Bass Help!!: How to approach my problem with a Behringer Feedback Destroyer?

Discussion in 'Archived Threads 2001-2004' started by Graeme Shiomi, Jul 29, 2001.

  1. Graeme Shiomi

    Graeme Shiomi Stunt Coordinator

    Jan 7, 2001
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    Okay, I did a careful sweep of the room as it is right now, and got some results. Basically, it looks like my response is far from flat. [​IMG]
    Now, when you look at my results, you see that it isn't one specific frequency that is killing me. It's a range, where the response goes down from 74-60 Hz and up from 58-30 Hz. However, as far as I can tell, a Behringer Feedback Destroyer only picks out specific frequencies. The number of offending frequencies I seem to have is far more than the 24 available settings.
    Am I missing something here? Is there a different method of attacking the problem? Help!
    Here are the results (from 25-80):
    Hz dB
    80 80.5
    79 80.5
    78 80.5
    77 80.5
    76 79.5
    75 80.5
    74 77.5
    73 76.5
    72 75.5
    71 73.5
    70 73.5
    69 71.5
    68 69.5
    67 68.5
    66 70.5
    65 71.5
    64 71.5
    63 73.5
    62 75.5
    61 77.5
    60 79.5
    59 80.5
    58 83.5
    57 84.5
    56 85.5
    55 85.5
    54 85.5
    53 85.5
    52 84.5
    51 84.5
    50 84.5
    49 86.5
    48 86.5
    47 85.5
    46 85.5
    45 85.5
    44 85.5
    43 85.5
    42 85.5
    41 86.5
    40 86.5
    39 86
    38 86
    37 85
    36 84
    35 84
    34 83
    33 82
    32 81
    31 82
    30 81
    29 80
    28 79
    27 79
    26 78
    25 77
  2. Brian_J

    Brian_J Second Unit

    Mar 3, 2001
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    Yes, you are missing something. For one thing, your resonse is pretty flat. You really only have one hole, centered at about 67 hz +-7hz. You really cannot address dips but only peaks anyway. But, when you cut or add with the bfd you will do it over a range of frequencies based on the bandwidth you select.
    Problems really are like 15-20 db jumps or dips, not 5. Thats not bad and barely audible. I am assuming you adjusted for the rs mic with the compensation values.
    BTW, a post in the DIY section would be more helpful to you.
    Anyway, you need to do some research into the bfd and freq charts before you post because I dont think you understand the purpose and objectives. There is a pretty steep learning curve involved. The first thing you might do is download Spectraplus and the BFD software.
    Zed's Dead Baby...
    [Edited last by Brian_J on July 29, 2001 at 07:30 PM]
  3. BruceD

    BruceD Screenwriter

    Apr 12, 1999
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    I think you should check out this site to get a better understanding of room-speaker interaction. It provides a demo HT room and does a great job of explaining exactly what you hear and see on the graphs as examples. A very good education in terms and technology.
    Also this program (ETF5) can help you do a greatjob of optimizing your low frequency response. It's a pretty good deal at $149 (compared to spectra plus) and you can use your RS SPL meter as a microphone.
    [Edited last by BruceD on July 29, 2001 at 08:11 PM]
  4. Graeme Shiomi

    Graeme Shiomi Stunt Coordinator

    Jan 7, 2001
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  5. Rick P.

    Rick P. Stunt Coordinator

    Mar 2, 1999
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    As others have said, you can't boost a null. It doesn't matter whether you try the FBD, a passive crossover or room treatment, you're not likely to see much of a difference. The FBD is however very good at cutting peaks at any frequency above 20Hz. Furthermore, since you can select a specific frequency, bandwidth and gain to work on it is very definately not limited to a few different settings. From what you described, it sounds like you were trying to enter gains for individual frequencies (i.e. use one program for 67 Hz, another for 68 Hz, etc ...). Although this is possible, it is not at all necessary. I suggest moving your sub until your 67 Hz null disappears - probably into a corner somewhere. Then, use the FBD to control any peak introduced when you move the sub.
    As an example, suppose you corner-load your sub and find that the 67 Hz null disappears, but that you now have a 15 dB peak centred at 42 Hz. Suppose further that this peak starts to form at 33 HZ on the low end and ends at 50 Hz on the upper end. You would then
    1. Set perhaps a 10 dB cut (press "GAIN" the turn the thumbwheel to -10) into the FBD. Note: You won't enter a full 15 dB cut because your sub is corner loaded.
    2. Set the centre frequency to 42 Hz. To do so, press "FREQUENCY" and select ISO frequency 40 HZ, then press "FINE (1/60 OCT)" and turn the thumbwheel until the display reads +4/60. Finally,
    3. Enter the bandwidth to be adjusted. In this case, your Q factor is 42 / (50-33) = 2.5 (a little more than 1/2 octave, or about 35/60 octave). Store your changes.
    You'll find that most if not all of your peak around 42 Hz is gone. Then, repeat the above steps for other peaks that you have. You'll have to do a few iterations to get everything just right. The tables in the FBD manual are very helpful once you know how to interpret them.
    Hope this helps,
    [Edited last by Rick P. on July 29, 2001 at 09:42 PM]
  6. Jay Mitchosky

