A reasonable BFD question

Discussion in 'Archived Threads 2001-2004' started by George W, Feb 28, 2003.

  1. George W

    George W Stunt Coordinator

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    Hey guys,

    The other night I finally got around to measure my in room frequency response from 22 to 150 Hz. Here it is:

    [​IMG]

    This is graphed from an "uncorrected" RS spl meter. Now, it seems to me that this would benefit from the BFD. It doesn't sound boomy to me right now, but I'd like to bring the overall volume to match where my 60hz peak is right now. I realize that I'd have to lower all the peaks and then turn up the volume on the sub amp. I think I should have enough headroom to do this, since I'm only at 1/4 on the dial right now. However, and this is my question, I am concerned about my sub bottoming. I usually listen to movies at about 75 dB. With my 85L ported shiva sub in a corner to maximize output, I can't put it up more than 2 or 3 decibels above it's Avia calibrated level or it will bottom on disks like LoTR SE. So I'm worried that if I try to bring all of the frequencies up to match the volume I have 60 hertz up now, I will end up with gratuitous bottoming of the sub. Does this make sense? Let me restate it more clearly. Right now I'm happy with the spl level at 60hz, but I can tell during listening that some other bass frequencies are quieter; I would use the bfd to ultimately raise all of the frequencies to match that peak, and I'm wondering if doing so will bottom the sub. Perhaps I could bring most of the frequencies up and then slope off on the lower end, but that might not sound any better than it does now. I did a lot of experimenting when I first installed the sub and I'm reasonably sure I have it in the position to maximize output. Now that my roomates have moved in though I won't be able to move it around much and I think wall treatments are probably out of the question. I'd say the room is about 13' by 20' with a open walled kitchen in one end and a hallway leading out in the middle. Realistically I probably need a bigger sub for this room but that won't happen for a long while so I'd like to get the most use out of what I have. Do you guys think the bfd will help me or is it an excersize in futility? I appreciate your thoughts.


    George
     
  2. Brian Fellmeth

    Brian Fellmeth Supporting Actor

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    Are you sure the problem is bottoming and not amp clipping ? If you are clipping, a BFD will make it worse. If the problem is botteming, a BFD MAY make it better. The botteming may be via that peak at 60 Hz. By leveling the FR, you can get more deep bass energy overall without the cone having to hit so hard at 60 Hz in particular.
     
  3. George W

    George W Stunt Coordinator

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    Brian,

    I'm reasonably sure the problem is bottoming. I can hear the noise coming from the driver that sounds like it's hitting something, so I don't think it's clipping, though I don't know exactly what that would sound like. I haven't heard of other people having their shivas clip though with the ava250 amp that I'm using.
    As far as 60hz vs. lower causing the bottoming I can't say for sure. To test I used several scenes from LoTR extended edition, specifically the black wringraith at the tree and frodo wearing the ring in the Prancing Pony. I thought that I had heard that these scenes had a lot of very low bass in them, so I assume that was the problem, but they also have higher bass in them as well. If it is bottoming on lower stuff, do you think I'm out of luck?
     
  4. BruceD

    BruceD Screenwriter

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    George,

    Your graph is a little hard to interpret for me.

    You might try changing the spreadsheet "X" axis frequency to 1/3 octave steps with a logarithmic scale.

    It would be easier to interpret, and would probably be easier for you to match with your test tones.
     
  5. George W

    George W Stunt Coordinator

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    Bruce,

    I tried putting the x axis into logarithmic scale but that made it harder to read for me. I was unable to figure out how to incorporate 1/3 octave steps into the equation. Perhaps if I include the raw data, it would be easier to interpret.

    Hz dB
    2261
    2368
    2471
    2672
    2871
    3071
    3272
    3464
    3669
    3867
    4065
    4266
    4466
    4668
    4870
    5072
    5274
    5477
    5680
    5882
    6083
    6280
    6482
    6682
    6881
    7080
    7278
    7477
    7674
    7875
    8078
    8278
    8478
    8677
    8874
    9069
    9264
    9460
    9655
    9857
    10060
    10270
    10474
    10675
    10875
    11076
    11276
    11474
    11674
    11874
    12072
    12269
    12461
    12654
    12862
    13065
    13267
    13469
    13673
    13875
    14075
    14274
    14473
    14670
    14868
    15068

    Remember that this is uncorrected data from the rs spl meter. Thanks for your help guys.
     
