As Griesinger writes stream separation happens in auditory system when original sound harmonics are preserved well enough so that all the harmonics line up in phase and superimpose forming a huge amplitude peak on every fundamental period, which makes the sound source stick out above all sounds around, providing enough SNR. Basically, auditory system notices there is sound in close proximity, provides it with it's own neural stream, makes your mind pay attention to it because it's likely something that's important for survival. Now, there is more clarity (and envelopment, engagement, sharp localization, focus, the adjectives) in your perception.
It feels intuitive to me to think auditory system a layer below our consciousness and what we perceive is what the subconscious auditory system let's us to perceive, let's into our reality we have our mind observing on. I like to think our conscious mind listens the auditory system, not the pressure variation at ear drum. The stream separation is just a tool for auditory system to prioritize sound over another, allocate more processing power and attention to important sounds, while suppressing the other sounds, the unimportant ambience.
Allright, wrangled harmonics could be a property of speakers, like edge diffraction, bad crossovers, what ever issues that mess up original harmonics. But it could be also the room, early reflections do this as well as per Griesinger studies on Auditory Proximity. Stereo setup is particularly fragile as the phantom image depends on two sound sources and not just one. The worse the speaker, the worse left/right are matched, the louder and shorter the early reflections, the closer you'd need to get to speakers in order to have a chance for well enough preserved harmonics to have auditory system handle provide full detail. At least this is how I've understood it.
What I'm trying to promote here is that knowing the auditory system is a filter to your perception, you can go and exploit it to your advantage: if you want to hear clarity just get close enough to speakers to force your auditory system to enable it for you. Or, if you want perceive washed relaxed sound and focus on something else, just stay far enough not to allow stream separation happen to prevent too much attention.
Room sound in spades? no stream separation is my bet while also adaptation to sound plays a role, there is rarely just one effect at play but multitude of. Have you tried to find the transition where stream separation happens? Which side are you making the quoted observation, or both, stream separation or without? I think I've asked this before but not sure if you returned
I'd bet that all of your speakers have slightly different max stereo triangle size where stream separation happens. Ideal omnis would have smallest, ideal dipoles would have largest as long as toe-in and position is optimized. It could be possible, that the real world speakers themselves hamper original harmonics and alter the results though.
It would be great if you could find transition distance (size of stereo triangle) for each of the different radiating speakers you have!
I have only one pair currently... so all I write has a lot of reliance on what Griesinger writes is true, and what I perceive with my setup is what Griesinger writes about.
If the stream separation is true thing I would think immersion can only happen with sharp image localization and precision! I think immersion as having a feeling of me inside the recording. Not having sharp localization and precision means it's spacious sound of the local environment, and not that which is embedded in the recording, which to me means there cannot be immersion, you stay in your room and not drift away to somewhere else. Perhaps you and I have different meaning for immersion? nevertheless a case of confusion that could be removed with common understanding about how the perception changes with stream separation.
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I'm sorry the post got very very long, but it's hard to write about the stuff shorty and that's because I think this subject makes great confusion all around the forums, seen almost daily, many many threads I have no time to write on, so hopefully occasional long post gets some attention