Yes, it's already been stated here but is worth repeating - what we hear is a construct of our brain processing. Fundamentally, there is not enough data in the signals that are picked up by the two ears to fully construct the auditory scene that we construct - we need to use all sorts of pattern-matching, extrapolation, experience of the behaviour of sounds in the real world, sight, etc. to generate the fairly robust auditory scene that we continuously do.
There is a whole hot area of research into this called "auditory scene analysis" (ASA) which is trying to ascertain the rules/techniques that allows us to successfully do this in real time - it's quiet a processing feat.
One of the important points that comes from the research is that we are continually processing the auditory data & updating our best-guess auditory scene.
People who interpret psychoacoustics as being the illusional part of hearing & what makes it untrustworthy are completely missing this fundamental point - psychoacoustics is what allows us to make sense of the jumble of pressure waves impinging on our eardrums. It's what allows us to pick out the auditory objects, such as the bassoon in the orchestra & be able to follow it's musical line through a performance or be able to switch to listening to the string section.
As Adamea says, stereo reproduction is itself a trick - a trick that uses some learned knowledge about psychoacoustics to present an acceptable illusion of a real auditory scene. However, not knowing the full rules/techniques that our brains use in psychoacoustics somewhat hampers this goal of realistic audio reproduction. As a result, we can find that small discoveries are stumbled upon which audibly improve matters in a small way but we have no clear explanation yet for how they are working at the psychoacoustic level.
Without this knowledge of psychoacoustic rules, we are also stumbling around using unsophisticated measurements & I believe, incorrect concepts about the limits of audibility. A lot of the improvements that I hear reported in audio are about increased realism, increased clarity, etc. - in other words they are no longer about frequency/amplitude improvements - they are improvements in other factors which our psychoacoustic rules are picking up on & we are perceiving as more realistic. Or, maybe they are small changes in freq/amplitude that currently are dismissed as inaudible but further knowledge about psychoacoustic workings may well reveal them to be audible when part of the dynamics of music & not when tested in a lab with simple tones?
I don't want to get into an obj Vs subj debate but I feel that this is at the heart of this conflict - what is important in the dynamic waveform to our psychoacoustic processing is not known & therefore not easily measured. It is also very difficult to A/B these sorts of improvements as they are not freq/amplitude/timing differences which are easily A/Bed.
It always strikes me that we are involved in a hobby that touches on these very complex, current areas of research - but they are only using test tones & we are using music signals
. Realising this might bring a deeper understanding of why there are so many disagreements.
