Cool topic!
 
For a variety of reasons that I'll spare you all, I have been working in a truly horrible environment for quite a while now, but I was really focused on composing, arranging, and just generally developing a new work-flow, so I tolerated it.
 
Now I am still not in a position to build my next purpose built studio, but I did need to do something, so I did.
 
My room is not yet treated, and yet I was able to place loudspeakers and ears such that I have a great stereo image, and smooth frequency response to the limits that my monitors can produce. As I've learned one too many times, placement will get you closer to usable than almost any other factor. In fact, I think placement is the first step in room treatment.
 
So, guidelines that I've used many times to optimize placement.
 
Start with ergonomics. Unless you have software that can accurately model a small critical listening space you have to start somewhere, and this is as good a place as any. Place your keyboard, computer, and other gear someplace that makes sense, and arrange them so that you can use them.
 
It isn't quite that simple, there are some rules of thumb that you'll want to follow as you set things up:
 
1) symmetry is critical. It isn't always possible, so if you have to make concessions make them so that your ears are equidistant from the walls. In my case that meant moving everything back so that the bump out would not directly affect reflections from the side walls. Note that this violates the rule of thumb that suggests that your ears should not be half way between any two boundaries. While this is true with respect to low frequency problems, it is still better to keep this symmetry in tact - it preserves the stereo image, and at higher frequencies you won't have problems with room modes.
 
1) symmetry is critical. (no, that's not a typo) The loudspeakers must be equidistant from your ears.
 
2) distance behind your head is more important than distance in front of your head. Ideally you want at least 12 feet from the back of your head to the back wall. I was able to do that, but you may not. If you can't, the try to place your ears somewhere between 1/4 and 1/3 of the way from the front wall.
 
3) try to place computer monitors, or anything else that will be in the same vertical plane as the loudspeakers so that you don't create obstacles.
 
4) with the exception of left/right symmetry at your ears try to avoid placing the loudspeakers halfway between boundaries. Lateral should be easy, since you have two loudspeakers<G>, front to back can be a little trickier.
 
5) the whole bit about placing the loudspeakers at ear level is, I think, a myth. Sometimes you have to place them higher (or lower) to get rid of bad early reflections. I usually go for higher, and angle them down, but I have worked in rooms where they were lower, and angled up, and that works too.
 
6) then there is the question of orientation - tweeter above woofer vs tweeter aside woofer. They can both work, but in small rooms the aside configuration seems to work better - for me. Even tweeters on the inside vs outside will generate debate, I prefer them on the outside. Again all of these are starting points, and it takes only a few seconds to change orientation as a test.
 
7) now to your original question... try to place the loudspeakers somewhere between 8 and 12 times the diameter of your low frequency driver - for a 6-inch driver that'll be 4 ft to 6 ft. It isn't critical that you create an equilateral triangle, again it is a good place to start, but it is not critical, and in fact I've not found it to work any more often than not. As a rule, the distance between the speakers will have a large impact on the stereo image, and you'll just have to play with that distance till you get your optimal image.
 
Now listen to some of your favorite recordings. Pay attention to the sound stage and the low frequencies. One or both may exhibit problems.
 
Even if you LOVE the results of your initial placement you'll want to move things around a bit, to verify you got it right if nothing else.
 
Once you have optimized loudspeaker and ear placement you can start to think about treatments.
 
Hope this helps...

I stll feel not enough is spoken about the correct distance between monitors. It is too easy to fill up the distance betwen the speakers with computer screens and such and not enough attention is put into distance between monitor speakers. In my opinion it is important.
 
When the distance between the monitors is excessive you get a poor centre phantom image. It will make you push things pannned centre up too high in order to correct for it. On other systems you will then hear the centere panned stuff too loud. Too close and you get an excess of the phantom image and it will be slightly loud making you pull centre panned stuff down a little to compensate.
 
Here is a simple experiment to get this right. Try it and you will hear what I am talking about. You cannot rely on any mathematical approach to determine this distance, it is an audbile use your ears thing really. I also feel that speakers or tweeters at ear level is also important. It sounds better to me compared to speakers being up high or down low as well.
 
