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How far out into room for rear ported speakers

Discussion in 'Archived Threads 2001-2004' started by ChrisAG, Sep 23, 2001.

  1. ChrisAG

    ChrisAG Supporting Actor

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    I've heard of the 1/3 rule, where the speakers are 1/3 into the room from the front wall, and the listener is 1/3 from the back wall, but this is not always practical. Right now my MS-906's sound great at 18" from the wall (measured from the rear of the speaker), but I'm wondering if this is enough. Due to furniture considerations it isn't possible to move them out much more. My rear bookshelf 902s are only six inches away from the back wall - less than I want, but they sound OK when my receiver's bass management is set to "Small." Again, it is not possible to move them out much more. Any thoughts, especially for the front?
     
  2. Bob McElfresh

    Bob McElfresh Producer

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    Hi Chris.
    Here is the trick: You want the back-fire sound to hit the walls and bounce so it reaches your ears some time after the front fire sound.
    Some PsychoAcoustics research has found some interesting things about a sound, followed by an echo. They looked at what the ear perceived based on the ammount of delay for the echo:
    0-5 miliSeconds: The ear registers a single sound.
    6-11 miliSeconds: Here is where the ear registers the distortion, but cannot tell anything about placement and assumes 1 sound source.
    12-20 miliSeconds: The ear registers a single sound, but tells the listener that the sound is several feet farther away than it really is.
    > 20 miliSeconds: The ear recognizes an echo.
    So pulling the speakers into the room, you want to get that 12 miliSecond delay to give you the sense of "spaciousness" or the effect of the walls of the room falling away.
    Sound travels in your living room at about 1.1 feet per milisecond. So you can try to measure the distance to the rear walls, then the distance to your ears and calculate the delay using this number.
    Hope this helps.
     
  3. Rick Radford

    Rick Radford Supporting Actor

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    Bob,
    That's really interesting info.
    But if my math is correct, I'd hafta pull the speakers in 5.5 feet to get that 12 ms delay (5.5 x 2 x 1.1 = 12.1)... in which case, I imagine most HT rooms don't have that much space to play with.
    at least, my little 11.5x12 room doesn't!
    ------------------
    --RR
     
  4. ChrisAG

    ChrisAG Supporting Actor

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    Hi Bob,
    So do you mean that if we can't achieve 12-20 miliseconds for spaciousness, the next best thing would be 0-5 miliseconds, because anything in the middle is registered as distortion and would thus sound worse?
    Chris
     
  5. Bob McElfresh

    Bob McElfresh Producer

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    Yes, it does seem like a lot but the effect is staggering.
    I had to strip everything out of my living room to have wall-paper stripped, re-texture and painted. I managed to move back the HT equipment and nothing else for a few days.
    With the speakers pulled into the room about 4 feet, all positioned correctly and level-adjusted, I watched several movies. But one disk, U.S. Marshals stood out.
    There are several scenes with a phone call that toggle between a open-swamp area, and a closed-in, stuffy office. When the scene would switch, it felt like the walls of the room exploded outwards. I thought my ears were going to pop. Then the walls rushed back in for the office scenes.
    So, do you need 5 feet of space? No. But a good 3 feet of space is better than 18 inches. Most sound-tracks have a complex mix of sounds. And the sound engineer can help this effect by simply putting in a delay for the L/R copy of a sound. So when the center speaker fires a sound, a delay in the L/R will produce a similar effect.
    ChrisAG: you said
     

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