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Thread: Destructively Driving Modes

  1. #1
    Member Sponsor Addicted to Best! dallasjustice's Avatar
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    Lightbulb Destructively Driving Modes

    Is it really possible to simply "destroy" a room mode? I mean like I waived my magic wand and that nasty 13db peak at 24hz just disappeared into thin air? Yes, it's possible.

    I wanted to show a quick measurement just to demonstrate how a very nasty room mode can be eliminated. Why would I call a mode "nasty?" IME, any room modes below about 35hz really cannot be reduced with any kind of room treatments. Okay, maybe if you built an enormous Helmholz resonator into your wall, it could be reduced. But anything else? Nope. There are NO off-the-shelf room treatments which will touch it.

    One of two ways to deal with such a nasty mode is to use EQ. Of course, that can make a big difference. But I want the "best" solution on whatsbestforum. RIGHT?

    I had some free time to measure my jl audio subs lately and I wanted to share something I think is pretty cool. I wanted to see how much I could reduce a 1st order axial room mode (one half wavelength) without using ANY DSP or room treatments. Normally, my system uses DSP, digital crossovers, four strategically placed subs for a total of 6 channels in stereo listening. The following measurement is NOT my normal system measurement. I say this upfront, because I know somebody will post something like; "man your system sux. I mean your bass is all over the place." The bass measurements in my room are very flat at the seated position once everything is measured together with DSP, delay, digital crossovers and four mono subs. The following ONLY show two subs.

    The concept of room mode cancellation is not new. All mathematical modeling has been done many years ago by Welti and written about extensively by Floyd Toole in his Sound Reproduction books. The following is a REW plot which shows three measurements in my room. I've labelled all measurements so the graph is pretty much self-explanatory. These are unfiltered measurements all at the same seated position with very close to the same level (I had to reduce the level to compensate for the two subs together measurement). There are only two subs used for this purpose (my system has a total of four subs). My room is 24' long. My room has a 1st order (half wave length) axial room mode at 24-25hz. All four of my subs exhibit the same 13db peak at 24-25hz when individually measured. My subs are spaced one quarter width on the front and backwalls. This is a "very good" configuration according to Welti's models.

    To eliminate an odd order axial mode (eg. 1st axial mode), only a pair of subs used in opposite polarity together are needed. In this case, I used the front right and rear right subwoofers run in opposite polarity to demonstrate how this works.These measurements aren't using any EQ or time correction, so they would look MUCH better after correction. The 24-25hz 13db peak is totally eliminated when both subs are measured together in opposite polarity. That is, the axial mode is reduced by about 13db due to this combined subwoofer mode cancellation setup.

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    I didn't post the decay plots, but things are very much cleaned up at 24hz. The time domain looks MUCH better there. Also, the even order modes weren't really increased using this method. If I had run both subs in the same polarity, the odd order mode would have been increased. That is, the 1st axial mode would be even bigger. I didn't post that plot but I did measure it; trust me I'm a lawyer.
    Last edited by dallasjustice; 01-21-2016 at 06:38 PM.
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    Quote Originally Posted by dallasjustice View Post
    ... trust me I'm a lawyer.
    That is precisely why we shouldn't trust you.

    Your example is a perfect illustration of applied acoustics. It seems counter-intuitive to put a sub out of phase - everyone knows that subs have to be in phase, right?

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    four subs vs. one sub; no DSP

    Just for fun. The following REW plot shows four subs driven at the same time from the "very good" locations mentioned in the OP compared to a single subwoofer. Remember, I am not using any DSP or other EQ tricks. These are just four subs played together. I adjusted polarity and phase a little on three of the subs to get this response. Of course, I would get much better response with DSP. I am just showing what's possible without any DSP using four subs.

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    Last edited by dallasjustice; 01-21-2016 at 06:16 PM.
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    Member Sponsor [WBF Founding Member] FrantzM's Avatar
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    Multi subs is a practice that more audiophiles will benefit from. It is in most cases physically impossible to obtain ideal bass response in most residential rooms, even the most palatial. In my experience the subs need not be the same.. 4 <$500 subwoofer have the potential of providing more even bass response and likely higher SPL than a single $5K sub in most rooms.. Judicious use of DSP and EQ propels those cheapo subs to surprisingly good SQ and that with even (and compared to) full range speakers with deep bass reach and capabilities.. The immediate u pshoot is better clarity of sound overall.. throughout the entire spectrum, even the highs will be clearer!

    I must add that I wouldn't have thought about reversing phase on one of the subs...
    Frantz
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    Quote Originally Posted by FrantzM View Post

    I must add that I wouldn't have thought about reversing phase on one of the subs...
    I used inverted polarity on the rear subs to destroy the peak. The phase was used just to flatten out the rest of the band.
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    Taking it one step further, and assuming a rectangular room, if you were to place one sub in a corner and an out-of-phase sub in the diagonally opposite corner (ceiling mount), you could in theory kill multiple resonances.

