Desperately seeking BFD help!

Discussion in 'Home Theater Projects' started by Jeff Elliott, Oct 31, 2004.

  1. Jeff Elliott

    Jeff Elliott Stunt Coordinator

    Joined:
    Nov 5, 2002
    Messages:
    59
    Likes Received:
    0
    Trophy Points:
    0
    Hi Folks:

    I've almost finished my new theater space, and I'm not too happy with the low freq. response in my seating position. I've sought help and got lots of advice, but so far, the ideas haven't been feasible or in some cases haven't worked out. I have a BFD now and am hoping that it holds much of the solution to my problems.

    Here's the room setup: room drawing side elevation
    Note that the subs have been moved to the corners of the stage as that yielded a more even response. Predicted modes for this space were 26hz, 42, 50, 52, 66, 68, 70, 78, 79, 83, 84.

    The equipment in question is as follows: 2 AV-15's, each in 7ft^3 cabinets tuned to 18hz. Each is powered by a rythmik 350 plate amp.

    Here's the response I'm getting at my seating position with the mains off: no mains

    Here with the mains on: with mains

    I've played with the BFD and have been able to get some minor improvements. However, even with major cuts (-40db), I can't seem to get some of the peaks down. Now, I'm ready to start over with my filters... hopefully with expert help from you guys!

    Also, I'm curious as to the crossover frequency for my subs based on these graphs. I thought I was crossed over at about 60hz as my mains are supposed to be -3db @ 50hz. However, based on the no mains repsonse graph, I'm not too sure.

    Any and all suggestions are welcome!

    Jeff
     
  2. GrahamT

    GrahamT Supporting Actor

    Joined:
    Sep 13, 2003
    Messages:
    556
    Likes Received:
    0
    Trophy Points:
    0
    Somethin' aint right. Your graphs with and without the mains are almost the same.
    As far as I know, cutting always works decibel for decibel. Maybe you have some settings wrong, are both engines on, are both engines set to the right filter, are the filters set to the right frequencies (the decimals and the Hz, kHz can be hard to see) are you in bypass etc. Dont take anything I say as condescending, I have made all of the above mistakes and then some.

    I would suggest you move the subs again, maybe co-locate them. You have a big ass null at about 35 Hz is it? That will be hard to fix without moving the subs or building traps.

    Hope that helps until the BFD pros get here.
     
  3. Allen Ross

    Allen Ross Supporting Actor

    Joined:
    Sep 30, 2002
    Messages:
    819
    Likes Received:
    0
    Trophy Points:
    0
    And seeing that his room is pretty tricked out with treatments i am not sure how much traps would help.

    I'd say move them around, possible to move them from the front wall?
     
  4. Jeff Elliott

    Jeff Elliott Stunt Coordinator

    Joined:
    Nov 5, 2002
    Messages:
    59
    Likes Received:
    0
    Trophy Points:
    0

    No room to move the subs out into the room. This pic should illustrate why... front of room

    Thanks for all the suggestions so far! Keep em' coming!
    Jeff
     
  5. Jake K

    Jake K Stunt Coordinator

    Joined:
    May 22, 2003
    Messages:
    132
    Likes Received:
    0
    Trophy Points:
    0
    What about turning the subs upside down, or just one of them? Might be worth a try.
     
  6. Wayne A. Pflughaupt

    Moderator

    Joined:
    Aug 5, 1999
    Messages:
    6,187
    Likes Received:
    51
    Trophy Points:
    9,110
    Location:
    Katy, TX
    Real Name:
    Wayne
    Jeff,

    Since you have the subs separated, it’s probably best to EQ them separately. This will be easy as the BFD is two-channel. If you turn off one of them and EQ the other, I’ll bet you’ll see that the cut filters will then become effective.

    Regards,
    Wayne A. Pflughaupt
     
  7. Jeff Elliott

    Jeff Elliott Stunt Coordinator

    Joined:
    Nov 5, 2002
    Messages:
    59
    Likes Received:
    0
    Trophy Points:
    0


    This was good advice... and it worked when dealing with each sub separately. I ran into problems again when I turned on both subs and remeasured.

    I first corrected the left sub:

    Left Before -
    [​IMG]
    Left after correction -
    [​IMG]
    Next, I worked with the right sub:

    Right before -
    [​IMG]
    Right after correction -
    [​IMG]
    While not perfect, I was reasonably pleased with the curves of each sub when operating independantly. When I turned on both subs and measured at the same listening position, the results were not nearly as appealing:

    Both subs operating together after correction -
    [​IMG]
    It's not a change in the BFD as I can physically turn off each sub amp and achieve the curves I previously got when correcting each individual sub.

    What the heck should I do next? I've got tons of money in this room and the bass stinks unless you sit in the cheap seats (back of the room).
     
  8. Max F

    Max F Second Unit

    Joined:
    Jun 26, 2004
    Messages:
    250
    Likes Received:
    0
    Trophy Points:
    0
    Are the two subs out of phase with each other?

    Also, try testing at lower volumne. Don't want to kill your ears.
     
  9. ThomasW

    ThomasW Cinematographer

    Joined:
    Nov 6, 1999
    Messages:
    2,282
    Likes Received:
    0
    Trophy Points:
    0
    Here are a few options, unfortunately you won't like any of them.

    1)move the subs to the sides of the center channel speaker
    2)Rebuild the subs, put both drivers into one box
    3)change the geometry of the room
    4)add additional subs at the back and or sides of the room
    5)invest in a digital processing unit that can control the frequency response in the time/space domain.

