It would help if we knew what type of speakers and amplifier you have.
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My current speakers are Avalon Opus with all Spectral electronics (DMA-250 amp/ 30SS pre). I am currently considering Magico speakers for which I would be willing and eager to optimize the room. I had a reputable dealer indicate that one needs a wall behind the speakers, albeit 4-5 feet back. I felt that having essentially no wall behind the speakers (i.e. no chance for reflected sound mixing with direct) would be preferred; but I am not the expert. I have pretty good imaging and excellent soundstage depth (width is reasonable). I have carefully consider first (side) wall reflections, floor reflection, etc. in the placement of the speakers.
What I have found, while setting up both panels and boxes is that the existence of a wall behind the speakers is best so long as your speakers are about 5+ feet away. Adds a bit of 'body' that can disappear if there is nothing there provided the surfaces match the speakers design. Exactly the opposite for listening chair. I greatly appreciate dead space behind a chair (or at least 5 feet from a wall). I can do with out side walls, :-).
The 'body' component is I believe the strongest supporting argument for having a rear wall. Of course we have the floor, side walls, and ceiling for some level of body. I was wondering if a good recording is to accurately reproduce a 'live' sound, it should also include the recorded characteristics of the room or recording studio. Mixing in different listening room characteristics adds some coloration (which may be desirable), but it is not accurate reproduction. I do realize that a completely dead room lacking no 'body' can lead to an equally dead or flat sound. One may say "whatever sounds great to you is fine." I would usually agree with this. My point in this thread is that it is impractical to (reversibly) add or subtract walls to find a preferred listening character. Fundamentals of acoustics/physics must play a role in understanding and addressing the limitations of any room that has not been purpose built for listening.
Problem is that 'accurate reproduction' is such a moving target, especially when most recordings are hardly 'accurate reproductions' of a live event in the first place. Most recording efforts to produce accurate recordings come from classical music department but it only works there when well engineered performances are small scale which seems to overcome some of effects of the compression and downsizing of larger orchestral events. So it is a compromise. Even most soundstaging effects having their orgins in the recorded music, as opposed to set up, are artificially recreated by mic placement which often gives undue prominance to some instruments which is something you never hear in concert. But it sounds nice :-).
So, I would also make a case for the thought that 'if it sounds good to you, then it is good' because sounding 'real' is virtually impossible anyway, and faithful reproduction of what is on the disc of the majority of the music you listen to may not really be what you want to hear. The exception to this may be the few really anal audio enthusiasts who really do want a perfect chain, cradle to grave and they listen to principally train wistles and jets taking off. :-)
Different. Maybe better. 4 feet is too close to be optimum for many speakers - 5 feet, 6 feet, more is often better.
You'll push the front-wall reflections out in time which helps sound-stage depth, drop the front wall reflection intensity which may improve timbre because frequency response isn't as uniform at extreme off-axis angles, have less bass (which can be good or bad depending on how the speaker's baffle step was designed to compensate for wall proximity), move the SBIR null lower in frequency and diminish it (good), and energize room modes differently (which can be good or bad depending on where the speakers and listener rare).
You want the speakers you mention at least 5 feet out from the wall. The more the better. Mine are 7.5 feet and the sound is glorious. I measure from the front of the speaker. Perhaps 13 is too much, but I would have to hear them. The rule of 1/3's works very well based on my experience and current set up. My room is 24 feet long and my speakers are 7.5 feet out from the wall.
I bet 8-10 feet would be ideal for you. You should still have body without bass overhang and great imaging a focus.
I'd think that there will likely be (at least) 2 audible effects:
Overall bass response will likely be attenuated as room re-inforcement from the "front" wall is eliminated.
Bass response should be smoother as frequency specific destructive interference from reflections off that wall are eliminated. (There will still be some destructive effects from the other walls, floor, and ceiling, but that wall is often the worst offender.)
As to the "better or worse" analysis of the net impact, that will be case specific and will turn, in part, on whether there'd be too much or too little bass from that speaker in that room, with the missing wall "restored".
