And outdoors? If I’m judging the “naturalness” of sound I think outdoors has to be the reference. If I walk out of my kitchen door to the deck, it is clear instantly the sound inside is much more reverberant. Finger snaps and hand claps inside have some ring and decay. In my music room before the remodel, the decay was clearly audible. Stand out in my yard, not so much. In my music room now the sound of a hand clap or finger snap is clear and clean without any ringing.
So to me, as I try to interpret this "natural" standard, I think I have some idea of what is intended, but getting there is not so clear. If you limit echo and ringing (highly desirable IMO for clean, organized music reproduction) you're going to give up a little bit of "liveliness". Obviously you can go too far.
And I'm sure the iterations could be nearly infinite... if I make a change to my room, do I need to then make a change to speaker position to balance what that change does to the "energy" in the room, i.e. there could become a real chicken/egg dichotomy. As Peter has described, this has been a lengthy process for him. Patience of Job as they say.
Imo, there is no 'absolute natural' or 'standardized natural' in audio, but there is in the real world.
Consider that almost every concert hall sounds different - put the same orchestra, musicians, conductor and music in any one and the sound is at least slightly different, sometimes more different. Natural is our experience of live music in the world, not necessarily outside although a concert outside is an example of it. Here, some of us are talking about what we use as a reference or guide for assesing the sound of our stereos.
Not sure if this is a good example or not: we've read about changes Peter made to his room set-up and small changes to his equipment with the goal of achieving 'a more natural sound' for the music he plays. For simplicity let's say his room before those changes was state A and after the changes state B. Imagine a quartett could fit in his room. Would the quartett's music sound more natural played in his room under state B than the room of state A? I would argue no - the sound will be
equally natural in both cases. Both instances present live acoustic music we hear directly. We might hear some difference between the performances given that there were tube traps in A and not in B, and the listening couch in a slightly different position.
If there was a
recording of the A performance and the B performance, which would sound
more natural when reproduced in Peter's room under state A or state B. Assuming Peter achieved his goal with the changes he made, both performances will sound more natural in the state B room. And the reproduction hopefully will let us hear the slight differences between performance A and performance B.
When talking about reproduction, "naturalness" (apologies to the nominalists) is always relative. The live acoustic music goal can not be achieved through reproduction if for no other reason than not being physically present at the live event. No one knows the live acoustic music experience better than you - you have that experience. And you are the gauge of how relatively close your own system comes to that
if you chose it as your reference. And if the assessment is in terms of more or less, you are the only one that needs to be satisfied with the degree of natural sound you achieve.