There are MANY formulas for speaker placement, but unless they take into account the variables such as individual room acoustics, speaker radiation patterns, irregularities such as furniture, room openings, alcove's, archways, adjoining rooms, etc... they are all but useless. They might get you part of the way there if you have what would be considered a relatively "square" or "rectangular" room, but anything else is going to be strictly hit or miss.
Here's a "plan of attack" that i've found to work pretty well. This works whether you have speakers on the long or short wall too. Personally, i almost always prefer speakers on the long wall but it is not always practical given room uses and traffic patterns.
To start off with, you will need a good quality recording of a solo vocalist ( preferably female with minimal reverb applied to her voice ) and a mono switch on your preamp. If you don't have a mono switch ( which you might not on a receiver ), you MIGHT be able to get away with connecting a "Y" cord from one channel of your CD player into both the left and right inputs on your preamp. I've never tried it that way, so don't know how well it would work. Listen to the disc this way using one channel at a time and see which channel ( left or right ) of the cd sounds more natural i.e. like the person is in the room. Use that channel to split into the input jacks and the other channel would simply remain unused for now.
All "funky knobs" ( tone controls, spatial effects, HT modes, etc... ) should be set to neutral, bypassed or minimized and only the two front "mains" working. In effect, we are looking for "straight wire with gain" settings for a normal 2 channel system.
First of all, we are going to work on getting the distance between the speakers in the ballpark. Start off with the speakers appr 4' apart and out a reasonable distance from the wall behind them. The speakers should be firing straight ahead ( aka "flat faced" ) with no toe-in for now.
Now start the recording of the solo singer and listen to this at your seated position. Obviously, you will have NO "dimensionality" to the sound as the speakers are way too close together. The one thing that you should hear is a very centralized point of origin for the vocalist. You should gradually work on spreading the speakers apart and sit down and listen after each move. When you get to the point that your central image is now weakened to the point of being able to pick out individual left / right placement, you need to move them back in just slightly. You should now have a pretty solid center image with just a bit of channel separation taking place. If you move them back in too much you'll minimize the stereo "spread" or "width of soundstage". If you keep them too far apart, you'll have a complete lack of center image to anchor the sound and poor imaging characteristics.
Once you've done this, now we can work on achieving a good tonal balance and optimum soundstaging within this specific installation. To do this, you need to put in a disc that you are highly familiar with that offers good output at both frequency extremes. Don't go over the top with big "boom boom" discs or something that will rip your ears and eyes out with "hot" treble. We are looking for a GOOD recording that will represent an "average" for all the discs that you will play. At this time, put the system back into normal 2 channel stereo mode.
Put the selected disc on and listen for a bit at your seat. If it sounds bass heavy, try moving the speakers further out into the room. If it sounds thin, try moving them back towards the walls. Do your best to maintain the proper distance between them and also make sure that they remain "flat faced". You don't want to go too far back towards the rear wall as this will "probably" play too many games with imaging. This is due to having a bunch of gear and a tv perched between the speakers ( as in an average HT or non dedicated listening room ). This is commonly referred to as a "nearfield reflection" and the results are never good.
If all went well, you have now found a ballpark point that should give you a wide soundstage, relatively solid imaging and good tonal balance. If you don't have that, you need to get to that point BEFORE going any further. Check it again just to make sure and i'll wait here : )
Now that we've gotten this far, we need to determine what your personal preferences are for several different aspects of listening.
If you like a more spacious sound with relaxed treble response, you may be okay with the speakers running "flat faced" and where they're at now. This should offer a good overall balance between width and pinpoint imaging with neither coming across as being dominant over the other. Soundstage depth will appear to come from somewhere relatively even with the speaker baffles ( the front mounting board that holds the drivers ) to somewhere slightly in front of them. Since the sound is projected directly forward, one can almost seem to be sitting at the very edge of the stage if you move in nearfield while staying centrally located between the speakers.
If you prefer a more intimate presentation with increased detail and "hotter" highs, you can play with toeing the speakers in. By this, i'm talking about aiming them more towards the center of the listening area. You have to be careful with toe-in, so don't go crazy. Tonal balance, soundstage, imaging, etc.. are all DRASTICALLY effected here, so take your time and work in small increments.
Since the tweeters are more on axis ( firing directly at you ), treble output will increase and the L-R signals will have a more pronounced "point of intersection" near your listening position. This moves the soundstage inwards, which reduces "spaciousness" or width of the sound stage. You also have a narrower "sweet spot" but it can drastically increase the ability to do "amazing imaging tricks".
Since the point of intersection between speaker baffles is now angled inward, the soundstage can appear slightly deeper and more centrally focused. In extreme cases, the sound can appear to eminate from well behind the center of the speakers. Obviously, different amounts of toe-in will vary how severe this is and how wide the soundstage is. If you move in nearfield with speakers that are toe'd in, it almost seems as if you are on stage with the performers.
Since toeing the speakers in to achieve the desired amount of imaging will affect the tonal balance ( increased treble response ), you might have to play with the front to back depth of the speakers again to achieve a little more bass output to keep things tonally balanced. Once again, be careful to maintain the width between speakers and the appr angle of toe in. If you have to move the speakers back quite a bit, you might also have to re-adjust the amount of toe-in also. Like i said before, don't go crazy as it will only take you longer and make you a LOT more frustrated.
One thing that i also want to mention is that toe-in will tend to lower the effects of the primary point of reflection. Since more sound is focused inward, you have a little less "spray" bouncing off the side walls. The "spray" is still there though, so don't think that you should overlook that aspect of achieving optimum performance or applying room treatments. Then again, that's another subject all together : )
This approach takes into account all of the variables of your individual system. It factors in the specific room acoustics ( including all of the aforementioned variables ), your speakers radiation pattern and your personal preferences in terms of soundstaging, imaging and tonal balance. I've used this approach more than a few times when helping set up systems for friends and it has never let me down. Obviously, others may have different approaches and ideas and that is fine too. The bottom line is finding something that works well and offers consistent performance from system to system while being easily repeatable. Sean
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