To what does the "Absolute" in "Absolute Threshold of Hearing" refer?

It is the standard "hearing sensitivity curve" that I believe Dolby
used to help develop their Noise Reduction systems. It is a curve
showing how the ear is most sensitive to mid-range frequencies (around
1-2 kHz) and how the sensitivity decreases quite a bit above 10 kHz
(in order words, high frequency sounds have to be LOUDER in order to
he heard). This curve attempts to show the QUIETEST sound levels that
the average human ear can detect across its frequency range (20 -
20,000 Hz).

Also, this spectral graph might help explain how MP3 encoding behaves:

http://rvcc2.raritanval.edu/ktek9053/noisetest.gif

The top half is the spectrum analysis of an uncompressed WAV file
containing white noise that fades out gradually, with a constant 2000
Hz triangle wave applied on top at a constant level (the "bars" are
the harmonics that this triangle wave generates).

The lower half is this same audio, as run through the popular
Fraunhofer "Fast" MP3 encoder at 96 kbps. You can see how it starts
to increasingly "cut out" the white noise as the tone begins to "mask"
more of it.

What do these nasty artifacts sound like? It will help me if I know for
what to listen.

It almost sounds like dead spots on magnetic tape -- the piano will
simply drop out momentarily, creating a "chattering" effect which is
actually quite common in low-quality lossy audio compression.