    Have you read the Harman International whitepaper about the placement of mutliple subs in a room? If not you should. HERE's a link
     
  10. Wayne A. Pflughaupt

    Moderator

    Joined:
    Aug 5, 1999
    Messages:
    6,187
    Likes Received:
    51
    Trophy Points:
    9,110
    Location:
    Katy, TX
    Real Name:
    Wayne
    It can be pretty difficult to get uniform response in symmetrical rooms with shoe-box dimensions. If you can’t get satisfactory results with only one sub or with both co-located, I’d look into bass traps.

    Regards,
    Wayne A. Pflughaupt
     
  11. Jeff Elliott

    Jeff Elliott Stunt Coordinator

    Joined:
    Nov 5, 2002
    Messages:
    59
    Likes Received:
    0
    Trophy Points:
    0


    Well, the front row always sounds terrible as compared to the second row. perhaps I should have said this earlier, but I listen to concert DVD's in this room quite a bit. That's what sounds so bad... music. Let the sub fall off the cliff in finding nemo and your teeth will rattle out. But kick drums have no impact in my front row at all... like the subs aren't even on. The back row has good impact but is against the wall (so I guess it should have impact). A good note from the bass guitar or a kick drum seems realistic in the second row... one sub or two. In the front row... nothing.

    I would have thought that two AV-15's in 7 ft^3 a piece, tuned to 18hz with 350 continuous watts would have been outstanding in a 2100 cubic foot room. Am I crazy?

    Any further suggestions or comments are greatly appreciated!
     
  12. Wayne A. Pflughaupt

    Moderator

    Joined:
    Aug 5, 1999
    Messages:
    6,187
    Likes Received:
    51
    Trophy Points:
    9,110
    Location:
    Katy, TX
    Real Name:
    Wayne
    Not really – one of them should be more than enough, actually.

    The problem with symmetrical shoebox rooms often is a dead spot in the center, with prominent bass along the boundaries. You might want to contact Forum member Ethan Winer at realtraps.com about some bass traps. Possibly they could absorb some of the excess bass chasing the boundaries and give you more uniform response.

    Regards,
    Wayne A. Pflughaupt
     
  13. Jeff Elliott

    Jeff Elliott Stunt Coordinator

    Joined:
    Nov 5, 2002
    Messages:
    59
    Likes Received:
    0
    Trophy Points:
    0


    Well, I'm willing to try anything at this point. However, I'm wondering if using traps would require me to add even more output from the subs (since there's no bass in the front row anyway)?

    Thanks
    Jeff
     
  14. Wayne A. Pflughaupt

    Moderator

    Joined:
    Aug 5, 1999
    Messages:
    6,187
    Likes Received:
    51
    Trophy Points:
    9,110
    Location:
    Katy, TX
    Real Name:
    Wayne
    Ethan could tell you for sure, but I expect so. However, I expect you have more than enough head room to handle it.

    Regards,
    Wayne A. Pflughaupt
     
  15. Brian Fellmeth

    Brian Fellmeth Supporting Actor

    Joined:
    Jul 30, 2000
    Messages:
    789
    Likes Received:
    1
    Trophy Points:
    0


    Probably not. Its a paradox, but the traps could easily add SPL to the center of the room at the same power. That is because they remove some of the energy that is causing destructive interfearence (summation of out of phase reflections that cancil)
     
  16. Greg Yeatts

    Greg Yeatts Second Unit

    Joined:
    Feb 26, 2002
    Messages:
    300
    Likes Received:
    0
    Trophy Points:
    0
    Jeff

    I may be wrong here, but we don't listen to test tones. I had a similar problem as yours. I had a 20db peak at around 45 hertz. I did not get a db for db cut when I tried to reduce the peak. I would have to cut 29 db to reduce the 20 db peak. I do not have twin subs, I only have one.

    The first thing you may do is use test tones that are only 1 hertz apart. That is what I did. With my previous tones (1/3 octave) I could tell I had peak, but could not pinpoint it.

    The thing is, when I listened to music, the bass was lifeless. I think this is because most music is transient and those test tones are the same frequencies for up to a minute on end. When I dialed my cut back to 20db rather than 29db to get the flat measurement, my system sounded more balanced even though it measured didn't measure as well.

    I was basing the balance on a CD cut that I have that has a meandering bassline from a acoustic bass. With the 29 db cut this bassline would get too thin in places (a good guess would be around 45db).

    Thomas W

    This is my experience. Am I way off base (bass) here?
     
  17. ThomasW

    ThomasW Cinematographer

    Joined:
    Nov 6, 1999
    Messages:
    2,282
    Likes Received:
    0
    Trophy Points:
    0
    Most people find an absolutely flat system a little dull. That's why the concept of a 'house curve' was developed.
     
  18. Brian Fellmeth

    Brian Fellmeth Supporting Actor

    Joined:
    Jul 30, 2000
    Messages:
    789
    Likes Received:
    1
    Trophy Points:
    0
    Greg,

    Your experience may be because your ears when listening were not located exactly where the meter was. The peaks can vary widely in amplitude over just a few inches in space. I suspect that the 29 dB cut was perfect for the meter location, but that your 45 hz peak is not as hot where your ears are, so it overcorrected for that point in space. I doupt that the duration of the tones is the culpret here- equlibrium is established very quickly.
     

Share This Page