A good thing to do is to set your speakers up on a long plank at the right height for this test. (that way once you find the distance if you are using stands you can put them back on stands.) You need two friends to do this. Feed a mono signal to both speakers (music pref something very nicely mixed eg Steely Dan) Start with them reasonably close togther eg 3 feet and centered in front of you. You close your eyes and listen for the phantom image in the centre. The two friends gently slide your monitors apart while you listen with your eyes closed. At some point the centre image will break up and fade away. That is the point when they are too far apart. The idea is to slide them back again so the phantom image re appears strongly. (and it magically does!) And maybe touch more in for extra measure. I have done this and this all happens at 1.1 meters for me and my room.
 
You should do this test first and then see if two computer screens will fit in between. (if you have two screens that is) They may not. If they don't and you have to widen the monitors to fit them in then this is a compromise. You have weakened your centre image. Everything sounds better and more robust when the monitors are the right distance apart. This is a very important aspect of monitor placement. I have got three screens actually but only one in between the speakers. The other two are on the right of the right speaker and it all works well.
 
 
 
 

Jeff,
 
That makes good sense, except one problem I'm dealing with a Mackie 32*8 mixer between the speakers.
 
I am getting a feeling while listening the speakers need to sit a bit closer together, but they'd have to float above the mixer to do that. The mixer sits on a table 28'' from the ground. The table is the exact width of the mixer.
 
Ok, so let's forget computer screens for a second, suppose I abandon the idea of being able to use the computer screen/mouse/keyboard while looking in the general direction of the mixer. I'm ok with that for several reasons actually but even when just dealing with a mixer, it's on a table 28'' off the ground. Not sure how I'd physically get the monitors in closer to create say a 3' triangle... as the mixer is blocking the way. I mean, what else can I put a mixer on aside from a table to allow room for monitors to sit closer together? :p

I could lower the mixer by setting it on a sturdy keyboard stand, but still couldn't get the speakers in closer? Would it be a good idea to lower the mixer 5 or 6'' by getting it off the table, regardless?
 
So I'm stuck with a super huge 6' triangle that takes up my entire room and leaves a dispersed stereo image, or, an isosceles triangle that also has a dispersed center image? I'm not the first person to use a mixer, on a table between 2 nearfields that need to sit on stands or magically float - is there an elegant solution to this? I'm much prefer to get them closer in, somehow, if possible.

those pesky computer screens!
 
Jeff presents an excellent technique to find the optimal space between the loudspeakers. Except it isn't quite that simple.
 
We are dealing with a geometry problem with several variables:
1) the distance between the loudspeakers
2) the distance from your ears to the loudspeakers (this can be measured from each ear to each loudspeaker, or from your nose to the line that connects the loudspeakers)
3) the distance from the loudspeakers to the room boundaries
4) any and all surfaces (console, screens, racks, etc) that can reflect sound
5) the absorption of the boundaries (and this will be frequency dependent)
6) the absorption of everything else in the room, including you, the listener, and of course any treatments.
7) the geometry of the room
8) the size of the room
9) the transmission loss across the room boundaries (another pesky frequency dependent factor)
 
(NB: note that we do not talk about the reverberation time of the room - small rooms are not actually reverberant.)
 
So, that's a mess of variables! You can typically change only a few of them, most likely those affected by the placement of the loudspeakers and the ears.
 
So Jeff's test is an excellent one, although I'd make a small change to the method. Find a recording that has a nice stereo image, and make a copy of that some by adding the two channels together (most audio programs will let you sum to mono, usually with a 3dB or 6dB attenuator inserted into each channel.)
 
Place the loudspeakers at some arbitrary spots that address as many problems as you can - your starting point - and then play the mono version of the recording. Pay attention to where the 'phantom' image sits, and whether it sits still or not. Now listen to the stereo version and compare. Now move the loudspeakers closer together by about a foot and repeat. Did the image change, and if so did it get better or worse? Now move them back to the original position and repeat. Now move them a foot further apart and repeat. And so on. One of these three positions will sound better than the others. Use that as a starting point and repeat the whole process again, and maybe even again.
 
IF there are limiting factors - furniture, monitors, a mixer, whatever, use that to your advantage. If you want to have two 24 inch monitors in front of you then you are going to have to start with the loudspeakers at least 2 feet (approx) apart. Sometimes limitations can make the job easier!
 
