Bass traps and/or other forms of acoustic treatments

Discussion in 'Home Theater Projects' started by David Strand, Jul 14, 2004.

  1. David Strand

    David Strand Agent

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    I am having a problem with very poor bass response at my main listening position in my living room / HT. The room is very odd shaped, I will try to post a diagram of it soon. Behind the main seating is a dining room and I believe this is the root of the problem.

    I have been trying to talk my wife into getting some acoustic treatment, but they are very very very low on the wife acceptance scale. Are there any decent looking corner fitting bass traps out there?
     
  2. Ethan Winer

    Ethan Winer Stunt Coordinator

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    David,

    > The room is very odd shaped <

    That's less of a factor than you might think.

    > Are there any decent looking corner fitting bass traps out there? <

    Aye, there's the rub. There are small and unobtrusive products you can buy to put into your corners. The problem is the smaller they are, the less effective they are. Some of the corner absorbers I see offered for sale are so small they couldn't possibly do anything useful.

    That said, the key to effective bass trapping is rigid fiberglass panels mounted straddling the room corners, the thicker the better. At the minimum you'll want four 2x4 foot panels - one in each corner - if you ever hope to obtain good bass in that room.

    --Ethan
     
  3. Wayne A. Pflughaupt

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    David,

    I suggest getting a Behringer Feedback Destroyer parametric equalizer. Has a high WAF and will greatly improve the sub’s room response. Do a search on the Speakers and Subs and DIY forums for “BFD” and you’ll get enough reading material to keep you busy for days.

    Regards,
    Wayne A. Pflughaupt
     
  4. David Strand

    David Strand Agent

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    Thanks for the help Ethan and Wayne. I have been thinking about getting a BFD, but I'm afraid it won't be able to do much for the nulls at my listening position. Here are some links to my living room, a picture towards the front of the room, and one towards the back of the room.

    http://users.adelphia.net/~dpstrand/front.jpg

    http://users.adelphia.net/~dpstrand/back.jpg

    I ended up moving my sub into the entryway on the right side of the railing... which sounds MUCH better than anywhere near the front wall (including along the wall just on the other side of the rail), but isn't so pleasing to look at since it's the first thing you see when you walk in the door.

    So that is my dilema. If I turn the sub off and set my front speakers to large I have the same weak bass response from them (polk audio lsi-15). If I go up the stairs to the back of the room it sounds great...

    I am thinking the main problem is the dining room area, and that is why moving the sub far enough to the right made such a drastic difference. I was thinking about trying some bass traps in the corners in the very back (in the dining room). Does that sound like the best place to start?

    The dimensions are about 27 feet from the front wall to the back wall in the dining room. The dining room area is about 9' 4", and the width of the room at the listening position is about 16' 1". Also, the room opens up into the family room which adds another 12' 9" to the width.
     
  5. Adam_R

    Adam_R Second Unit

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    That is a HUGE room. You may find that you just need more sub to move all that air.

    I am currently in a similar size room with all kinds of openings into the kitchen, hallways and dining area and a vaulted ceiling. It's a tough room to fill with bass for sure. And now I'm moving the HT is to the exact opposite, a small room. UGH.

    BTW - What kind of sub are you running?
     
  6. Ethan Winer

    Ethan Winer Stunt Coordinator

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    David,

    > I have been thinking about getting a BFD, but I'm afraid it won't be able to do much for the nulls at my listening position. <

    Exactly right - no EQ can compensate for deep nulls.

    > about 27 feet from the front wall to the back wall <

    Where are you sitting in relation to the front and rear walls? I can't tell from the picture, but it looks like you might be about halfway back. That's the worst place for bass.

    Have you tried putting the subwoofer in the front corner?

    --Ethan
     
  7. chuckg

    chuckg Supporting Actor

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    Keep trying different positions for the sub, you might find one spot that works for both appearance and for sound. I'm not too sure that _absorbing_ the lows will increase the volume elsewhere.....

    Trivia - Led Zeppelin drums were recorded with mics all over the studio, including one that was halfway up a stairway where there was a nice strong bass peak.
     
  8. Wayne A. Pflughaupt

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    David,

    The dining room might not be as much of a problem as you think. I’ve found that the more irregularities you have in a room – i.e., departures from straight-up “shoebox” dimensions - the easier it is to get good bass response.

    As Ethan noted, center-room is typically the spot with the weakest bass. That doesn’t necessarily mean you’re in a true null or a hopeless situation. It’s just that bass tends to “chase” boundaries, so the point in the room farthest from the boundaries will see reduced levels. I have much the same situation as you do in my living room, and it equalized nicely.

    If you haven’t already, you might want to take some measurements with at least 1/6-octave resolution. Otherwise you’re just guessing as to how bad response it (or not).

