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Ideal Placement of Surround Speakers Using Psychoacoustics

Discussion in 'Configuration & Setup' started by Flint, Apr 4, 2018.

  1. Flint

    Flint "Do you know who I am?" Superstar

    The brain is an amazing thing. It can process sight and sound in unbelievably ways to create a mental awareness of the world around us which we unconsciously rely on to know where we are and the location of things around us which might be a threat or a treat. Our ears are placed wide on our heads to allow for stereo location which is enhanced by using our necks to move our heads around if we want to pinpoint a sound. Since our outer ears are essentially directional filters - focusing our hearing towards sounds in front of us - we instinctively turn to face a sound if we have cause to pinpoint its location in case we need to escape it, hunt it, or merely identify it with our eyes to determine what to do, if anything. This mechanism is amazing!


    Stereo sound reproduction takes advantage of most of our physiological skills are judging location in front of us by one speaker not only feeding a signal to one ear, but by each stereo speaker also feeding sound to the other ear. Our brains measure the difference in distance from one ear to the other by the time delay between the two sound sources, but also based on the loudness of sound as it reaches each ear. That’s why a sound played equally loud from two stereo speakers in a well-aligned listening room will appear to be coming from directly in front of us – a phantom center speaker. That effect can allow a sound engineer to place a sound anywhere in the virtual soundstage in front of us with just two speakers, and in a great room with great speakers the virtual soundstage can even extend beyond the width of the physical speakers. With the application of fancy DSP algorithms, two stereo speakers can even fool us into believing the sound is to the side and even behind us. But that effect is only for the primary listener in the perfect location in the room.


    Surround sound through the use of multiple speakers spread around the room can ensure multiple people experience the effect of sounds coming from anywhere in the 2-dimensional 360-degree plane at eye level and with new surround formats which place speakers above the listener some of the entire 3D audio environment can be produced in the room. But, there are a few issues which must be dealt with concerning the placement if speakers.


    Several years ago, I wrote a long essay on the reasons a single rear surround speaker is inferior to having dual rear surround speakers, and I am recreating the basic knowledge today.


    First off, we are pretty much incapable of holding our heads completely still while focusing on sound. We move our heads in tiny amounts all the time and sometimes quite consciously shift, tilt, and turn our heads when we are trying to understand what we are hearing. Our brains know how much we move our heads as we are doing it and comparing that motion to what sound is being processed to confirm or correct the mental mapping of the sounds being heard. This is why true binaural headphone recordings are not perfect at fooling a listener into believing they are in the audio environment of the recording (though some new VR technology which tracks head motion and adjusts the signal feeding the headphones is starting to solve the traditional binaural issues). So, fixed speakers in a room is often better than headphones at creating the impression of an ambient space with all the sounds seeming to come from their logical locations around a listener.



    So, if you go with a traditional 5.1, 6.1, or 7.1 setup, where should you put the speakers?


    Ideally, one would avoid a 6.1 setup if the money spent on the single rear surround speaker could easily be spent on two rear surround speakers in a 7.1 arrangement, or it could be applied to other parts of the system which are more impactful. I’ll get into that now.


    Every comment below is based on direct radiating speakers. Using dipolar speakers for surround is a completely different conversation.
     
    PaulyT, heeman and Zing like this.
  2. Flint

    Flint "Do you know who I am?" Superstar

    FRONT

    In a traditional home theater configuration, the three front speakers can create a soundstage where any instrument, voice, or effect can appear to exist anywhere in front of the listener, as shown in the drawing below by the yellow-green oval. Because the front speakers can effectively be heard by both ears of the listener, the soundstage is deep, broad, and full. This is also where our brains look to find details from what we are hearing due to both the ability to locate sound very well when they are in front of our faces, and it allows us to add our vision to the task of identifying and locating what we are hearing.

    Soundfields_Front.jpg


    SIDE

    Two side surround speakers, slightly behind the listening plane, can create a soundstage directly to the sides of the listener with a little rearward tilt, as shown in the drawing below. Since we are not facing the speakers, the ear on the opposite side from each speaker cannot offer much help in locating sound over a broad area, so our brains just place the sound to the side, left or right, and if both side speakers are playing the same sound at the same level, our brains will struggle a bit but usually place the sound generally behind us.


    Soundfields_Side.jpg


    SIDE / FRONT

    However, a sound can be sent to both the front left stereo speaker and the left-side surround speaker and add a sense of location farther to the left. The area of perceived sound locations with the front left and side left speakers working together are shown in the drawing below. If the speakers blend well together, the effect can be extremely realistic and convincing.

    Soundfields_Leftside.jpg
     
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  3. Flint

    Flint "Do you know who I am?" Superstar

    SINGLE REAR SURROUND

    To complete the entire soundstage to include the area behind the listener, rear surround speakers are used. Most surround decoders will let you choose a single, center rear surround speaker or two, spaced rear surround speakers. If you choose a single surround speaker, the way the brain works actually hinders that speaker’s ability to fully fill in the rear soundstage. Our ears are aiming forward, and our brains are designed to pinpoint sounds quickly and accurately, so a single source of sound from behind us will be strange and confusing until we move our heads a bit to listen to it. After we determine the sound is somewhere behind us, in the real world we will turn our heads around to locate the sound accurately and hopefully see what is making the sound. However, if we over-ride our instincts keep our heads facing forward, like we would do in our HT, the sound from directly behind us will seem to come from immediately behind our head, if not seemingly inside our head. This is shown in the drawing below. Like the front center channel, the center rear has no ability to go wide, but since our brains are not good at rear placement, any sense of depth or space is nearly impossible. Even if mixes with one of the side speakers, the side speaker will quickly overpower the perception of location because our brains can comprehend side placement vastly superior to sounds directly us.

