I have a problem with IB sub location...

Discussion in 'Archived Threads 2001-2004' started by Rick Guynn, May 13, 2002.

  1. Rick Guynn

    Rick Guynn Second Unit

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    Well, after being inspired to strive for an IB, I have encountered a problem... The 'ideal' area (above the TV between the mains) is covered by the A/C unit (completely). I was planning on making a 2-tempest unit like Rich Kraus', but even moving it off to one side or another is pretty much out.
    So I need an alternative location. Should I put it in the rear of the room, centered? Another possibility is maybe trying to slide the drivers under the unit and have them fire directly into the room instead of using a manifold.
    My room is 14x22 and the layout looks like this:
    [​IMG]
    [​IMG]
    Anyone have any suggestions?
    RG
     
  2. ThomasW

    ThomasW Cinematographer

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    If you can put it in the room behind the TV that's a good location. Or consider the attic above the listening position. If you have 8' of higher ceilings the wave will be traveling a distance that would be close to that of the mains.

    I wouldn't not recommend putting it behind the listening position.
     
  3. Rick Guynn

    Rick Guynn Second Unit

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    Thanks Thomas. The area above the listening position is the best possibility so far. The room behind the mains is a hallway.. so no-go there [​IMG]
    I will be using the PE 794 plate amp to power this, btw. Does the variable phase give me more flexibility on placement?
    RG
     
  4. Wayne A. Pflughaupt

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    Rick,

    In this room the sub will get the best performance – extension, SPL and response - in a corner with the longest interrupted wall length in both directions. Unless the doorway opening near the right speaker can be closed off, I recommend turning the room around and setting up everything on the opposite wall. The way the room is set up now the sub will be near a room opening, which is never good.

    The phase switch has nothing to do with placement. It is to compensate for phase problems around the crossover frequency that may arise when using mismatched crossovers.

    Another tip: Moving the L/R speakers inward away from the side walls will improve imaging.

    Love the 3-D pictures, by the way!

    Regards,

    Wayne A. Pflughaupt
     
  5. Rick Guynn

    Rick Guynn Second Unit

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    Thanks Wayne! I'm not sure that rotating the setup will meet with good WAF. The 'not good' with the doorway there would explain the readings I have been getting with the sub I have over there now.

    As to the mains, I am planning on building the rack/entertainment center shown in the images soon and I hope that with that and the IB, I can free up some floor space to move those things in.

    The pictures were done with Rhino 3D. I went hunting it after I saw someone else here post about it a while back.

    I will certainly post up when/if I ever get this IB going ;P

    RG
     
  6. Rick Guynn

    Rick Guynn Second Unit

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    Wayne, I was crawling around in the attic and I think I *might* be able to get the box over the left corner area of the room. There is a closeable door on that opening. Would this proved long enough walls? Even with the window in that wall?

    Thomas, do you think locating it to such an extreme side-position will 'muddy' the image too much?

    Thanks again guys,

    RG
     
  7. ThomasW

    ThomasW Cinematographer

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    Actually a crossover with a variable phase function can assist with matching the off center sub to the mains. This is why the Paradign X-30 is often recommended to people making IB subs.

    I don't think the corner mounting will 'muddy' the bass it will increase the output though.

    There are potential issues with phase delay if there is a significant difference of the distance between the mains and the IB and the listener. Trying putting an existing sub in the listening position then climb a ladder to the proposed spot of the IB. That's the best test of whether or not the IB location will be suitable.
     
  8. Wayne A. Pflughaupt

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    Rick,
    The window won’t matter – unless you leave it open during movies... [​IMG]
    Regards,
    Wayne Pflughaupt
     
  9. Rick Guynn

    Rick Guynn Second Unit

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    Thanks guys. I will get to do all my measurements and decide on a final location over the Memorial Day weekend. After that, it should only be a matter of getting the drivers in.....