    As to whether or not the BFD can equalize out nulls, it’s true, it can’t. But you also have to keep in mind that not under-represented areas in response are true nulls. You’ll probably find that most low spots, especially in your irregular room, will equalize out nicely, as they did for Sonnie Parker, the guy who put the BFD Comprehensive Set-Up Guide up. Here’s a “before and after” of what the BFD did for him:


    [​IMG]


    Regards,
    Wayne A. Pflughaupt
     
  9. Ethan Winer

    Ethan Winer Stunt Coordinator

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    Chuck,

    > I'm not too sure that _absorbing_ the lows will increase the volume elsewhere. <

    This is a common misconception. In truth, adding bass traps to a room generally increases the perceived level of bass. Since the damaging nulls are caused by reflections from the room boundaries, a bass trap is designed to absorb sound rather than reflect it. And when the reflections are reduced, the low frequency response becomes flatter. At locations where certain bass frequencies were cancelled, adding bass traps increases the level. Of course, other places in the room experience peaks in the response, so in that case bass traps do reduce the level. But the real point is that bass traps tend to flatten the response, whether by increasing or decreasing the level at various frequencies.

    --Ethan
     
  10. PaulDA

    PaulDA Cinematographer

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    I have a 21x11 space in which to build an HT and a storage area. I planned to make a 11x14 space for HT and 7x11 (I know these aren't exact as the wall has thickness, of course) for storage (the room divides easiest this way as there is a small closet already in place that is 7x2). For attenuating bass problems (I will also use room treatments, but bear with me) would it be better to leave an opening to the storage area without a door, or with a door, or hang a curtain in the opening, or not build a wall and just hang a curtain to "wall off" the storage space and thus get the most out of the irregularities of the whole space? Any option is viable, as construction hasn't started yet. My budget constrains me from knocking down existing walls and doing major changes to the configuration of the basement.
     
  11. Ethan Winer

    Ethan Winer Stunt Coordinator

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    Paul,

    > I have a 21x11 space in which to build an HT and a storage area <

    Yes, a curtain might be a good idea, but you should also consider all of the room dimensions together. Without knowing the ceiling height it's impossible to determine the best length.

    Have a look at the Acoustics FAQ, second in the list on my Articles page:

    www.ethanwiner.com/articles.html

    In particular, see the section about room modes and also the sidebar that describes the ModeCalc program (PC only) you can download.

    --Ethan
     
  12. PaulDA

    PaulDA Cinematographer

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    The room height is about 7'5", a drop ceiling above which I will be stuffing the joists with R-19, most likely.
     
  13. David Strand

    David Strand Agent

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    Thanks for the help guys. I am working on a scale drawing of my living room so I can show you how I've positioned things.

    The spot where I have the sub now (you can barely see it in the front pic on the right hand side in the entryway) definately sounds better than any other spot I've found, but It has 6 db peaks at about 60hz and 40hz, and a nice 15db null at 30hz (and then weak sub response below that, velodyne cht-12) so it's a bit boomy and lacks really low end response.

    My real problem stems from the cancellation I get from any speakers placed on the wall with the TV. This gives me 2 problems:

    1. My mains playing music don't sound so hot by themselves.

    2. I have been thinking about getting an SVS, and none of their subs will fit in the location I have the sub in now...


    Since I don't really know where the most significant cancelling reflections are coming from, would it make sense for me to pick up a few rolls of fiberglass from Home Depot and position them in various corners while testing frequency response? Will (temporary) rolled bails of fiberglass work well enough for me to at least determine where to try some real bass traps? I don't want to waste money on traps where they have little effect.
     
  14. Adam_R

    Adam_R Second Unit

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  15. BrianKR

    BrianKR Second Unit

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    I would like to know the answer to this question as well.
    I saw them on eBay before and I almost purchased a couple pairs of the 4' lengths myself. I need bass traps and I am too busy/lazy to build them myself.
     
  16. Adam_R

    Adam_R Second Unit

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    ^not to mention they are very affordable!
     
  17. Ethan Winer

    Ethan Winer Stunt Coordinator

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    Paul,

    > The room height is about 7'5", a drop ceiling above which I will be stuffing the joists with R-19 <

    The true height for mode calculations is to the rigid surface above. In this case it's to the bottom of the floor boards. What's that dimension?

    Better, download my calculator and play around with various lengths. There's no one perfect ratio, or perfect size, so only you can decide where to draw the line between improving the sound further versus having the room be too short to be practical.

    --Ethan
     
  18. Ethan Winer

    Ethan Winer Stunt Coordinator

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    Adam and Brian,

    > has anyone used these? ... Would one 4-footer in each corner of a 12x12x10 room help a great deal? <

    Understand that I sell acoustic treatment, so take this as you see fit:

    In my opinion foam is not a good absorber for low frequencies unless you use a huge amount of it. For DIY bass traps I always suggest that people buy panels made of rigid fiberglass. For the same physical size, rigid fiberglass is 2-4 times more effective than foam corners at 125 Hz and below. Likewise, commercial treatment based on rigid fiberglass enjoys a similar advantage. That is, more absorption and to a lower frequency.

    --Ethan
     
  19. Adam_R

    Adam_R Second Unit

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    ^thanks!
     
  20. David Strand

    David Strand Agent

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    Here is a scale drawing of my living room setup. You can see the openings to other rooms on the left (thinner lines), and the bridge that you see in the above linked pics as the dashed lines.

    The corners I am thinking about trying to treat are circled in red. I think those will make the most difference since having the sub anywhere near the tv gave me extremely weak bass, and moving it to where it is now made a night and day difference.

    Is there any way to determine where the most effective areas are to treat without spending a fortune with experimentation?

    I'm thinking about trying some of those corner foam wedges linked above with a 1" panel of rigid fiberglass lining the exposed edge (in the circled corners).

    [​IMG]
     

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