Mythtrip -
Let's take a look at what's going on in the concert hall, and then come back and look at speakers that bring some of those elements into the listening room.
In the concert hall, the direct sound from the instruments arrives first (naturally), from which we get our directional cues. Then there's a bit of a delay before the reflections start to arrive. There is a great deal of diffuse energy in the reflections, which set up a very powerful reverberant field that actually contains many times more energy than the first-arrival sound (although the first arrival sound is louder, its presence is brief).
This powerful, diffuse reverberant field contributes to the rich timbre we experience in a concert hall. There is a tradeoff relationship in halls between precise localization of sound sources and ambience, just as in the home. Most concert halls excel at rich timbre and luscious ambience rather than "imaging".
Okay, now let's step into the typical listening room. Just as in the concert hall, the first arrival sounds give us our directional cues (and primary timbral cues). But then there is relatively little delay between the first arrival sound and the onset of reflections. And typically, the reverberant field in the home has a very different tonal balance than the direct sound (unlike in the concert hall), because loudspeakers' directional characteristcs change with frequency. Compared to the concert hall, the typical reverberant field in the home arrives too soon; often contains undesirable strong, distinct early reflections; isn't powerful or diffuse enough; has too much bass relative to the midrange and treble energy; and decays too quickly.
A surround sound system seeks to approximate the concert hall by reproducing delayed ambience energy from the rear speakers. Whether or not this is more realistic than a comparably-priced two-channel system is debatable.
The problem with using a second pair of speakers to the sides, without delay, is that the ear localizes sound primarily by arrival time, and secondarily by intensity. Unless the path length to the second pair of speakers is at least a foot longer, they will be perceived as distinct sound sources. And even if they are a little farther away, their "reverberant" energy arrives way too soon, and will still cause image shift. Unless the signal to the second pair of speakers is delayed, in practice it will probably do more harm than good.
Some speakers are designed to put a great deal of energy out into the reverberant field. Omnidirectional speakers do so, for instance. You might look at designs by Shahinian, Wolcott, and MBL, for example. The bi-polars from Mirage and Definitive Technology also generated well-energized reverberant fields.
If placed fairly far out from the wall, dipole speakers can create a good sense of ambience because the backwave contributes to the reverberant energy in the room, and the farther out the speaker is from the wall the longer the delay before the onset of that additional ambient energy.
Treating the first reflection points is a good idea, whether with absorption or diffusion. Remember that in a small room a little absorption goes a long way (because sound waves will bounce it many times in fairly short time interval), and that in general diffusion is the better approach because it preserves the reverberant energy as long as possible instead of swallowing it up.
I hope this helps a little.
Duke