    Soundfields_1Rear.jpg



    FULL 6.1 SOUND FIELD

    As shown in the drawing below, a 6.1 speaker system can create a very immersive soundstage and pretty much fool your brain into believing you are sitting in the middle of the environment a sound engineer designed for you, except for the space behind the listener. It may be a little better than not having a rear surround, but it won’t deliver nearly the results of a 7.1 configuration as I will get into next.

    Soundfields_61.jpg


    DUAL REAR SURROUNDS

    With two rear surround speakers placed apart from one another, the same stereo skills we have for front listening will operate for rear listening, but with less detail and precision. Just like a good stereo setup can create a broad and detailed soundstage, two rear speakers can as well, but some limitations on specific location and the illusion of a phantom speaker. The breadth of the soundstage is represented in the drawing below.

    Soundfields_2Rear.jpg


    FULL 7.1 SOUND FIELD

    So, by utilizing two rear surrounds to expand the rear effects of a good 5.1 system, you can create an almost completely immersive 360-degree soundstage that seems to fill the room around you. This is shown in the drawing below.

    Soundfields_71.jpg
     
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  4. Flint

    Flint "Do you know who I am?" Superstar

    CONCLUSION

    If you are considering rear surrounds, it makes sense to do what you can to get two rear speakers instead of just one. I personally would not buy just one rear speaker if there were any reason I couldn’t go with two, such as cost, room limitations, WAF, or mounting restrictions. If, however, I already have the 6th speaker and a place to put it, I would install it and use it rather than limit myself to 5.1. I guess I am saying, making a great effort to upgrade to 6.1 may not be worth it if that money, effort, and time can be used to greater effect elsewhere. Waiting until one can upgrade to a proper 7.1 system makes sense. Again, I am not trying to say 6.1 is a waste. However, I am suggesting it offers much less benefit over 5.1 than going to 7.1.


    CAVEAT: We all must make compromises and sometimes there are hard restrictions on what we can do with our home theaters. I get that. No one should feel shunned or stupid for installing a 6.1 system. Please don’t take this thread as an attack on anyone or any system. Thanks.
     
    heeman likes this.
  5. Dentman

    Dentman Well-Known Member

    I've never been interested in moving to 7.1 but then again I've had little to no exposer to it. In my room now I have no place for it since my couch is about 20" from the rear wall.

    By the way I heard some of those subtle sound effects you spoke of in a TV show I watched the other day. I'll be ordering rear speaker mounts soon so I can correctly mount my rear channels.

    Funny how little the poor placement affects big action scenes but stands out like crazy with the fine detail stuff.
     
  6. Flint

    Flint "Do you know who I am?" Superstar

    Generally speaking, TV shows often have soundtracks which have specifically placed detailed sounds in the rear channels while movies tend to have non-distinct ambient sounds coming from areas of the room to the side and behind you.

    These differences are easy to understand, in large theaters they use arrays of many side and rear surround speakers which create a diffuse sound field which is relatively balanced for everyone in the audience regardless of where they are seated. Since the speakers can only produce a diffuse sound field, the ability to accurately place a specific detailed sound, like a ringing phone, the loading of a pump action shotgun, or a door knock, is greatly hindered as it will almost never be reproduced in a way which clearly and specifically places the sound at a certain point. In a home theater, however, most people have one direct radiating speaker per channel which thusly offers the ability to place a specific sound in an exact location in the room. This means placing a crack of branch during an intense forest chase/hunting scene in the rear left corner of the room is possible.

    I still vividly remember watching the TV show, Numbers, back in the day and every time the scene was in the big work room at the FBI they would often have the sound of the glass door opening or a knock on the door behind me which was so realistic I was regularly startled and looked back to see what was behind me.
     
  7. CMonster

    CMonster Lazy Individual

    What's really fun is the new 3D sound formats like Atmos and DTS:X specify a base layer and a top layer so the traditional 5/7 speakers all need to be at ear level. All of you placing your surrounds and rears up high on the wall will have to lower them if you make the jump.
     
    bmwuk likes this.
  8. Dentman

    Dentman Well-Known Member

    I'm now more open to these formats but having the correct room is paramount for them.
     
  9. bmwuk

    bmwuk Well-Known Member

    And with 8 ft ceilings that starts to become impractical
     
    Dentman likes this.
  10. Flint

    Flint "Do you know who I am?" Superstar

    I get the impression they chose speaker placements which were easy to comprehend for users and easy to implement with directional speakers being added to standard speakers, like the Klipsch Atmos speakers which have a regular tower with another upward facing speaker in the cabinet.

    Yamaha did the same thing when they attempted to sell the concept of full auditorium/theater/hall ambience recreation with their very powerful and effective DSPs in receivers and pre-pros. Like Dolby Atmos they placed the "height" channels directly above several of the standard channels.

    Research from the 1970s showed that a couple of speakers directly above the listener was more than enough to get the full experience (the height surround channels), but it isn't easy to mount a speaker directly above the listener in most rooms, especially in aftermarket installations. So, we got the approach Atmos is offering. It works pretty well, from the demos I've seen. But, I fear, we are frantically pounding on the door of the point of diminishing returns.
     
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