I don't know quite how to put this, but hearing is not simply a matter of an auditory apparatus (the ear) sensing a signal. Hearing is a very complex process using many neural pathways throughout the brain. The pitches we prefer for listening to CW no doubt reflect a complex process of perception and interpretation.
This pretty well says it..... to the unprofessional audiologist, it's very difficult to explain. My memory has failed me here but I seem to recall that the lower frequencies have more power than the higher frequencies. In which case frequencies around 600-1KHz contain more power that those frequencies around 3KHz and higher.
It also seems to me that sounds around 3KHz are very similar to the other noise one hears on shortwave. This makes the lower frequency sounds much easier to hear.
Because of my poor hearing I have taken many hearing tests down through the years. In each case, it has always been easier to hear the low frequency sounds than the high frequency sounds....even those around 3KHz.
I presently use a homebrew device that boosts the 3KHz frequency range amplitude to I can operate SSB. Once again, I think this is because the higher frequencies don't have the power to stimulate the "hairs" in the inner ear.
It would be interesting to hear an explanation from an audiologist on this subject.
Al - K8AXW