Prescription Speaker Placement

cmjones, Great question.  I am pretty much in agreement with a lot that has been said so far, but I will encourage you do some work around getting speaker placement and listening position right before you worry about dialing in imaging and soundstage.  This is because what you need to do to get the spacial clues dialed in is relevant only to one particular speaker position within a room.   If you get your imaging dialed in but still have huge problems with low frequency ringing and deep nulls you are not going to be happy.

If you want to get serious about your room, the best thing to do is to download REW room measurement software (free) and learn how to use it.  Specifically with respect to imaging, if you learn to use and interpret the impulse graphs you will have an incredibly powerful tool to use when trying to optimize spacial clues.

If you want a quick and dirty approach without doing actual room measurement, I would suggest the following.

1. As mentioned by MC, EVERYTHING in your room forward of the listening position should be symmetrical right to left.
2. In many rooms, the best position for speakers will be 20% of the room length out from the front wall, and 20% of the room width from the left and right sidewalls.  There is a dynaudio you tube video describing this.  I set my room up for optimal frequency response through a very laborious process of measuring and critical listening and ended up with my speakers within inches of the 20% rule.  In the same way, in many rooms you will find the optimal listening position for frequency response is 20% off the back wall.  
3. Once the speaker and listening position is established, you can begin to get serious about treating your side and back walls to improve imaging.  Human hearing is highly sensitive to spacial clues derived from reflections generated in the horizontal plane.  So ceiling and floor reflections are less important than side wall and back wall treatments.  You can use a mirror to determine first reflection points on side walls by having a second person hold a mirror along the side wall while you sit in the listening position and shine a laser light into the mirror.  When the reflected light hits each of your speaker, you have located the first reflection points on the side walls.  These points are not points, but really are zones, meaning you will need to effectively treat a foot or two foot wide area on your side wall.
4. The early reflections that confuse spacial clues and mess up imaging are those that arrive at your ear within the first 12 milliseconds or so later than the direct signal coming from the loudspeaker.  Any reflection in that time frame, and especially those that are derived from the side and back walls, should be treated so that the SPL of the reflected sound is 15 -20 dB lower than the sound coming directly from the loudspeaker.  
5. I prefer not to treat sidewalls between the front wall and the first reflection point.  Sometimes a little treatment is necessary if the room is excessively bright, but no treatment in that front side wall area usually gives the widest soundstage.  You should be able to hear images wider than the width of the room if you get this right.
6. If your listening position is more than 12 ft  away from your back wall, back wall treatment is less important for imaging.  The same is true of sidewalls, but we rarely have rooms that are 30 ft or more wide.

Thanks to everyone so far.

MC, thanks for your straightforward approach to getting good imaging. In the reply you mention the steps to get the right frequency balance. I know there are apps which assist this, but I wonder if you have a straightforward approach that gets you most of the way to a good balance.

C

millercarbon,

 I find you speaker setup very interesting. I never thought of the string trick. I always measured from the back wall. I would love to hear the rest of your speaker setup  guide.

The method I use is the classic one that has been around for years. I merely refined it to improve precision. But it all starts with figuring out where to put the speakers- and equally important where to sit!- for the best frequency balance.

Two things about that. First, where you sit is just as important as where the speakers go. We have one thread right now with a guy complaining about bass. Look at the picture, he is sitting right against the wall! Well of course you are going to have awful bass sitting right by the wall! Why not sit right in a corner?! So you get the point. Where you sit matters just as much as where the speakers go.

Any normal room there are all kinds of limitations on where things can go. You may be unable to bring the speakers, or the listening chair, very far out into the room. Whatever. It is all about trade-offs anyway and so in that sense these limitations hardly matter.

So, the classical method, starts with speakers and chair somewhere. Hardly matters where. In this phase we are not concerned with image focus or sound stage at all, so forget precision the speakers just need to point sort of the same way. Play music with a good varied bass content, something like Dire Straits Ride Across the River. Listen for a good smooth bass response. The biggest effect on this is distance from walls. So try moving them, a half foot or so at a time, closer or further away from the front wall. Or the side walls. Or both.

Where you sit matters too. So for each speaker location try listening from different places closer or further away. Try not to be too distracted by imaging. We will fix that later. Try and focus on bass response, and midrange balance.

Almost certainly you will find the best sound with the speakers several feet out from the front and side walls, and your listening chair way out into near the middle of the room. This is the basis of prescriptions like the one above, such and such a percent into the room. That is probably about where you will wind up. What I just had you do explains why this is so. It has to do with room boundary reinforcement. Which you just heard when moving everything around, the closer to walls the greater the bass reinforcement. But also the lumpier the bass. Trade-offs. It is all about trade-offs.

Notice your best location for response winds up with speakers several feet from walls, which is also good for imaging! Now with imaging we go back to my first post, to which only one thing to add. There is a tendency to move things a lot, probably because this makes differences bigger and easier to hear. The closer we get to having things dialed in however the more very small changes matter. My speaker placement is within 1/16" and that is why people who hear it always gush about the pinpoint imaging. There’s other things going on but to a large extent anyone can do this simply by following these same steps and being precise about it.

When it comes to toe-in this can be huge. My Moabs were initially set up identical to the prior speakers because I know this is the ideal location for balance. But after a while I began to wonder about toe-in. With very small changes, probably only about one degree or even less, it is possible to adjust imaging to deep and precise or wide and diffuse. A very, very small change can add great depth with these speakers!

This is probably not a question of right or wrong. Straight ahead would be so diffuse just about everyone would say this cannot be right. Even then, notice I said just about everyone. Some will like it. Between straight ahead and straight at your ears is a judgment call.

All of this stuff is a judgment call. The more you do it, the better your judgment! ;)

PS- lostbears, I measure from the walls in my room too. But that is only because I know the room, and the walls. Giving advice on the interzones you never know who has what. Using the string method I have set up systems for beautiful imaging in rooms as diverse as either side of a fireplace in a living room to a large divided screwball walled conference room at CES. It just works.