Interesting article regarding NOS DACs, measurements, etc...written by the guys at Metrum (DACs)...very interesting to read, not only outlining where some measurements dont always link back to the way humans hear, or how the electronics are producing sound...but then also readjusting those ways of measuring to more closely link to human hearing...interesting read.
http://www.metrum-acoustics.com/Design Philosophy Metrum Acoustics.pdf
interesting quotes about measurements:
"Truthfully, the filtering used by the microphone is not quite sharp
enough to simulate our ears. This deserves some explanation...
...Pictured above is a part of our ears. It concerns the cochlea with is located in the inner ear. Every spot in the
cochlea is sensitive to a certain specific frequency. The highest frequencies are registered at the front whil
the lowest are registered at the end of the cochlea. The basilar membrame, which is part of the cochlea,
functions as a base for 15 to 20 thousand hair cells. Each of these hair cells are connected to a nerve which
is connected to the brain. This is only a (very) short summary of the working of the ear because in truth, the
ear is infinitely more complex. We however will limit ourself to the sensory cells (hair cells) that each have
describe a limited range of frequencies in such a way that they all overlap.
The sum of all of these determines the range of our hearing, which on average is between 20 Hz and 20.000 Hz. With 20.000 hair cells their bandwith is very small and thus very selective. This means that for one specific hair cell, excepticing the
specific frequency that it is tuned to, a signal gets filtered out at 40 dB per octave. This means that our
hearing on this level behaves much like a band-pass filter, comparable to the filters found in CD-players.
Implementing filters of this level in a CD-player can thus be described as overkill..."
"...Because the basilair membrane [of the human ear] behaves as a sharp filter for our hearing, it has become, in a certain way, part of the DAC. The measurements on a NOS DAC are therefore not measured at its logical end-point, but
before the filter (our hearing). Herein lies the problem, which exists when comparing the actual time domain
results and the way in which the Jtest interprets and shows results..."