Proper speaker placement is one of the most important parts of an audio system setup. According to Jim Smith, the author of Get Better Sound, the speaker placement alone has 50% impact on the overall system performance. My experience confirms that notion. The most important points of the placement are the distances between speakers, between speakers and the wall behind them and between speakers and the listener. One should not spare the distance to the back wall. I think that a minimum distance of 20 centimeters is necessary in order to have a proper soundstage portrayal and right tonal balance.
What one have to listen to while adjusting the back wall distance is not just the bass quality and the tonal balance but also soundstage width and depth. Proper three dimensional soundstage is very important when we consider realism in sound reproduction. When put to close to the back wall the soundstage often feels somehow compressed, the performers seems to overlap and the whole presentation is somehow flat and lacking depth.
The same space compression effects can be observed in the X axis when the distance between speakers is too small. One can feel that the placement of the performers on the soundstage is unrealistically portrayed. I suggest while setting the speaker position to leave the toe angle for the very end, because even with the speakers pointed straight ahead the overall image quality and tonal balance should be there. Trying to adjust toe-in angle and distances simultaneously becomes harder task, because the variables are increasing exponentially. It is always a good strategy to break down a single complex task into several simpler ones so that step-by-step adjusting process is being used.
In my room for example the speakers are nearly parallel. This is because the speakers I use have a wide dispersion, but also because of the specific room situation. In my room the distance between speakers and listening position is around 1.8 times bigger as the distance between speakers themselves. This is due to the room limitations, because I just can’t simply move the door one meter on the left or put my sofa in the middle of the room. Most rooms are constraining and one don’t have the freedom to move speakers and listening position as he likes, because then all the other domestic functions of the room will be affected. If you are not one of these lucky guys that have dedicated listening room you have to make compromises and try to get the best out of your room without disturbing your everyday life and free move. The room should be still usable and one should feel himself comfortable especially if it is a living room like mine.
So in summary the toe-in angle is very fine instrument for adjusting the soundstage dimensions and the final tonal balance, but already with parallel speakers the soundstage should not feel compressed but naturally fitting in the room. I want to mention also that even small increments of half centimetre could bring big differences in the presentation. Sound weird at first, but one have to consider that in an average room of 25 square meters the audio system reproduces the sound of a whole band or even an orchestra and realise that by reproduction in such a room the real space of the recording venue have been compressed several times.
We can think about the space compression as a scale 1 to 5, 1 to 10 or even more. In accordance to the space compression ratio, moving the speakers 1 centimetre apart could result in 5, 10 or even more centimetres increasement of the soundstage dimensions. In certain situations when one of the speakers is close to the side wall but the other not, damping of the side wall have to be considered so that the natural balance typical for symmetrical spaces could be established.
One shouldn’t also take for granted what specifications in regard of speaker placement requirements are claiming, because every room is different. It could be useful to consider speaker placement requirements given as an index in magazines like the German Audio where speakers are categorised according to the minimal back wall distance into 3 groups, but not following them blindly. In Hobby HiFi for example Bernd Timmermann claims that Fostex BK108 horn speakers require no more than 10 cm from the back wall. Putting them on 10 or even 15 centimetres in my room the soundstage these speakers are throwing is rather poor compared to the one they deliver at the back wall distance starting from 20 centimetres.
Don’t forget that once the setup is made this does not mean final state. By every component change the tonal balance of the system is changing too which means that small placement adjustments are necessary. Even an interconnect cable could bring difference in the tonal balance that needs to be addressed. For example by my latest notch filter experiment the frequency response of the speakers was affected and it was necessary to increase the back wall distance with around 2 centimetres to reach the full soundstage dept and tonal balance.