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Surround Sound speaker placement and surround imaging


Prodigal Son
Humans are engineered to locate sounds when we hear them. This is why we have two ears on the sides of our heads. If we hear a twig snap in a forest, we can usually very quickly turn our heads and face the sound to see what made the twig snap, like a Tiger (A tiger in Africa?). However, our eyes are on the front of our heads, and our outer ears are shaped to focus on sound coming from in front of us. When we hear a twig snap behind us, all we can really tell is that a something made a noise behind our line of sight. It is difficult to pinpoint the source of the sound if it is in the region behind our heads.

These physiological facts play a big role in how we attempt to recreate the illusion of a full acoustical experience via multiple speakers in a surround sound setup. We are very, very good at locating sounds in front of us, but behind us it is difficult. Our skills are locating sound in front of us defines the common desire for a dedicated center channel to go along with the two "stereo" front speakers. Rear speakers don’t need to be directly behind us because our brains have a difficult time interpreting acoustic information behind us.

Since we have such a difficult time locating sound behind us, the first successful commercial surround sound solutions placed the "surround" channels to the left and right of the listener, not directly behind them. The illusion of enveloping 3D sound can almost be perfectly created with side surround speakers.

THX engineers (and other brilliant scientist movie industry types) have found that placing surround speakers only to the side of the room leaves a gap in the illusion of sound coming from behind the listener. If a signal is equally produced by both the left & right side rear speaker, the listener thinks the sound is coming from the center of his head instead of thinking the sound is coming from behind him. So, they thought placing a speaker behind the listener would solve this problem. It turned out that sound from a speaker directly behind the listener will also be interpreted as coming from inside the head of the listener instead of coming from behind the listener, unless the listener were to move his head while the sound was being made.

They tried placing two speakers behind the listener to see what happened. What the found was that a mono signal reproduced by two rear speakers was interpreted as coming from behind the listener.

Why??? It is because the human ears are aimed forward. Sound coming from directly behind a person cannot be deduced as coming from behind because is sounds identical to sounds coming from exactly in front of the listener, except slightly muffled since the outer ear is aimed forward. The only way for a listener to know a sound is coming from directly behind the listener is in fact behind is if he moves his head while the sound is being made and gets other clues from the sound shifting from the moving head. Only very subtle changes to the head's position can make it easy to know where sound comes from (a reason why headphone listening ends to lack in soundstage realism when compared to equally excellent stereo speaker listening).

I created some diagrams to visualize how we perceive the location of reproduced sound coming from the speakers of a surround sound system.

Front stereo speakers:

As shown, we can be fooled into believing a sound coming from just three (or two) front speakers is located anywhere in front of us betwen the speakers. In some cases we can think a sound is coming from just outside the width of the front speakers without any additional speakers.

Side surround speakers:

Side surround speakers create illusions over a much smaller area due to the way we hear sound.

Left side front & side surround speakers:

If you combine the left speaker with the left surround speaker, a reproduced sound can appear to come from just about anywhere to the left of the listener, but not behing the listener.

Center rear speaker:

If a single rear surround speaker is placed directly behind the speaker the sound will only appear to come from inside the listener's head. If the listener turns his head quite a bit while a single sound is being produced from that speaker, he will recognize the location of the speaker with near pinpoint accuracy, so the illusion of a full surround sound behind the listener is very limited.

6.1 surround system:

In a full system, a single rear surround speaker will moderately enhance the rear sound perception, but it is very limited.

Dual rear surround speakers:

Using two rear surround speakers the illusion of rear sound is vastly expanded to cover the area directly behind the listener. The listener doesn't have to move his head around to realize the sound is behind him, and with discreet dual rear channel signals, the placement of those rear sounds can be somewhat controlled (but the limitation of how we hear is still a critical factor in reducing the overall soundstage).

7.1 surround system:

With a 7.1 system, the rear soundfield is considerably larger and the listener can get nearly a full 180 degree surround sound experience. Note that the rear surround experience is still limted, but real life is like that anyway. Adding more speakers behind the listener can help alleviate the limtations, but humans care mostly about front sounds and directors and mix engineers know that. Just like most video content has the important stuff to see mostly clustered in the center of the screen since that is the center of our visual focus, so too do sound engineers put the most imporant audio information in the front of the listening area. Surround tends to be to enhance the realism of the experience.

