucky (or unlucky) kids. It's also not unknown for the filaments in incandescent bulbs to vibrate (I've heard it myself) at some harmonic of 60hz, and there used to be PWM dimmers that could make this worse.
There was a clear reward system in learning to tune it out...
