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Thread: Destructively Driving Modes

  1. #11
    Michael,

    Quote Originally Posted by dallasjustice View Post
    Thanks Klaus. I've been curious about reading a variety of AES papers for a while. I bought a subscription so I can read them now. I appreciate the references. I want to learn more about the "sink" subwoofer setup used in the first paper you mentioned. The paper doesn't go into too much detail about how it works. Maybe I'll search the database for other papers on various subwoofer setups.
    Fazenda mentions 3 more sources that might be interesting in the respect:

    [24] A. Celestinos and S. Nielsen, “Controlled Acoustic
    Bass System (CABS)—A Method to Achieve Uniform
    Sound Field Distribution at Low Frequencies in Rectangular
    Rooms,” J. Audio Eng. Soc., vol. 56, pp. 915–931 (2008
    Nov.).

    [25] A. Makivirta and A. Varla, Sound Reproduction
    Equipment and Method for Reducing the Level
    of Acoustical Reflections in a Room - United States
    Patent 6795557, application: 2000, published: 2004. URLhttp://
    patent.ipexl.com/US/6795557.html.

    [26] M.Wolff A. Goertz and L. Naumann, “Optimierung
    der Wiedergabe von Surround Lautsprecheranordnungen
    in Tonstudios und Abhorraumen”, Tonmeistertagung,
    22, English translation available at http://forums.
    klipsch.com/forums/storage/8/1485263/tmt2002 eng.pdf,
    2001.

    plus

    Celestinos et al., “Optimizing placement and equalization of multiple low frequency loudspeakers in rooms”, Audio Engineering Society Preprint 6545 (2005)

    Celestinos et al., “Low frequency sound field enhancement system for rectangular rooms using multiple low frequency loudspeakers, Audio Engineering Society Preprint 6688 (2006)

    Celestinos et al., “Low-frequency loudspeaker-room simulation using finite differences in the time domain – Part 1: Analysis”, J. of the Audio Engineering Society 2008, S.772

    Klaus
    Speakers: Klein + Hummel O500C, Electronics: Funk MTX preamp, Rane PS1 phonostage, Analog: Michell Gyrodec, SME 309, Shure V15VxMR, Digital: Tascam CD-RW4U, Tascam MD-801R

  2. #12
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    Quote Originally Posted by KlausR. View Post
    Michael,



    Fazenda mentions 3 more sources that might be interesting in the respect:

    [24] A. Celestinos and S. Nielsen, “Controlled Acoustic
    Bass System (CABS)—A Method to Achieve Uniform
    Sound Field Distribution at Low Frequencies in Rectangular
    Rooms,” J. Audio Eng. Soc., vol. 56, pp. 915–931 (2008
    Nov.).

    [25] A. Makivirta and A. Varla, Sound Reproduction
    Equipment and Method for Reducing the Level
    of Acoustical Reflections in a Room - United States
    Patent 6795557, application: 2000, published: 2004. URLhttp://
    patent.ipexl.com/US/6795557.html.

    [26] M.Wolff A. Goertz and L. Naumann, “Optimierung
    der Wiedergabe von Surround Lautsprecheranordnungen
    in Tonstudios und Abhorraumen”, Tonmeistertagung,
    22, English translation available at http://forums.
    klipsch.com/forums/storage/8/1485263/tmt2002 eng.pdf,
    2001.

    plus

    Celestinos et al., “Optimizing placement and equalization of multiple low frequency loudspeakers in rooms”, Audio Engineering Society Preprint 6545 (2005)

    Celestinos et al., “Low frequency sound field enhancement system for rectangular rooms using multiple low frequency loudspeakers, Audio Engineering Society Preprint 6688 (2006)

    Celestinos et al., “Low-frequency loudspeaker-room simulation using finite differences in the time domain – Part 1: Analysis”, J. of the Audio Engineering Society 2008, S.772

    Klaus
    Thanks Klaus! I've got some work to do.

    I can see the benefit to using four subs. In particular, it's easy to see how out-of-polarity opposing wall setups can destroy a nasty room mode which isn't susceptible to room treatments.

    My wheels have been turning now. I am trying to imagine how I would set things up in Acourate. I have a long email into to Uli now. I like to time/phase align my subs with my R/L speakers. However, if I run four subs (two out of polarity from the R/L speakers), it would be a mess to time/phase align all 6 six channels the the same crossover point with the R/L speakes. I am considering whether it would make more sense to crossover the out-of-polarity subs at a much lower frequency than the in-polarity subs. This makes sense to me because I could then easily time/phase align the in-polarity subs with the R/L at the crossover and not worry about time/phase aligning the R/L speakers with the out-of-polarity subs. This way, the out-of-polarity subs are mostly used to destroy the nasty mode and aren't really heard throughout the crossover region.


