Standing waves + Bass Traps?

Discussion in 'Home Theater Projects' started by Jack*Mains, Jan 20, 2004.

  1. Jack*Mains

    Jack*Mains Stunt Coordinator

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    Id like to ask this one last time before I commit to making a few test bass traps for myself.

    I have an L-shaped HT room with the sub positioned in the furthest part of the shorter 'leg' of the L. This appears to be the best location, but unfortunately it creates a few standing waves which cancel the bass out completely in the secondary listening area, while having peaks that sounds quite nice in other parts of the room.

    Would putting a few bass traps in the corners of the room even out the responce throughout the room?

    I dont think an EQ is what I need here, since the responce if fairly even at every point of the room, some areas just have less SPL in every frequency (below 100 or so) than the others.

    So.. to bass trap or not to bass trap? And if so, are there any fairly simple designs to test them out on?
     
  2. Torgny Nilsson

    Torgny Nilsson Second Unit

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    I can't answer your question, but I have one of my own. How do you tell if you have a bass problem? I just moved my system to a new room and can't tell if I have the sub in the right place. The thought of just moving my big SVS sub around the room to see where it sounds best seems like a lot of work. Is there any better way to tell with a sound meter and AVIA DVD?
     
  3. Wayne A. Pflughaupt

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    Yes. Get a test disc with at least 1/6-octave sine-wave tones. If you can’t find one, you can download some tones at the following site (you might have to dig a little to find the link). Once you have the test tones you can use the meter and some graph paper to map your sub’s room response.
    http://www.snapbug.ws/bfd.htm

    Regards,
    Wayne A. Pflughaupt
     
  4. Tim Bargar

    Tim Bargar Agent

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    There is some debate as to whether or not broad band bass absorption is appropriate. It has been argued that since only a few frequencies need to be dealt with, absorption of other frequencies by broad band bass traps will require the need for extra amplification to compensate for the loss of the other frequencies....which will require more bass trap for the problem frequency....which will require more amplification...etc. That is how Dennis Erskine over on AVSforum described the situation anyway. He advocates tuned bass absorbers (resonant peaks) and equalization (resonant nulls) to deal with problematic bass.

    A way to avoid moving the sub all over the room is to place it at the prime seating location(s), play specific frequencies through it and walk around the room to find where in the room the various frequencies sound best. Where they sound best is the location for the sub.
     
  5. Jack*Mains

    Jack*Mains Stunt Coordinator

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    The problem is definitely not phase related. Ive tried runnign the sub with the main speakers off and the same behavior occurs.

    I only have a few places in the room I can put the sub and all of them have their own problems.

    I understand that the low points in the responce are caused by destructive interference of the waves reflected off the walls? Eslecially in the corners. So wouldnt placing a few bass traps get rid of the reflected ways and even out the responce?
     
  6. Chris White

    Chris White Second Unit

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    I don't know what kind of bass traps you were planning on making, but there is a really easy and inexpensive way to determine the effects of bass traps.

    Stop by your local building supply and pick up a dozen rolls of insulation. The R-factor doesn't really matter, so just get whatever is the cheapest. Don't bother taking the insulation out of the plastic wrapper since it doesn't affect low frequency anyway. Stack three rolls of insulation in each corner. Voile! You now have a very effective bass trap in each corner. Did it help? If so, just cover the rolls in cloth (preferably something that's relatively acoustically transparent). If not, just return the insulation.
     
  7. Jack*Mains

    Jack*Mains Stunt Coordinator

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    I might just try that..

    What else would be possible to reduce reflections and destructive interference to get rid of them valleys? [​IMG]
     
  8. Cees Alons

    Cees Alons Moderator
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    You could place your subwoofer at your preferred seating position ("sweet spot") and walk through the room to hear (measure) where the best bass is found. If you find a place that's also appropriate to place the sub there, you've found it!

    (This may also be a good solution to Torgny's problem.)

    You know, Jack, somehow I wonder if this is caused by reflections and standing waves. Interference with other speakers could be possible, but can easily be ruled out (or not) by switching those other channels off and hear how your bass is then.

    Cees
     
  9. Jack*Mains

    Jack*Mains Stunt Coordinator

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    I've already tried doing that, and the bass responce is not affected by other speakers...

    Also done the sweet spot thing, and unfortunately I can't place the sub in the area where it sounds best...
     
  10. Cees Alons

    Cees Alons Moderator
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    That's bad. Did you try switching the phase of the sub?

    Cees
     
  11. Jack*Mains

    Jack*Mains Stunt Coordinator

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    Im thinking the reason the other speakers don't affect the bass responce is because they're Mini Monitors set to small, and smply can't play frequencies that low.

    Im sure if I had different speakers the phase would matter, but right now its all up to the sub.
     
  12. Cees Alons

    Cees Alons Moderator
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    Jack,

    The phase can play a role in standing waves and the like. So there may be an effect at the place where you're listening. If you were free to place the sub almost anywhere, it would probably matter less, but because you're forced to place it where it is now, it just may help.

    Cees
     

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