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Thread: Destructively Driving Modes

  1. #21
    Industry Expert [Industry Expert] Nyal Mellor's Avatar
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  2. #22
    Member Sponsor Addicted to Best! dallasjustice's Avatar
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    That's not what I measure. To drive 1,0,0 destructively, the subs need to be in opposite polarity. When I measure and they are in the same electrical polarity the 1st gets much bigger. If they are in opposite electrical polarity, the 1st goes away. I read that bluray forum post before and I don't think that's accurate info.

    I will post a REW plot later showing this.
    Quote Originally Posted by Nyal Mellor View Post
    Technical correction from acoustics geek...

    There are two polarities here....

    If you place the subs with same electrical polarity in opposite polarities of the room mode then you will drive it destructively. This is the best online resource that explains it: http://forum.blu-ray.com/showthread.php?t=48286. Also explained well in the Toole book.

    I think what you did is different, which is what Klaus refers to as "Single Source-to-Sink" (though have not read that AES paper).

    The above assuming both subs are run with no electrical delay. The Welti simulations are based on all subs fed same signal, no delay, same polarity.

    With electrical delay (e.g. delay rear xx ms) then you are changing how the subs are driving the standing wave. Different delays will cause different effects. You can use this to your advantage along with individual sub level adjustments to "move" the peaks and dips in the room modes where you want them.

    Some simulations: two subs on front wall (dots in middle of room are seats, this is a HT). You can clearly see the 20Hz 1st axial length room mode (low SPL center of room, high walls front and back)
    Attachment 25222

    four subs, two on front wall, two on rears (rears delayed 2ms and run -3dB relative to fronts)
    Attachment 25223

    One interesting thing about multi-subs is the debatable importance of time alignment. People like Geddes are purposely advocating NOT aligning the subs in order to control the room modes. The Welti methods also have no delay adjustments for individual sub channels relative to any listening position.
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  3. #23
    VIP/Donor [VIP/Donor] amirm's Avatar
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    Quote Originally Posted by dallasjustice View Post
    That's not what I measure. To drive 1,0,0 destructively, the subs need to be in opposite polarity. When I measure and they are in the same electrical polarity the 1st gets much bigger. If they are in opposite electrical polarity, the 1st goes away. I read that bluray forum post before and I don't think that's accurate info.
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  4. #24
    Member Sponsor Addicted to Best! dallasjustice's Avatar
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    Quote Originally Posted by amirm View Post
    You have bad water in Texas so it screws up everything including your audio equipment....
    No. I think the polarity is reversed on my front subs because I get cheap electricity.
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  5. #25
    Industry Expert [Industry Expert] Nyal Mellor's Avatar
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    Quote Originally Posted by dallasjustice View Post
    That's not what I measure. To drive 1,0,0 destructively, the subs need to be in opposite polarity. When I measure and they are in the same electrical polarity the 1st gets much bigger. If they are in opposite electrical polarity, the 1st goes away. I read that bluray forum post before and I don't think that's accurate info.

    I will post a REW plot later showing this.
    I'm not sure what is going on in your setup but reference Toole pg. 221 diagram (b). "two subwoofers, one on each side, radiating identical signals, destructively drive the mode reducing the amplitude. This is "mode cancellation".

    This is the behavior I get from acoustical modeling simulations.

    Are the subs fed exactly same signal (no digital delay of rears)? If you are using the old F112 front and the new F112v2 rear it's possible that the amplifiers are different and one is polarity inverting and the other is not. It's more likely something in your setup than the physics being wrong.
    Acoustic Frontiers - specialists in the design and creation of listening rooms and home theaters. Experts in room design, sound isolation, acoustic treatment and AV electrical systems.

