dB drop out in sub tuning

Discussion in 'Speakers & Subwoofers' started by Keith Hyde, Feb 10, 2004.

  1. Keith Hyde

    Keith Hyde Stunt Coordinator

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    Avia phase sweep. SPL meter. Crossover toggled "disabled" on sub. "Large" mains. Bass out to Mains and sub. Main listening position. 10+ dB drop out once or twice in the sweep. Can't recall specific tonal ranges at the moment, but I thik middle of the sub range 70-80Hz is the weak spot. Speaker size settings pretty insignificant. Crossover point changes pretty insignificant. Lows are great - they actually grow into the next higher dB range near the end of the sweep. High sub tones are great.

    I take it this is the main result of positioning of the sub in the room, and/or the room style/size itself (acoustic cancellation)? It's in a small space for now, until I move. What is a common acceptable peak and valley range? The theoretical ideal I'd imagine to be within +/-2/3dB the whole way through the sweep.

    More experimenting to do, but thought I'd look for a second.
     
  2. GrahamT

    GrahamT Supporting Actor

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    Your right it is the room and position. A common fix is to get a parametric equalizer like the BFD. I have one and my response is now all within 3 dB from 16 to 90 Hz. I would also suggest that first you try to find the flattest spot in the room, probably away from a corner.
     
  3. Keith Hyde

    Keith Hyde Stunt Coordinator

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    Graham, only questions: Are you happy with the parametric equalizer? I take it it is different than a graphic equalizer. ANY negative experiences? I don't know much about them but I believe the theory is you can boost or weaken amplitude at certain selectable frequency ranges to "drive you through" the lull or "ease up" on a boomy range? What type do you have?

    Anyhow, I did lots of playing around last night and it was the position. The best spot sonically is a bit odd along the right wall, but it works and is out of the way somehwat - though my sub is now very much the centerpiece of the room (ahhh, the reasoning behind the PB2+ Ultra - you can't hide big coffin of a sub as well without the nice finished wood grain). Another advantage is the rear panel for tuning is facilitated more now than anywhere it was before. Funny though, a lot of bass has a mild left-side feel to it even thought the sub is plainly on the right wall and behind the right main. Those long waves...

    I understated the dB drop. After playing with Avia and the SPL meter last night to tune to 85, there were times that it went off the charts below 75, and off above 95 (I have the digital version unfortunately). Now it generally hangs in there anywhere from 78 to 90 which is much better. And the music playback is much much better. Anywhere in the room sounds about the same. Ah well, when you are tight on space, you have to get very creative, which is a headache sometimes but also twice the fun. I hadn't realized how effectively those low frequencies could cancel each other. One spot is NO bass, another is boomy. Now I know firsthand.

    At least I live alone and can give sonic performance PRIORITY in room/speaker arrangment, THEN work in the furniture second unlike a girlfriend might have it. Just so long as there's a cushion wide enough for two in the sweet spot, I'm happy.
     
  4. Nathan Stohler

    Nathan Stohler Second Unit

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    I noticed the same problem when calibrating my new sub. I have a dropout around 100 Hz. I adjusted with the built-in equalizer on my receiver, but it's still about a 7 Hz drop. I will experiment with moving the sub to different locations.

    I would really like to leave my sub in the corner, so I'll probably just live with the drop. Other than that, the response was pretty smooth.

    By the way, nice signature Keith. One of my favorite Neil Young songs.

    --Nathan
     
  5. TimRP

    TimRP Stunt Coordinator

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    When you measure the output, are you accounting for the meters non-flat response in the frequency range of the sub?
     
  6. GrahamT

    GrahamT Supporting Actor

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    Keith,

    I use the Behringer Feedback Destroyer Pro DSP1124, very popular here because of low-cost. It is a Parametric Equilizer so you pick the frequency, gain (amplitude), bandwith. You can boost it up to 16 dB or cut it up to 46 dB for up to 12 filters. It is not used as a feedback destroyer. You can also set up to 10 programs such as music, HT, TV etc if you want. Check out: http://www.snapbug.ws/bfd.htm I followed Sonnie's intructions and had no problems.

    Nathan,

    If you are using the sub out on the receiver it is likely that you have it set to 100 Hz. The filter will slope down from after 95 Hz-ish and that is where the mains should pick up so when they are summed you will have a flat response.
     
  7. Roger Q

    Roger Q Stunt Coordinator

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    Keith,

    If I recall from other posts you have a PB2+. If that's true and you have a small room then extra dB is probably not an issue. Try your sub away from a corner. 1/3 of the distance from one corner to the other.
     
  8. Nathan Stohler

    Nathan Stohler Second Unit

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    Thanks, Graham.

    Actually, my main speakers have response down to 51 Hz, so I have the crossover set to 70 Hz.

    When I'm testing, I'm testing the combined output of the subwoofer and main speakers, so that the crossover is taken into account. In other words, I'm not calibrating the subwoofer by itself.

    So, now that I think about it, my problem is with the frequency response of my main speakers, not my sub, since at 100 Hz, my sub should not even be contributing to the sound.

    --Nathan
     
  9. Keith Hyde

    Keith Hyde Stunt Coordinator

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    Oops. No. I thought the SPL was flat. What's the compensation formula? I guess it might be in my SPL manual. Should have known it wasn't as easy as just turning it on and hitting "slow" and "c-weighted".
     
  10. Keith Hyde

    Keith Hyde Stunt Coordinator

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    Graham,

    What a link! But I can tell I need some computer hardware first. I've got it marked for the future though. Definitely on the to-do list to plot one of those dB charts sometime. More arts of fine tuning - sweet.

    Keith
     
  11. Justin_D

    Justin_D Stunt Coordinator

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    How exactly does a Parametric Equilizer work?
    Also, how does it hook up to the sub? Do you just put the RCA cables into it, then back out to the sub?
     
  12. Brian L

    Brian L Cinematographer

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    Just a general comment...

    If your dips are due to nulls in the room, you can not fix it with EQ. The only solution is to move your sub, your chair, or both.

    EQ is great to kill peaks, and also when dips are relatively small, but for big holes in the FR, you need to move things around.

    BGL
     
  13. GrahamT

    GrahamT Supporting Actor

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    Keith,

    Plotting a FR graph is easy. All you need are test tones which you can download for free. Then a spreadsheet. The correction factors are:

    10Hz add 20
    12.5Hz add 16.5
    16Hz add 11.5
    20Hz add 7.5
    25Hz add 5
    31.5Hz add 3
    40Hz add 2.5
    50Hz add 1.5
    63Hz add 1.5
    80Hz add 1.5
    100Hz add 2

    As Brian said, the BFD will only help with small nulls and finding the flattest position is the first step.

    Justin,

    A PE just boosts or cuts the frequecies within its filters before the signals get to the sub. You can hook the BFD with rca to 1/4" connectors.
     

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