Best soundstage depth I heard out of my system, I placed my speakers a third of the way into my 13x18 room (on the short wall) with a wide, open double doorway behind them, a 10x13 entryway beyond that, and beyond that through another open set of double doors, the living room. The distance from the speakers to the nearest full rear wall was 36 feet. Despite the speakers being closer to side walls, I do think I was hearing the depth of the soundstage on each recording more accurately than I ever had. Unfortunately, bass was MIA (lacking body and depth) and the placement was a bit awkward to live with.
I'd imagine you're probably getting most of the benefit already with a 14 foot distance? You could try some diffusion on the back wall--like a jungle's worth of plants. They help a lot.
Hickory, I just wanted to add my voice to those who say that you'll get better depth if you have your speakers away from the wall. It will increase until they're about 15' out. The same thing is true of width and side walls. The reason for this is that we judge the size of an acoustical space by the time delay between the direct sound and its reflections. Bigger room, longer delay. Since most recordings are made in fairly big spaces (or have artificial reverb to imitate them), having the speakers close to the wall means that the ear will hear the reflection from the room first, and will either decide that the space is a small one or compromise between the size of the two spaces.
So, from the perspective of imaging, far from the walls is often best.
At the same time, I agree with those who pointed out that you'll lose some bass reinforcement if the speakers are far from the walls, though whether you have too much/too little bass will also depend on placement and your room (because of modes). Speakers never seem to have the best bass in the same place as they have the best image! I usually end up positioning mine for best imaging, but YMMV.
Another consideration is that if you build a wall and make the room smaller, the bass will become rougher because the room modes will occur at higher frequencies. Modes are hard to fix, whereas if you want more bass you can just add a couple of subs (which also have the advantage that you can move them independently, so you can position the mains for imaging and the subs for bass smoothness and extension).
I really can identify with your situation which sounds quite similar to my set up. We also have a double door opening (French doors removed) 3 feet behind the speakers with about a 3 foot wall section flanking the double doorway opening on either side. There is another room behind the speakers with the back wall mostly glass. The depth of the soundstage is way back into the second room and bass is not all that bad to my ears. I have been speculating that the combination of a partial wall with the large double door opening and lengthy distance to the rear wall might be an 'ideal' mix of minimizing reflected sound behind the speakers without losing too much 'body' or room ambiance.
Some well intended but misinformed suggestions here with reference to the goal of Avalon Acoustics. I think you already understand the basic principals of thirds as described in the Avalon manual when considering your own room.
As simple as it's described in the manual, if like me, you're dealing with an irregular shaped room it will take a degree of trial and listening. The right recordings and speaker mobility during this period of adjustment can be your biggest asset.
After months of this I was sure I had found the magic placement only to have a friend who is very experienced with Avalons separate the speakers a few more inches and add 1/4" of toe-in and bingo. The sound stage seemed to surround as if the timing of reflection found the listening position optimally. In the end it takes someone who has experience with the particular speakers potential to wring the most out of your room.
The affect this had on the speakers bass was also a result of timing and not about an increase or decrease in bass. In my case (or my taste) the addition of two small Velodyne subwoofers to blend seamlessly and control the lowest octave also had the effect of adding size to my room.
Good luck with it.
I don't know whether it's physics or psychoacoustics, but the only way I can perceive soundstage depth with any real effectiveness is by positioning my speakers as you suggest, Hickory. Bass response is fairly easily addressed by changing the proximity of your head to the wall behind you. That does pretty much the same thing as changing the proximity of the speakers to the wall behind them.
Thanks to all for the insightful suggestions. At a minimum, I have a few other considerations that do not involve major construction. I do not believe that bass in the current set up is lacking, so building a new wall may mess things up with respect to null and re-enforced modes as suggested by Josh358. Also, for Vicdamone (PS. I have a couple of records of the 'original' Vic Damone) I have spent countless hours playing with toe-in with my Avalons and with great benefit. I used to be disappointed in the soundstage width, in particular, which is currently much improved from all of the tweaking.