    RG
     
  10. Greg Monfort

    Greg Monfort Supporting Actor

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    >The phase switch has nothing to do with placement. It is to compensate for phase problems around the crossover frequency that may arise when using mismatched crossovers.
    ====
    Phase = time = distance. It's the mismatch in distance, and therefore time, that causes the phasing error. [​IMG] For instance, at 80Hz with no XO the two driver's acoustic centers should be vertically aligned if their signals are to arrive at the same point in space at the same time (in phase).
    Now let's divide them up with a 4th order XO: (speed of sound/crossover frequency)/(360/required phase).
    A 4th order XO leads by 360deg, so (13,560/80Hz)/(360/360) = 169.5" (14.12ft) between the acoustic centerline of the sub and the mains LF. IOW if boundaries, seating, etc. aren't considered, then ideally the sub should be positioned ~14ft in front of the mains. Conversely, if the sub is back where the mains are, then it should be wired 180deg out of phase WRT the mains.
    GM
     
  11. Rick Guynn

    Rick Guynn Second Unit

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    OK Greg, I have seen you use this equation before and I am trying to understand it... tell me if I have it right, please [​IMG]
    With a 4th-order XO, you essentially have a wave from one driver leading the other by a full wavelength? (360°) Does this assume that only one driver is involved in the XO? If so, then if both drivers are running through the XO (as in using the built-in receiver XO), then the drivers are in-phase at this point?
    Now, having said all of that, does this mean you *can* compensate for disparities in distance by adjusting the variable phase on the the sub? Looking at the math, wouldn't this phase adjustment only be useful for one given frequency?
    Sorry for the barrage, but I kinda like the mathematical side of it and want to understand how it works..
    Thanks,
    RG
     
  12. brucek

    brucek Second Unit

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    Greg,

    I certainly agree with your post and your phase distance calculations, but it's my understanding that the initial large compensation for the speaker placement distances to the listening position are through the processor setup.

    Whether the processor uses the typical 4th order HPF / 2nd order HPF, the delays required to compensate for the speed of sound with respect to speaker placement and crossover used in the processor are taken care of with "distance to listener" setups. This would then set the mathematically calculated "best phase" position for the sub (and all the speakers) to essentially an ideal zero.

    Then after that, as Thomas W points out, a variable phase control (such as the X-30) for your sub is useful to "trim" the phase (which as you point out yields distance), to compensate for real world practicalities like, off center effects that are introduced because it's not a perfect world.

    My point is that the processor assumes that all speakers are connected "in phase" when you enter your speaker distances. It knows the speed of sound and the basic formulas you have shown for distances vs speed of sound and frequency. I feel changing the phase of a sub 180 degrees should only be carried out after you find the resultant sound is not to your liking once your processor has been properly set up....

    brucek
     
  13. Wayne A. Pflughaupt

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    Greg,
    I must say I also questioned the premise of your post initially. The first thing that came to mind was what brucek mentioned: “If that’s true then why don’t we have phase controls to compensate for relative speaker distance?”
    But I thought it would be smart to put your theories to the test before dismissing it all out of hand.
    I have an outboard crossover with 24dB/octave slopes (the same as what you referred to), but it doesn’t have a phase switch. However, since you said 180o out of phase was “the ticket” I just flopped the speaker wires to both my subs (which accomplishes the same thing). Then I took 1/6-octave sine wave readings from 45Hz to 200Hz (essentially an octave above and below the crossover frequency of 90Hz) with both mains and subs on.
    I must say I was shocked at the results. I had two major problem areas in this range, an extreme null at 71Hz and a just-as-extreme spike at 45Hz. Well, they were both (insert sound of jaw dropping to floor)... GONE!
    As a result of reversing polarity, overall response between 45-200Hz improved from ±30dB (including the spike and null) to – are you sitting down? – ±6dB!
    That’s not all. Before, response from 63-45Hz increased by 12dB, no doubt driven by the 45Hz spike. With the mains and subs out of phase, response over the same range was now ruler flat!
    That’s not all. Another in-phase problem area was between 80Hz and 200Hz, which was a very ragged “saw tooth” pattern that varied ±13dB. After reversing phase, the “saw tooth” pattern remained, but tightened up considerably to only ±6dB.
    Greg, I’d certainly be interested to hear any thoughts you have on this!
    I still have questions about the time/distance/phase theory, however. Namely, the way bass sucks out when you reverse polarity on two full-range speakers even though there are no changes to time or distance. Or why that change seems to matter less and less the further up the frequency spectrum you go.
    I’m also curious to find out the effects of reversing polarity with other crossover slopes (like 12dB or 18dB/octave) and what would happen in instances of mismatched slopes (i.e., steeper low pass, etc.).
    Not that it matters - I’m keeping the polarity reversed on my subs!
    Regards,
    Wayne A. Pflughaupt
     
