Space...the Final Frontier
The performance hall in Boulder's Chautauqua park is a big, barn-like, wooden building that has no memorable acoustical signature, unlike, say, Boston's Symphony Hall or Philadelphia's Academy of Music. In fact, from a Row-H, center seat, I'm hardly aware the hall has any reverberation at all. But one July 4th concert, held outdoors on the green, left no doubt in my mind that the seemingly benign auditorium had been contributing immensely to the orchestra's indoor sound. Outside, it sounded completely different---cold, stark, and dead. The orchestra had no acoustical frame around it.
Real-life ambient space reaches our ears from all directions, as a multiplicity of diminishing echoes from the concert hall's sides and rear, and our ears expect to hear the echoes as such. As long as the reproduced soundstage is more or less bound by two frontally located speakers, we know, at least subconsciously, that what we're hearing is wrong. There's only one way to put the hall sound where it belongs---all the way around you---and that's with a surround-sound system.
Most of us who think of surround-sound tend to think of Home Theater and raucous blockbuster movies---with airplanes roaring over one shoulder, explosions blasting behind the other, and gunshots soaring all around. Except for those who remember the quadraphonic debacle of the '70s (footnote 1), most audiophiles are unaware that surround-sound can also be used to reproduce musical space the way it ought to be reproduced---from all directions.
Audiophiles often choose loudspeakers that excel in soundstage presentation over those that give a more convincing portrayal of the instruments themselves. Much of that "space" around and behind your loudspeakers, however, is ersatz. A reviewer's listening room greatly contributes to the sound's apparent realism---sounds reflecting from the wall behind the loudspeakers (footnote 2) enhance spaciousness, and reflections from the other room boundaries help to restore some of the enveloping ambience of a real space that the system can't reproduce by itself.
This becomes immediately apparent when you listen to the same loudspeakers outdoors or put sound-absorbing treatment on the walls behind them. I have found that when all you hear is the speakers themselves, a lot of the soundstage depth disappears. There's apparent distance but little "real" depth. That deep, wide soundstage in your listening room isn't coming from the recording, it's largely the result of interference between the reflected Left and Right signals, which gives rise to "comb-filtering."
Comb-filtering is the creation of narrow frequency-response dips (which resemble the teeth of a comb) through interference between two or more similar signals that are out of phase. This happens when the sounds from two loudspeakers reflect off the front wall; the reflections come from the entire area of the wall and follow many paths of different lengths on their way to your ears. If the speakers are symmetrically placed and the listener is dead-center between them, the combing dips will be at pretty much the same frequencies in both channels; there'll be little intra-channel difference due to the dips, and negligible spatial enhancement from mono recordings. But because the hall ambience on a stereo recording is very complex and is vastly different between the L and R channels, their front-wall combing produces tremendous differences in the reflections from the wall to the left and right of the speaker pair---differences I believe the ears interpret as greatly enhanced spaciousness.
This is why minimonitors so often throw such an impressive soundstage---they comb a wider range of middle frequencies than large speakers. A piston-type loudspeaker (as opposed to a dipole) radiates low frequencies in all directions, and high frequencies forward---toward the listener. The frequency at which radiation begins to change from omnidirectional to directional relates to the width of the enclosure's front panel, and becomes higher as the panel gets narrower. A very narrow speaker---and they don't come much narrower than a minimonitor---is omnidirectional well up into the midrange. This means an unusually wide range of those frequencies that our ears use for direction-finding is wrapping around the enclosure and being combed off the front wall. [See Sidebar.---Ed.]
Since combed wall reflections that add spaciousness are mainly a result of L- and R-channel ambience, they do relate to the acoustical environments of the recordings themselves. This is evident from the range of apparent soundstage "dimensions" that we hear from different recordings. But a lot of that soundstage information is actually disinformation; it's being synthesized by the front-wall combing. It's not a reproduction of what's on the recording, because two loudspeakers are incapable of reproducing all the recorded depth information. [See Sidebar.---Ed.]
Footnote 1: There were four competing four-channel "systems," and most quadraphonic recordings were used for hurling sounds at the listener from all directions rather than for trying to reproduce acoustical space. Classical listeners were offended, rock/pop listeners didn't give a damn, and the decoders didn't work very well. It died of natural causes, and the industry still avoids using the Q-word. The S-word carries no such stigma.
Footnote 2: These are often called rear-wall reflections, because the wall is behind the speakers. In surround-system parlance, the rear wall is the one behind you, so it's less confusing to call the one behind your speakers the front wall.