    Jay Mitchosky Producer

    Sep 6, 1998
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    Awakening an old thread. What source did you use for playing the individual frequencies?
    "No one can hear when you're screaming in digital."
    My Home Theatre Pictures...
    "You're no messiah. You're, you're a movie of the week. You're a ... t-shirt, at best."
  7. Graeme Shiomi

    Graeme Shiomi Stunt Coordinator

    Jan 7, 2001
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    I have two sources. One is on the computer, from a company called NCH Swift. It's freeware, which you can get from this site: Look for the "Tone Generator" under "Freeware".
    The other source is from Stryke Audio at It's their Bass-Test CD. Even though it's in the States, it still comes to around 20 bucks Canadian shipped, so it's pretty inexpensive. It's a little more comprehensive than the Tone Generator, since it has some different types of tracks.
    Personally, I'd suggest the Bass-Test CD, since it is easier to use, especially if you can't easily connect your PC to your gear.
  8. Jay Mitchosky

    Jay Mitchosky Producer

    Sep 6, 1998
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  9. Ray R

    Ray R Stunt Coordinator

    Jun 28, 2000
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    Just a suggestion about the null you have. I had a null around 75 hz and found that moving my mains only a few inches made for a flatter response around the null frequency.
    I spent a lot of time setting up my BFD. It's probably just me, but I had a hard time getting a handle on things until I graphed my results. I then decided that +-4db was what I was going to consider flat. That's an 8 db swing, but with very little room treatment I felt this was acceptable. I put this 8 db band on the graph so that most of my frequency readings were inside this band. Then all I had to do was look at the frequencies where the response was outside of this acceptable range. With a list of numbers I just didn't know where to start, but a graph helped a lot.
    One point about using the BFD, don't try to reduce a wide hump with just one filter. In your case, I would set a narrow filter at 39 hz and a wider filter at 43 hz. Then take some readings and make ajustments.
    HT pictures
  10. Zbigniew

    Zbigniew Stunt Coordinator

    Jan 8, 2000
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    Where can I find adjustment values for RS SPL ?
  11. brucek

    brucek Second Unit

    Dec 29, 1998
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    Below are the values that I use, I think they're accurate.
    I posted a review of my BFD a week ago at the end of the thread below. It may clear up a few questions on the BFD.
    * = added 1/6 OCTAVE APPROXIMATION
    10Hz +20.5
    *11Hz +18.0
    12.5Hz +16.5
    *14Hz +13.5
    16Hz +11.5
    *19Hz +8.0
    20Hz +7.5
    *22Hz +6.5
    25Hz +5.0
    *28Hz +4.0
    31.5Hz +3.0
    *36Hz +2.5
    40Hz +2.5
    *45Hz +2.0
    50Hz +1.5
    *56Hz +1.5
    63Hz +1.5
    *71Hz +1.5
    80Hz 1.5
    *89Hz 1.5
    100Hz 2.0
    *111Hz +1.0
    125Hz +0.5
    *142.5 Hz +0.5
    160Hz -0.5
    200Hz -0.5
    250Hz +0.5
    315Hz -0.5
    400Hz 0.0
    500Hz -0.5
    630Hz 0.0
    800Hz 0.0
    1.0KHz 0.0
    1.25Khz 0.0
    1.6KHz -0.5
    2.0Khz -1.5
    2.5Khz -1.5
    3.15Khz -1.5
    4.0KHz -2.0
    5.0KHz -2.0
    6.3KHz -2.0
    8.0KHz -2.0
    10Khz -1.0
    12.5KHz +0.5
    16KHz 0.0
    20KHz +1.0

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