Why train your hearing?
Mixers don't have magic ears
What they actually do is sit in front of a parametric EQ for years, sweeping a narrow bell up and down a track until they can do it without looking at the screen. After enough hours of that, certain numbers go from being numbers into something close to a sensation. You hear a frequency and your brain hands you the number before you have thought about it. No genetic shortcut, no mystery. Just reps with feedback, which is the part this page is here to provide.
The payoff lands the first time you put on a new pair of headphones and a 250 Hz dip jumps out on its own. Reviews stop hand-waving. Buying decisions stop being "I think this one sounds nicer." Words about audio start lining up with measurements other people publish about audio, which is the only common language honest comparison can happen in.
About the range
Healthy young hearing reaches roughly 20 Hz at the bottom and 20 kHz at the top. That ceiling drops with age and with cumulative loud exposure. Most people in their 40s have already lost the top octave, and by 60 the lid sits closer to 12 kHz. The Audiophile and Sweep modes here run all the way up to 22 kHz partly so you can find where your real ceiling sits, partly so you can put a teenager on the same chair and feel ancient about it.
Different parts of the spectrum do different work in a track. The lowest octave is closer to vibration than to tone unless your room can actually move air at those wavelengths. Voice and most melodic content live somewhere between 200 Hz and 5 kHz, and a small dip in there is what gives recordings the "distant" quality reviewers complain about. Up above 12 kHz is the air band. Lose it and a recording feels flat across the board rather than missing any one element.