From a previous post of mine:
“You mentioned full scale symphony. The ability to convey the hall ambience on a good recording again goes back to getting the reflection field correct, as the in-room reflections are in effect the "carriers" for the reverberation tails on the recording. I can go into more detail about this if you’d like.”
To which @phusis replied:
“I’m certainly all ears for an elaboration here.”
In the playback room there is in effect a “competition” between these the “venue spatial cues” on the recording (whether they be real or engineered or both) and the “small room signature” cues inherent to the playback room. The goal is to make the “venue spatial signature” perceptually dominant.
At the risk of over-simplifying, the ear/brain system gets room size information from the earliest reflections; from the reverberation tails; and from the “temporal center of gravity” of the reflections. The latter refers to the interval between the first-arrival sound and the “average” of the reflection energy; the further back in time the “temporal center of gravity” is relative to the first-arrival sound, the larger the indicated room size.
We can disrupt the “small room signature” by taking advantage of the directionality of horn loudspeakers. We can toe them in aggressively such that the speaker axes criss-cross in front of the listening area. This way the radiation pattern of each speaker does not illuminate the same-side wall; rather, the first significant lateral reflection for each speaker is the long, across-the-room bounce off the opposite side wall. This also pushes the “temporal center of gravity” of the reflections back in time somewhat, further disrupting the “small room signature” of the playback room.
At the same time, we WANT plenty of spectrally-correct later-arriving reflections arriving from many different directions. We want to preserve the spectral balance of our in-room reflections by using as little absorptive treatment as we can get away with. If the spectral balance of the late reflections is preserved, the ear/brain system will detect the recording venue reverberation tails therein by recognizing the overtone patterns. The ear/brain system is actually able to detect and follow the reverberation tails down into the noise floor IF the overtone patterns are intact. But if the higher overtones (the higher frequencies) have been absorbed, the recording’s reverbration tails will not be effectively presented to the ears by the in-room reflections; they will have ceased to be "signal" and will have become "noise". Also by preserving the later reflections instead of absorbing them, we are again pushing that “temporal center of gravity” of the reflections back in time a bit.
In my own designs I often cheat and inject a bit more late-onset reflection energy via rear-firing drivers.
One of the reasons I prefer constant-directivity horns is that their off-axis energy tracks their on-axis energy well, such that the later-arriving reflections have more intact overtone patterns than if the off-axis energy had started out with the highs already rolled off.
If all goes well, the playback room’s inherent “small room signature” package of cues will have been disrupted by the early reflection arrival times presenting an inconsistent (and therefore weakened) picture, and also by the “temporal center of gravity” of the reflections indicating a larger room size than the actual room. If at the same time we have reverberation tails coming from all around which are indicating the much larger acoustic space of the recording venue, this can tip our perception in favor of the recording venue package of spatial cues. When this happens, and it is to some extent recording-dependent, the result is a “you are there” presentation wherein, with eyes closed, one has the perception of being within the acoustic space portrayed by the recording. For instance instead of the soundstage extending several feet behind speakers, it extends as deep as the recording cues indicate.
These are not the only factors in play, and the relevant principles can be applied to non-horn systems, but the typically narrower and more consistent radiation patterns of horn speakers can contribute to that elusive “you are there” presentation.
