Room Modes?

Discussion in 'Speakers' started by Victor Ferguson, Jul 13, 2005.

  1. Victor Ferguson

    Victor Ferguson Stunt Coordinator

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    I have a problem with a room mode. First I guess I should explain the room. The room is 15x20x8. The room is layed out as follows. I have a FP throwing to the 15 foot wall. Due to screen size I have to sit about 15 feet back which puts me 5 feet from rear wall. I have a 6.1 setup and a rear center surround mounted close to the top of ceiling. My room mode is about 3~4 feet wide and ~12 feet long centered in the room. This puts my listening position in the mode. Close to the rear wall the mode subsides some and the bass is much better. Would changing my seating position to the rear wall do more harm than good? My rear surround would be directly over my head. I have a BFD in my system but it doesn't help room modes any. I have moved the sub all over the front of the room and the mode always exists.
    *Short of room treatments which are not an option does anyone have any suggestions?
    *Would a second sub located in a different part of the room help with the mode or is always going to be there due to the size of the room?
     
  2. John Garcia

    John Garcia Executive Producer

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    Sounds like you would be a good candidate for the "crawl" method of location. Put it where you sit and check possible locations for the sub. Where the strongest bass is, is where you should put the sub.

    Have you tried 2/3 of the way along the 20' wall or pulling the sub out a little further into the room?

    Sitting against the wall will give you more bass, it will most likely detract from the sound of the rest of your speakers
     
  3. Victor Ferguson

    Victor Ferguson Stunt Coordinator

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    The crawl method I have tried. I have put the sub 1/3 along the 15 foot wall, in the middle and corner loaded. All locations still give me poor response at the listening position. I have tried moving the sub to the back of the room and also along the side walls but it never intergrated well that way. Keeping the sub in the front soundstage gives me the best overall sound quality. Would adding another sub exicte the room in a way to cancel out this mode?
     
  4. John Garcia

    John Garcia Executive Producer

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    What are your other speakers and how are they setup in the room? What happens if you run your mains large with no sub? Do you have the same problem? What sub and speakers and what x-over are you using?

    I would think that with a BFD you would be able to correct at least some of the response issues. What did your FR look like and how many adjustments did you have to make with the BFD?

    Sorry, I couldn't really tell you if two subs would help.
     
  5. Victor Ferguson

    Victor Ferguson Stunt Coordinator

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    My other speakers are as follows:
    Mains - Polk RTi8's
    Center - Polk CSi5
    All surrounds - Polk RTi4's
    Sub - Polk PSW404 - it sucks but its what I got right now
    AVR - H/K 335
    x-over - 80hz for all speakers

    Speakers set to large alleviates much of the problem. But it still exists.

    Before BFD (tested with mains and sub w/ x-over at 80Hz) was as follows.
    *I had a pretty flat response from 28 - 100 Hz +/- 3db with the exception of a null of about 6 db at 40Hz, another null of 10db at 70Hz and 10db hot at 80Hz and 110Hz.
    *After the BFD (tested with same AVR settings) I had a +/- 3db from 25-100 Hz with the exception of a 7db drop at 70Hz and a 8 db rise at 80Hz and 110Hz.

    I added the following filters to get this,
    +3db at 25 Hz
    +3db at 40Hz
    -48db at 80Hz
    -48db at 110Hz

    The bandwidths I don't remember off the top of my head. The 80 and 110 Hz spikes have always been a problem and I'm sure they are room related. Setting the BFD to narrow bandwidth and using max reduction at least tames the sub in these frequencies leaving only my mains creating the spike. I have heard its bad to boost any and I did it as moderately as possible. In the beginning I just used allot of filters to cut everything down to my low point at 70Hz and ended up with my sub turned all the way up and it sounding like crap. Well I am going to try placement one more time to see if it helps. Thanks for all the responses.
     
  6. John Garcia

    John Garcia Executive Producer

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    Well, rather than getting a second one of those subs or even a second sub, I'd say you might be better of with one good one, aka: SVS or HSU. I installed a 202 and had a pretty tough time getting it to sound right; placement turned out to be critical for that one.

    I have mode very similar to that, because my room is nearly square, and the lovely null happened to be right at ear level on my couch. I ended up slightly adjusting my seating position - still have a bit of a null, but not as bad. I have not gone the BFD route yet becase I intend to move soon, but I have heard what one can do for a difficult room, and the PB-10 boosted +1dB gives me enough oomph.

    Generally speaking, two subs could actually make the problem worse.
     
  7. Wayne A. Pflughaupt

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    Victor,

    That’s contrary to common wisdom. It’s true that equalizers can’t eliminate room modes, but they can deprive them of energy, which has virtually the same effect. Needless to say, you can’t do that with narrow filters.

    That aside, with a 48 dB cut you really don’t have narrow filters anyway. With any equalizer, especially cheap ones like the BFD, the further you boost or cut, the wider the affected path gets. So if you started out with say, a 1/12-octave filter, by the time it’s cut 48 dB it could easily end up affecting an area between 1/3-2/3-octave wide, possibly even more.

    If cutting the peaks didn’t eliminate them, that can only mean one thing: Your mains ore overwhelming your sub down there. Taking a look at the specs at Polk’s website, it’s easy to see that the Rti8’s are far more capable than the PSW404, which explains the problem you’re having.

    Adding insult to injury, your mode is smack in the middle of the crossover region, so it’s substantially reducing both the high and low pass filtering. The net result is that your system is almost behaving like you’re running all speakers full-range.

    I can offer a suggestion, but I imagine you won’t like it. If the receiver’s tone controls operate independently of the sub output, I suggest using it to reduce the main’s bass output, possibly the center channel’s too, so the sub can do the low freq work by itself. If you have selectable frequency points for the bass control, I suggest setting it around 120-140 Hz and cutting 8-10 dB. Experiment with both the frequency and cut value for best results.

    If the sub has a built-in crossover, it would probably help to move its frequency setting down to 60 Hz or so. Combined with the bass cut to the mains, this will essentially create an underlapping crossover function, the goal of which is to create a sizable hole in response in the region of the mode, thus robbing it of energy.

    The problem your going to end up with, I suspect, is that you’ll find out just now inadequate the PSW is without all the help it’s getting from the other speakers.

    Regards,
    Wayne A. Pflughaupt
     
  8. Victor Ferguson

    Victor Ferguson Stunt Coordinator

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    Update for everyone. I have changed allot of settings and it has helped.
    I have changed my Polk RTi8 mains to "large" (this helped allot) with sub crossed over at 80Hz
    I moved the sub a little closer to the corner. Recalibrated levels.
    I ran sweeps and found that the sub + mains gave me much better FR.
    Tweaked sub with BFD and got +/- 3db from 30 - 120Hz with the exception of
    a null centered around 63 hz that goes from -5db to -7db
    a null at ~70hz that is -6db
    and a peak of +8 db at 80hz and +5db around 55Hz
    I ran separate FR charts for center and surrounds and ended up crossing them over at 80hz.
    After all this I have come to the conclusion that due to my room size it created nodes that were pretty much predictable and right on the mark. My sub wasn't good enough to hold its own and needed my mains playing full range to make up for its lack of performance. By playing my mains full range and having multiple locations outputting bass it seemed to reduce the effects the nodes had. After making the changes no boosts were made to BFD only cuts. I didn't get my whole response within +/- 3db but the majority of it is and room modes are causing the rest of my problems. Short of room treatments and getting a better sub I think its about as good as its going to get right now. .
     
  9. John Garcia

    John Garcia Executive Producer

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