Front wall/back wall? I'm from downunder remember, it all looks different from here. I haven't had any chance to use RHUBARB yet on this post, which is my only disappointment. Otherwise this is a very good read.
A friend asked me the question that I started this post with because he wanted to spend a weekend setting up his first high-end system, and most of it was easy to lay out for him, except for this speaker placement issue. So I tried to think about how I could describe what to listen for, but despite setting up speakers loads of times I realised I didn't really have a method, or at least it was too intuitive for me to write down. Without pushing my speakers around the room to remind myself, I will post some things that I do remember about setting up speakers.
First of all, I believe that amp/speaker/room size relationships change the results. That is - for my room, changing the amp or speaker can (not always) result in different optimum positions. I think the root of this is that moving speakers around can affect the relationship between the sounds that define the outline of a singer or instrument and the sounds that define the body of a singer or instrument. The perceived "reality" of the sound and image are therefore affected if we do not hear the right combination of primary sounds as well as resonances and the harmonics of both of those. A similar relationship of sounds affects depth, width and height perspectives, and it seems to me that our brain recognises the very subtle ways in which propogation of sound through air, and multipath (ie. the combination of direct and reflections) affects the waveform, and that these relationships have to be right for us to perceive the "space" clues in the recording. But all of those relationships can also be affected by going to a speaker that moves significantly more or less air (which I mean in a way that is different than sheer loudness), in that a speaker that is too "small" for a room may need to be moved closer to boundaries than it would in a smaller room. Similarly a "large" speaker can overpower a room unless it is moved out towards HP's "thirds" position. Similarly a very powerful amp can make a "small" speaker perform more like a "big" speaker. I am not sure precisely what I am talking about. Is it just tonal balance? No. Perhaps there is something to do with relationships between impulse and decay (whoops I almost spelt that with a "k"). Perhaps it is something to do with phase errors. Perhaps it is something to do with reducing the smearing effect of reflections in the room that destroy resolution. Most likely it is all of these and some others too. Either way I think we may be trying by empirical experiment to optimise the value of a multinomial equation. So my second point is that getting an optimum result can be severely inhibited in the first place if the amp/speaker/room size equation is poor. So you end up facing a compromise like tonal balance is best with the speaker very close to the wall but soundstage is flattened. Obviously you also have a problem if either the equipment or the room is poor, but that is another matter.
So... The first thing I do is some very quick guages of a room, by just plonking the speakers in a likely place and moving them three or four times only with a brief listen in between. I listen to soundstage shape, gross tonal balance issue (ie. way too much or too little bass) and how images float free of speakers and boundaries. I do this to check which of a room's walls I want the speakers against, or the wall I will work with first.
So... Moving speakers forward and back (back and forward for you northern hemisphere people). The problem with sorting this out is that two effects come into play. The first is that, too close to the wall and a female vocalist's image can flatten and the outlines and nuance(the sound of the parting of a tongue from a lip or the breath that accompanies the vibration of the vocal cords) become recessed. This is what I call too much body and not enough outline. The voice just becomes less interesting because there is less going on - the steak with no sizzle. Too far from the wall and the opposite occurs. More outline seems like more resolution, and soundstage depth can be great, but without the body of the voice or instrument, so it sounds lacking in substance and seems to stir the soul less - all sizzle and no steak. What mucks this up is the fact that what gets the right balance for all those midrange voices and instruments may not be right in the bass - for the reasons I raised earlier about size of speaker and size of room etc. With too much bass going on the sound is no longer coherent - there is a transition from a tonally right mid and treble to an overly warm bass that lacks outlines (definition). This is arguably worse than too little bass going on, but on the other hand too much bass can be dealt with by room treatment (which I prefer to do than add a sub-woofer).
So... Lateral movement - ie. moving them closer together or further apart. Firing across a room I am in some agreement with how it was put in one of the first posts here (Abstract7 it was) that it is about opening the soundstage up without it falling apart or images wandering. But it is also about the very same things that I mentioned in moving speakers back and forth, and even more so when firing down a room - ie too far apart, which is arguably also too close to walls and images go flat and undefined, too close together and images lack body. So both of these are going on. And what's more our bass problem is moving around in the equation too. So here it seems to me we are playing around with three outcomes which may not optimise at the same point.
As if that was not enough, if you literally tried the above to try and achieve an optimum result, all your observations only apply to the position you were seated in! Where you sit can be a very useful way to solve that bass problem. But I also find I just like sitting about 10 feet away. Closer and it becomes like listening to headphones, further and it feels like listening to the room. Only in a very well treated room do I like sitting much further away. So moving back and forth can put you too far in, or too far out of the soundstage.
This post is way too long so I will be brief and then stop soon and come back later.
And then there is toe-in. Too close to side walls and more toe-in is required. Similarly more toe-in is required if you have the speakers too far apart or if you are very close to the speakers. The sound of more or less toe-in is covered in posts above.
Which leaves - what seems to work? But I will add my thoughts to the excellent posts already posted before me on this another day.