I have a great 7.1 direct monopole system with identical sounding speakers in each location. My experience with well recorded soundtracks, mostly TV or remastered DTS movie soundtracks, is that my wife and I are fooled by sounds behind or above us quite often. The 3D audio experience can be so lifelike that we forget where we are sometimes and are shocked by a door opening behind us or a click, thud, or gunshot occurs from anywhere in the room. My poor wife is tired of me laughing when she often turns to me with wide eyes while we are watching TV and urgently asks, "did you hear that? Is Joseph (son) home?"
Ok, with us expecting the reverb and rear room side effects in the sound from the rear. What would be the effect of using a set of bose 901 with out the forward fired component on the perceieved sound field? Would the added scattering and first reflections add to the sound field in a constructive way? After all in the acoustics work done in your space you have added diffusers to scatter the rear of the room? How is this different from the 901 model?

I see that having the 6.1 book self speakers with first reflections coverd in the rears also as a good rule of thumb because I feel the matched 6 would be the best in creating the 360 degree illusion. I will be setting up for a dolby model of 60 degrees, center, 60 degrees, 110, center rear, 110 .

We have also had that expierence where someone has picked up the phone to make sure it was not ringing. Look to see who is nocking on the door.
So Flint what are your views on the new 9.1 (with height speakers) or (ultra wide fronts) To me we should go back to using four side surrounds in a standard rectangular shaped room along with two rear channels. I had an old receiver that allowed time shifting on surround speakers giving the illusion of the room being far larger so with two side surrounds on each side, I feel with time shifting the audio, gunshots could appear to travel along the wall in a more indepth manner than what we currently have.
Yes they would but when doing this you also need to ensure that the rear speakers on the back wall have a longer time shift also. Most of the movies have a specified time shift for the rear speakers on the back wall. One of the other problems would be the reverse direction. When the sound starts from the rear wall and works forward. The time delays are out of sync with the sound travel. The sound effect would start in the rear wall move to side 1 delayed and move to side 2 then back to the front.

Oh come now sound hound you are one of the best with COF.

In the past Yamaha offered extra channels for adding to room ambience. This was not accepted very well by most listeners. Probably with SO did not like the added two speakers in the left and right front ceiling areas and the extra cost 15 years ago. Let me ask you this, When listening to a good movie of jets flying over head and the surround is working, Do you want to look up to see where that jet is coming from behind and over your head, How about a good helicopter with the chop of the blades. Were you ducking under the planes in Pearl Harbor. Does the sound from a rushing stream from rapids feel like it is beneath your feet. There are scenes of storms were we have checked the windows and doors to make sure the weather was not getting in. My modest JBL 2500 bookselfs are no match for the room Paul, COF, or Sound hound run. I did not forget the others, just to many great systems to list. I feel that some of this is good and has benifit, but the added cost and space and acceptance by the market will still be driven by the SO.
When I wrote the article above many years ago it was focused 100% on direct radiators.

With diffuse sound systems, the rules are a little different.
Yes I understand, one of my big complaints on the 901 theory is that it requires three solid walls to work correctly along with the spacing away from the three walls to allow the first reflections to work correctly. These first reflections would also have small delays from the direct sound creating the reverb field based on the added time of travel to the listener. The early reverb may cloud some of the staging in the sound field if used in the front speaker set.
The Bose 901 was developed during a time when people were desperately trying to remove some of the limitations of stereo listening such as the limited sweetspot, the lack of complete envelopment, and so on. Dr. Bose sold kits which were mounted in the upper corner of the front walls for awhile before developing the 901 to be sold commercially. It was an interesting idea, and for people who just want a much better experience with non-hyper-critical listening, it is pretty good at what it does. But in the days when it came out, true high-fidelity was only found in huge speakers like the K-Horn, home versions of Altec and JBL systems, and a few excellent electrostatic speakers. Since then much has improved in designing more affordable and home friendly speaker systems which offer amazing stereo imaging, depth, and soundfield to make them a little passe.

The 901 still does it's thing, but since that genre of speaker has otherwise completely vanshed, maybe the value of the flagship 901 is moot.