  6. Wayne A. Pflughaupt

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    Actually, George, 1/6-octave spacing would be much better. The target frequencies are listed below. If you are limited to the frequencies you posted, just get as close as possible.

    20Hz
    22Hz
    25Hz
    28Hz
    31.5Hz
    36Hz
    40Hz
    45Hz
    50Hz
    56Hz
    63Hz
    71Hz
    80Hz
    90Hz
    100Hz
    111Hz
    125Hz
    142.5Hz
    160Hz

    Regards,
    Wayne A. Pflughaupt
     
  7. Mat_M

    Mat_M Stunt Coordinator

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    George,

    Here's a link to a chart that is pre-made (I actually think BruceK made it) http://www.snapbug.ws/subtestmodel.xls

    All you have to do is plug in your readings from the Rat Shack meter in the 'raw' column. The graph is derived from the values in the right column, which has the Rat Shack corrected values. If you have any questions about it, this guide will help:

    http://www.snapbug.ws/bfd.htm
     
  8. George W

    George W Stunt Coordinator

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    Mat,

    That's excellent, I will make up some more test tones and try to take some readings tonight.

    George
     
  9. George W

    George W Stunt Coordinator

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    ok guys,

    I made up some more test tones and used the template that Mat provided. Here's the graph that was generated:

    [​IMG]

    According to the template these should be the corrected values.
    Looking at the graph makes me worry more about bottoming. Assuming I'd like to keep my 60hz spl around 80dB, it seems that if I applied a house curve I would need my 20hz spl to go up by at least 10 dB, which I fear is asking too much of my sub. Am I totally confused on this? Is it likely that once I equalize it I will be happier with a much lower level at 60hz? I listen to a lot of rock and pop in addition to other music but it doesn't need to sound boomy. I appreciate your input.

    George
     
  10. Mat_M

    Mat_M Stunt Coordinator

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    I think the best answer here is, just experiment with it. Try different sub positions in the room, and then try different filters (I can't remember if you said you have the BFD or not yet). One thing I do remember is that the guide said boosting frequencies lower than 50-60Hz could be taxing on the sub. So as for your 20Hz level, maybe try increasing it little by little. The BFD has a little meter on it that tells you when you're clipping or not. For your 60Hz peak: Yes it can be tamed. Will you be happy with it? That's up to your ears[​IMG]
     
  11. Darrin_R

    Darrin_R Stunt Coordinator

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    George

    You only want to use the BFD for cuts. Or any other EQ for that matter.

    Reduce your peaks and then recalibrate to desired level.
     
  12. George W

    George W Stunt Coordinator

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    Mat and Darrin,

    Ok, since I don't have a bfd yet I will try to pick one up soon. I'm still a little nervous that my sub won't be able to keep up but I guess I'll just have to try it. Thanks for the help guys,

    George
     
  13. Mat_M

    Mat_M Stunt Coordinator

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    If you end up having any questions, just give me an email. I'd be happy to give you suggestions or tell of my experience with the BFD.
     
  14. Wayne A. Pflughaupt

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  15. George W

    George W Stunt Coordinator

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    Wayne, thank you for the encouragement. My total space to fill is somewhere around 2800 ft^3, so hopefully it will cut the mustard. I'll be sure to report back to you guys when I get it up and running. If ultimately the sub can't keep up then I'll just sell the bfd on ebay so I won't be out too much.

    Mat, thanks for the offer, I'm sure I will take you up on it soon enough.

    Thanks again guys,

    George
     
  16. Darrin_R

    Darrin_R Stunt Coordinator

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    I think it is much better to use cuts. It eliminates taxing the amp if you try to boost a null.
     
  17. Jeff Kohn

    Jeff Kohn Supporting Actor

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  18. Wayne A. Pflughaupt

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    Darrin,

    True, it is useless to attempt to boost a null. However, most under-represented areas are not true nulls and can be equalized. Most nulls are very narrow – 1/6- to 1/3-octave or so - and are often hard to pinpoint with initial readings. If there are any, they will show up after equalizing.

    For your edification you might want to take a look at the various charts posted at this thread. If you compare the first and last charts, you can see the deepest low point in the initial readings (at about 87-88Hz) was not the true null. Note that this BFD user was able to successfully equalize all under-represented areas except the null at 111Hz.

    Jeff,

     

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