I find subjectively that a larger soundstage is partly associated with FR, specifically a less "mids-focused" (i.e. "U-shaped") FR. it may be that a larger "soundstage", defined as the listener's subjective impression of being in the audience at some distance from various instruments and performers, may be affected by directional or other cues from the headphones themselves (e.g. angled drivers), but also is affected by perceptual cues in the music associated with distance from the performers (tonal and amplitude balance among the instruments). The EQ applied to the recording or to playback may affect this.You see, that's the issue with whatever people call "soundstage" as long as it isn't operationally defined in a way similar to what we're seeing right now in virtual reality / surround sound simulation research.
Since it isn't defined, and can't as a result be measured in any way, we're left to subjective impressions only.
As a result we're left to believing or not others' impressions. It's perfectly fine that you're unwilling to believe my own (after all they're subjective impressions of a totally undefined characteristic, which makes them a lot less comprehensible than "I hear a peak at 6kHz" - which would be theoretically measurable), but why should yours then become more believable than mine ?
I would rather think that it's more plausible that they just happen to produce the right FR at my eardrum (or at least deviate from it in a more ideal way than other deviations) for most of the spectrum for me to subjectively find whatever I personally call "spatial qualities" or "soundstage" with stereo recordings better than other alternatives, but I would not presume that this would systematically apply to others.
In doing many A/B, bidirectional, Harman/Oratory matched, level matched comparisons, my pragmatic impression is that a significant part of a perception of soundstage is due to tonal balance. This is a perceptual cue in the recording that is affected by playback EQ. If I use headphones tuned to more to the mid's, then there is often more of a focus on just a few instruments or performers. If the recording is of a variety of acoustic instruments (e.g. an orchestra, or any acoustic performance with a variety of performers on a stage), this can cue the listener to imagine being close to or with the key performers (e.g. vocalists) or certain instruments. By contrast, a flatter tonal balance will bring a wider range of instruments into tonal (and often amplitude) balance, and cue the listener that s/he is more "in the audience". Ambience may also be conveyed by bringing reflections, resonances or more-physically peripheral sounds into the playback, particularly in the bass end of the FR.
Angled drivers, frontally-perforated earpads, open-back earcups, or other physical features may convey a wider soundstage. But my experience is that tonal and amplitude balance among instruments also plays a role. When doing A/B comparisons using EQ's that alternately give greater or lesser emphasis to the mids, I expected to find myself preferring one EQ or the other, depending on the musical material. For example, I appreciate close-miking of good female vocalists. But on the fly with A/B comparisons, even if the mid's-focused EQ sounds more energetic and revealing at first, I usually find myself preferring the U-shaped EQ with less mids-emphasis, because it makes the soundstage appear larger and the tonal/amplitude balance more like a live performance. Now, that is a personal preference; but it is definitely associated with a preference for a realistic soundstage.
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