  7. #7
    Active room mode treatment methods have been compared and ranked in

    Fazenda et al., "Subjective Preference of Modal Control Methods in Listening Rooms", J. of the Audio Engineering Society 2012, S.338

    Tests were done double-blind with music program material. The best contender was the 2-sub source-to-sink, subs on floor in centre position on front and rear walls, rear sub in opposite phase. No. 2 was CABS, i.e. 4-sub source-to-sink, all subs at 1/2 of room height, rear subs in opposite phase. No. 3 was single floor sub in centre position on front wall with amplitude EQ.

    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

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    Member Sponsor Addicted to Best! dallasjustice's Avatar
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    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.


    Quote Originally Posted by KlausR. View Post
    Active room mode treatment methods have been compared and ranked in

    Fazenda et al., "Subjective Preference of Modal Control Methods in Listening Rooms", J. of the Audio Engineering Society 2012, S.338

    Tests were done double-blind with music program material. The best contender was the 2-sub source-to-sink, subs on floor in centre position on front and rear walls, rear sub in opposite phase. No. 2 was CABS, i.e. 4-sub source-to-sink, all subs at 1/2 of room height, rear subs in opposite phase. No. 3 was single floor sub in centre position on front wall with amplitude EQ.

    Klaus
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  9. #9
    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.
    Welti, "How Many Subwoofers are Enough?", AES paper 5602

    Welti's study was concerned with optimal bass response (seat-to-seat variation) across a seating area with 16 chairs. The configuration you mention was best in the virtual room and calculated metrics. This configuration, however, was not included in the actual test room, with calculated and measured metrics. No listening was performed.

    Here are some of his conclusions:

    How many subwoofers are enough? Four subwoofers are enough
    to get the best results of any configuration tried. Two subwoofers
    at the wall midpoints is very nearly as good and has very good low
    frequency support as well.

    What is the optimal placement? One subwoofer at each wall
    midpoint is the best in terms of Std, Max-ave and Max-min but
    does not support low frequencies particularly well. Two
    subwoofers, at opposing wall midpoints, performs very nearly as
    well as four at the midpoints and gives a much better LF factor

    In Fazenda the following methods were compared:

    1.1 Subwoofer in Corner – No Equalization
    1.2 Subwoofer in Corner – With Magnitude Equalization
    1.3 Subwoofer in Front Wall Centre – No Equalization
    1.4 Subwoofer in Front Wall Centre – With Magnitude Equalization
    1.5 Controlled Acoustic Bass System
    1.6 ‘Welti’ Configuration (2 subs wall midpoint front and rear, in phase)
    1.7 Single Source-to-Sink
    1.8 Opposite Phase-Opposite Corner

    If interested in copies of those papers, drop me a mail.

    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

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    Member Sponsor Addicted to Best! dallasjustice's Avatar
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    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.

    I'm particularly interested in this topic now because I've been agitating Bernt from Audiolense to improve his measurement procedure to account for setups which do destructively drive modes. He will release a new version which can and will do that. He said he's coding now so it may be a while before it's ready. If it works well, it should be a nice improvement to already excellent DSP software.

    Michael.


    Quote Originally Posted by KlausR. View Post
    Welti, "How Many Subwoofers are Enough?", AES paper 5602

    Welti's study was concerned with optimal bass response (seat-to-seat variation) across a seating area with 16 chairs. The configuration you mention was best in the virtual room and calculated metrics. This configuration, however, was not included in the actual test room, with calculated and measured metrics. No listening was performed.

    Here are some of his conclusions:

    How many subwoofers are enough? Four subwoofers are enough
    to get the best results of any configuration tried. Two subwoofers
    at the wall midpoints is very nearly as good and has very good low
    frequency support as well.

    What is the optimal placement? One subwoofer at each wall
    midpoint is the best in terms of Std, Max-ave and Max-min but
    does not support low frequencies particularly well. Two
    subwoofers, at opposing wall midpoints, performs very nearly as
    well as four at the midpoints and gives a much better LF factor

    In Fazenda the following methods were compared:

    1.1 Subwoofer in Corner – No Equalization
    1.2 Subwoofer in Corner – With Magnitude Equalization
    1.3 Subwoofer in Front Wall Centre – No Equalization
    1.4 Subwoofer in Front Wall Centre – With Magnitude Equalization
    1.5 Controlled Acoustic Bass System
    1.6 ‘Welti’ Configuration (2 subs wall midpoint front and rear, in phase)
    1.7 Single Source-to-Sink
    1.8 Opposite Phase-Opposite Corner

    If interested in copies of those papers, drop me a mail.

    Klaus
    MUSIC IS GOOD

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