    I'm just thinking . . .
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  3. #13
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    Quote Originally Posted by dallasjustice View Post
    Thanks. I know Welti's most recommended four sub positions include all four subs at all four midwalls in a rectangular room. I can't really do that in room. It seems like the study you mentioned also recommends midwall position for two subs or front and back walls.
    Two subs in middle walls is actually superior to four in corners from flatness point of view (at least that is what I recall from the study). The reason it is NOT recommended is that you lose a lot sub power when it is not reinforced in the corners.

    Also all of Welti's recommendations rely on DSP as the idea is to cancel nulls, and let the peaks be managed by the DSP.

  4. #14
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    Quote Originally Posted by amirm View Post
    Two subs in middle walls is actually superior to four in corners from flatness point of view (at least that is what I recall from the study). The reason it is NOT recommended is that you lose a lot sub power when it is not reinforced in the corners.

    Also all of Welti's recommendations rely on DSP as the idea is to cancel nulls, and let the peaks be managed by the DSP.
    Interesting Amir. Thanks.

    Welti has a few different recommended setups. Obviously, midwall is best. In my setup the subs are one quarter width each on frontwalls and backwalls. They are not in the corners. I believe he refers to this setup as "very good." I would never try to integrate a sub without DSP. However, my objective is to use opposing wall sub placement to destroy a very low frequency mode, so that much less DSP is needed. It's my hope this will be a superior way to set the subs up. I don't really know yet since it may be a little more complicated once I try to time/phase align everything else as I mentioned in my previous post. We will see.
    MUSIC IS GOOD

  5. #15
    Quote Originally Posted by amirm View Post
    Two subs in middle walls is actually superior to four in corners from flatness point of view (at least that is what I recall from the study). The reason it is NOT recommended is that you lose a lot sub power when it is not reinforced in the corners.

    Also all of Welti's recommendations rely on DSP as the idea is to cancel nulls, and let the peaks be managed by the DSP.
    When you say the middle wall, are you referring to the side walls or the middle of the front walls in the same plain of the main speakers?

  6. #16
    Amir,

    Quote Originally Posted by amirm View Post
    Two subs in middle walls is actually superior to four in corners from flatness point of view (at least that is what I recall from the study). The reason it is NOT recommended is that you lose a lot sub power when it is not reinforced in the corners.
    Keep in mind that Welti's goal was to optimize for a seating area, not a single listening spot:

    "The first goal for low frequency optimization should be to minimize variation of frequency response from one location to the
    next within the seating area."

    "A secondary goal should be to maximize low frequency output of the subwoofers."

    Klaus
    Speakers: Klein + Hummel O500C, Electronics: Funk MTX preamp, Rane PS1 phonostage, Analog: Michell Gyrodec, SME 309, Shure V15VxMR, Digital: Tascam CD-RW4U, Tascam MD-801R

  7. #17
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    Quote Originally Posted by prerich View Post
    When you say the middle wall, are you referring to the side walls or the middle of the front walls in the same plain of the main speakers?
    They are the same in theory. In practice I found in my room the two side walls worked better because I sat closer to the subs.

    Just a quick theory when you put the sub in one corner, you have a null in the middle of the room. If you move the sub to the middle then by definition there is no null there. This takes care of that axis of your room. To get rid of the null in the other axis, you add a second sub and it will then fill in the center between the two subs. This all assumes perfectly rectangular room.

  8. #18
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    Quote Originally Posted by KlausR. View Post
    Amir,



    Keep in mind that Welti's goal was to optimize for a seating area, not a single listening spot:

    "The first goal for low frequency optimization should be to minimize variation of frequency response from one location to the
    next within the seating area."

    "A secondary goal should be to maximize low frequency output of the subwoofers."

    Klaus
    That is true but the principal of mode cancellation works just as well for one seat or we would lose the sweet spot in such optimizations.

    BTW, it is good to have you back Klaus. If there is someone who can give me a run for my money on these topics, it is you!

  9. #19
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    Quote Originally Posted by amirm View Post
    That is true but the principal of mode cancellation works just as well for one seat or we would lose the sweet spot in such optimizations.

    BTW, it is good to have you back Klaus. If there is someone who can give me a run for my money on these topics, it is you!
    He's giving my tiny brain a pretty good workout too!
    MUSIC IS GOOD

  10. #20
    Industry Expert [Industry Expert] Nyal Mellor's Avatar
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    Quote Originally Posted by dallasjustice View Post
    To eliminate an odd order axial mode (eg. 1st axial mode), only a pair of subs used in opposite polarity together are needed.
    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)
    Name:  20Hz.jpg
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    four subs, two on front wall, two on rears (rears delayed 2ms and run -3dB relative to fronts)
    Name:  FINAL_20.jpg
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    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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