  14. Greg Monfort

    Greg Monfort Supporting Actor

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    >With a 4th-order XO, you essentially have a wave from one driver leading the other by a full wavelength?

    ====

    Correct.

    ====

    > (360°) Does this assume that only one driver is involved in the XO?

    ====

    No.

    ====

    >Now, having said all of that, does this mean you *can* compensate for disparities in distance by adjusting the variable phase on the the sub?

    ====

    Yes, this is what the adjustable time delay option on active XOs and digital EQs do.

    ====

    > Looking at the math, wouldn't this phase adjustment only be useful for one given frequency?

    ====

    Correct, but it's not that simple. I just used the basics to show that phase = time = distance.

    ====

    >I certainly agree with your post and your phase distance calculations, but it's my understanding that the initial large compensation for the speaker placement distances to the listening position are through the processor setup.

    >Whether the processor uses the typical 4th order HPF / 2nd order HPF, the delays required to compensate for the speed of sound with respect to speaker placement and crossover used in the processor are taken care of with "distance to listener" setups. This would then set the mathematically calculated "best phase" position for the sub (and all the speakers) to essentially an ideal zero.

    ====

    This is strictly limited to SPL right? Not having a prepro or HT receiver, I'm not familiar with how it works. I mean if it could measure acoustic phase then it would be the equivalent of a TEF machine, which I seriously doubt.

    ====

    >Then after that, as Thomas W points out, a variable phase control (such as the X-30) for your sub is useful to "trim" the phase (which as you point out yields distance), to compensate for real world practicalities like, off center effects that are introduced because it's not a perfect world.

    ====

    Correct, and the only point I was trying to make.

    GM
     
  15. Greg Monfort

    Greg Monfort Supporting Actor

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    >But I thought it would be smart to put your theories to the test before dismissing it all out of hand.
    ====
    That's right, trust no one, but dismissing it out-of-hand helps no one either. [​IMG]
    ====
    >Greg, I’d certainly be interested to hear any thoughts you have on this!
    ====
    Not sure what you're looking for, I mean I'm not surprised at the results.
    ====
    >I still have questions about the time/distance/phase theory, however. Namely, the way bass sucks out when you reverse polarity on two full-range speakers even though there are no changes to time or distance. Or why that change seems to matter less and less the further up the frequency spectrum you go.
    ====
    Well, when two sources are within ~1/3WL their pressure sums, ergo if one is out-of-phase, they cancel. Since WL get smaller with increasing frequency they affect each other less n'less until one channel on average is ~3dB lower at the listening position.
    ====
    >I’m also curious to find out the effects of reversing polarity with other crossover slopes (like 12dB or 18dB/octave) and what would happen in instances of mismatched slopes (i.e., steeper low pass, etc.).
    ====
    Reversing polarity would be too much, 1st order would be only 45deg, 2nd/90deg, 3rd/135deg, and another reason why a variable phase switch is 'de riguer' IMO if proper physical placement isn't a viable option. If it's not available then the advice is to try it both ways and leave it whichever way it sounds loudest.
    ====
    >Not that it matters - I’m keeping the polarity reversed on my subs!
    ====
    Maybe one of these days you'll wise up and step up to an all ~phase coherent system. [​IMG]
    GM
     

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