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  6. #26
    VIP/Donor [VIP/Donor] amirm's Avatar
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    Quote Originally Posted by dallasjustice View Post
    No. I think the polarity is reversed on my front subs because I get cheap electricity.
    That too. And you kept defending the guy who did your wiring despite my feedback to the contrary:

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  7. #27
    Member Sponsor [Technical Expert] DonH50's Avatar
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    I am using four subs spaced roughly to provide the best overall frequency response, within the limits of the room and furniture. Two are on the side near the front wall, 3 or 4 feet out (beside the mains), and the other pair is on the back wall with each sub about 1/4 in from the side wall. I used REW and the multisub program by andyc56 on AVS (hope that's not a problem, if so I'll delete this and find my link to his site later) ti dial things in, then did lots of measurements and playing with sub phase and Dirac Live filters. Decently flat and with room gain down 3 dB around 7 Hz or so, though I have more fiddling to do, just no time!
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  8. #28
    Member Sponsor Addicted to Best! dallasjustice's Avatar
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    Quote Originally Posted by amirm View Post
    That too. And you kept defending the guy who did your wiring despite my feedback to the contrary:

    Yes, I do accept payment-in-kind. I once had my bathroom remodelled for a fee. I'm not above engaging in medieval economic transactions. I believe economists call barter a "coincidence of wants."
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  9. #29
    Member Sponsor Addicted to Best! dallasjustice's Avatar
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    Quote Originally Posted by Nyal Mellor View Post
    I'm not sure what is going on in your setup but reference Toole pg. 221 diagram (b). "two subwoofers, one on each side, radiating identical signals, destructively drive the mode reducing the amplitude. This is "mode cancellation".

    This is the behavior I get from acoustical modeling simulations.

    Are the subs fed exactly same signal (no digital delay of rears)? If you are using the old F112 front and the new F112v2 rear it's possible that the amplifiers are different and one is polarity inverting and the other is not. It's more likely something in your setup than the physics being wrong.
    "An effective method to equalize low frequencies
    in rectangular rooms has been simulated
    and implemented in these three rooms. The system
    uses two loudspeakers in the front wall of the room
    to create a traveling plane wave and an extra two
    low frequency loudspeakers in the back wall delayed
    and in opposite phase to remove the reflection of
    that wall. "

    Low frequency sound field enhancement
    system for rectangular rooms using multiple
    low frequency loudspeakers

    Adrian Celestinos and Sofus Birkedal Nielsen

    You can see what I'm talking about in section 3.2. "Removing the Reflection from the Back
    Wall"

    This deals with the method I'm using to destroy modal ringing by running a rear sub in "antiphase."

    In the paper's discussion:

    "By first creating a plane
    wave in only one direction of the room which implies
    exiting only the axial modes of that direction
    and secondly canceling that plane wave using loudspeakers
    delayed at the end wall in opposite phase
    with the traveling sound, optimal equalization can
    be obtained."
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  10. #30
    Member Sponsor Addicted to Best! dallasjustice's Avatar
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    Nyal,

    Here's some more from the AES paper authored by Fazenda et al to which Klaus originally made reference with regard to the "sink" method to cancelling room modes:

    "The ‘active’ aspect of this control configuration comes
    into place as the plane wave reaches the rear wall. At
    this location, the ‘sink’ speakers reproduce the same
    signal in anti-phase which has the effect of cancelling
    its reflection from the rear wall and thus any modes
    generated along this dimension."

    The rear subs act to absorb or "sink" the so called "plane wave" from the front of the room. My understanding of all this stuff and my own measurements makes me pretty confident that the rear subs need to be in "anti-phase" to the front subs. IOW, the rear subs can either be in opposite phase or opposite polarity and they act to "sink" the modal ringing between those two walls.

    To be sure, I will measure the impulse peaks for all four of my subs to make sure the polarity switch on each sub corresponds with the sub's actually polarity.

    Here's the "sink" configuration in the paper.

    Name:  sink.JPG
Views: 101
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    Michael.
    Last edited by dallasjustice; 01-22-2016 at 02:24 PM.
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