As you do this you also want to change the distance from your ears to the monitors. Start with an equilateral triangle for no other reason than it is easy. As you conduct each test move forward and backward, again in 1 foot increments. For each test the best combination is the one you want. And yes, there may be multiple combinations that work in your space. That leads us to...
 
Now mute one loudspeaker at a time and listen to swept sine waves or pink noise or even music - what you are looking for here is frequency response problems, excessive bumps in the curve. The nice thing about using your ears instead of an SPL meter is you won't go chasing the needle - you only have to trust your ears. I find swept sine waves to be the easiet source in which to detect problems, but once I find problems I still listen to noise, and then music to make sure that the problem is real.
 
For the most part, in small bedroom type spaces, it has been my experience that it is pretty easy to pick the optimal location for the stereo image, but that this often leads to frequency response problems. And when you solve those you usually end up collapsing the stereo field a little. Your priorities will determine which problem to favor.
 
In larger spaces, such as a basement or garage it is usually much easier to find a configuration that minimizes response problems without sacrificing the image.

Don't forget that loudspeaker orientation will also play a role, so once you find your sweet spot don't be afraid to try different orientations.
 
As I mentioned earlier, I recently (finally?) went through this exercise in my own basement studio. The stereo image is excellent, and I have no annoying bumps in the low frequency range. In fact I think I got a little lucky, and the early roll-off that I hear at the bottom is in fact the roll off from the small monitors. I have a set of larger monitors that I will start experimenting with as soon as I replace the surrounds, which finally rotted.

This exercise can be done in an afternoon - maybe a long afternoon depending on your room. And tolerances are forgiving, if you get the loudspeakers within 4-6 inches of that target you'll have a valid test. You can then get out the tape measure and place things exactly. You may not even hear a difference between approximate and exact locations.

The other comment that was made earlier that I'd like to underline is levels. In a smaller space you can overdrive the space itself, definitely take that into consideration. And since you won't be using fancy test gear, perform the tests at a level similar to the loudest level you are likely to work at.

Oh yeah, and have fun!

I agree with Jeff on the 3' wide placement.  I only have 1 22" monitor between them and my listening field is really open.  I have sound foam behind the speakers and they are about 6" from the wall.  My keyboard is to the left.

I just remeasured my speakers and from center cone to center cone I am at 44 inches.  

question... is it possible to have these current day monitors set in such a position that two or three people can all sit in a "sweet spot" for listening... in anyone's opinion?  Although "wide" spacing may not be the best solution, ( like 5-6ft), what would be the recommendations for using smallish, nearfield  monitors, so I could have a client or two listen to playback at the same time?  IOW...sitting beside me is preferred.
Seems the JBL LSR30X series addresses that with their "waveguide" horn lens...or whatever you call them nowadays.
So, if we're getting down to 40 inches or so, that seems to exclude anyone else after one person.
Or do I position another set of speakers, that cover a larger area... like mid-fields?

I agree that close spacing sounds great, and probably the best for using these types of speakers, but 25% of the time, I'm not listening alone.  (hand the other person your headphones!!)

In my opinion, if you need to handle both mixing and multiple listeners in the same room, you may want two sets of monitors.
 
I have my mixing monitors spaced 40 inches apart and toed in at 30 degrees, which provides excellent imaging for me sitting at the desk, but they just can't provide proper imaging for the "spectator seats" 3-7 feet behind me. I would get a mix to have a wonderful image when listening at the desk, but when listening to it from the "spectator seats",  the imaging wasn't nearly as good.
 
Specifically to address this, I recently added a second pair of monitors spaced 7 feet apart and toed in at 15 degrees, and this provides excellent imaging for the "spectator seats". By the way, these second monitors are JBL LSR2308P's, the somewhat similar predecessor to the LSR308's, and I'm very happy with them.
 
I also have about as much acoustic treatment as my fairly small room can hold installed, which is also very important. And lastly, I use IK ARC2 on both sets of monitors. It does an amazing job of fine tuning my monitors to my room. Both sets sound wonderful and virtually identical to each other at their respective listening positions. Others may disagree, but from my experience, I'd say that ARC2 can make any decent monitor you may have sound extremely accurate in your room, whether you spent $300 or $3000 or more on them.