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Thread: Comparison of Double Bass Array to Sound Field Management: THE MEATY PART!!!

  1. #1
    WBF Technical Expert (Subwoofers In Rooms) [Technical Expert]
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    Comparison of Double Bass Array to Sound Field Management: THE MEATY PART!!!

    NOTE: this is a part 2 post. Read
    http://www.whatsbestforum.com/showth...ement-Overview
    first.


    OK, now the meaty part: I want to compare the Double Bass Array to Sound Field Management on a point by point basis. Efficiency seems like a good place to start.

    Efficiency
    It might seem that the DBA approach would take an efficiency hit, since we are creating just a front to back plane wave (like in a free field), thus removing the room gain.

    The intended effect of the DBA is to remove the room and create a free field plane wave response. We are removing the room modes. It is clear that the room modes increase the average sound pressure level in the room, so removing them must reduce the average SPL in the room. Of course it may not be easy to see this, since at some locations/frequencies, when we remove the modes we are removing cancellation dips.

    Of course, subwoofer optimization can be said to rely on cancellation of room modes in one way or the other. For Sound Field Management, however, we are only trying to optimize the sound at a select number of seats, not the entire room. Remember that DBA effectively optimizes the entire room, there is no measurement at any particular seat. For SFM, it is entirely possible that efficiency can be increased at the selected seats even while the average SPL in the room is decreased. We are simply “moving the sound around”. With the “purest” version of SFM we ignore bass efficiency, and only try to optimize seat to seat consistency. However, if we change the selection criteria for SFM to choose among the best solutions, we can sacrifice a small amount of seat to seat consistency to get more bass efficiency and usually end up at least not losing any bass, often gaining. It’s a matter of changing one line of code.

    Interestingly, at frequencies just below the first mode, where the room acts as a lumped element system, I will contend that in the case of DBA the delayed and inverted rear speakers will be in phase with the front. So, at these very low frequencies, DBA won’t lose any bass. I will leave this as an exercise for the reader!

    Optimizing particular seats versus the entire room.
    DBA in effect attempts to optimize the entire room, due to the plane wave produced. This would be a particular advantage if/when you don’t know where the listeners will be. So, if there are no fixed seats or designated seating area DBA would have an advantage.

    Convenience
    In the author’s opinion, DBA has a major disadvantage in this category. Implementation requires 2 or 4 subwoofers at very particular positions, duplicated on both front and rear walls, not just sitting on the floor either but in or on the walls. Very small rooms could use 2 subs each on front and rear wall, but otherwise 4 subs on front and rear walls, for a total of 8 subs, is required.

    http://www.avsforum.com/avs-vb/showthread.php?t=837744

    DBA requires 4 or 8 identical subwoofers. The SFM algorithm has no particular requirements that the subs be identical, though they should be within a few dB of each other in max output capability.

    SFM normally uses subwoofers on the floor, measured at multiple locations, but locations which are preselected to meet WAF considerations. Normally 2 or 4 subwoofers are all that is required.

    As for configuring the system, the SFM method is at a disadvantage, requiring multiple impulse response measurements (no 1/3 octave RTA’s here). Each possible combination of potential subwoofer location and seat must be measured. For DBA the delay and gain setting can be calculated (though measured values might give a slightly better result).

    As for signal processing required, SFM does require a bit more. Level, delay and one biquad filter is required for each subwoofer channel. In DBA one global delay and gain would suffice.

    Applicability to non rectangular rooms
    SFM will work in any room, since it is based on actual room measurements and does not care about the shape of the room. DBA is based on the acoustics of a rectangular room and wont work in a non rectangular room. It also may not work as advertised in rooms which are acoustically non-rectangular. This could be the case for rooms with very different wall construction on different walls, for example.

    Performance
    Though no systematic comparison has been made between DBA and SFM, I will divulge my unbiased opinion on the matter and proclaim that SFM will give better performance. The reason I can say this is that if I allow longer delay values and polarity inversion in the SFM algorithm, the DBA solution will pop out if it indeed is the best solution. However, if there is a better solution, that alternate solution will pop out, and be an improvement over straight-up DBA. The author has done this and even in “rectangular” rooms (which are often not acoustically rectangular), DBA did not always give the best solution. DBA can thus be considered as a special case of the more general SFM approach. Still, to be fair, DBA does give a solution which is likely to be a very good one.

    SUMMARY
    I would summarize by saying that in rectangular rooms, if you don’t know where the seats will be, and you can get the subs located in/on the front and back walls, use DBA. Also if making impulse response measurements is too difficult, use DBA. In all other cases SFM is a better approach. For non-rectangular rooms, DBA is not an option.


    Afterthoughts
    Unfortunately, neither method discussed here is particularly easy to implement at present. For DBA, it’s because of the requirement for 4 or more likely 8 subs to placed in exact locations in/on the front and rear walls. For SFM it’s because the search algorithm is not widely commercially available yet. In the case of SFM, it is incorporated into the Harman Audio Test System, and available as part of a JBL Synthesis custom installation. An optimization algorithm which does what SFM does (albeit using a different method) is commercially available under the JBL Performance line. This is the BassQ 4-channel Automatic Room Mode Corrections Processor. Some other commercial algorithms have been around for a while (R.A.B.O.S for example), and more are coming on line all the time (Audyssey, Trinnov, Lyngdorf, etc., etc., etc., etc…..). Most of these do not address seat to seat consistency and/or do not use separate processing for different subs (or subwoofer groups).

    OK, if you’re still with me, and don’t just accept everything I just said in this missive, good for you! Fire away….
    Last edited by twelti; 06-10-2011 at 08:58 AM.

  2. #2
    Are you at liberty to discuss the difference in method between the "BassQ" and the JBL Synthesis approach?

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    Member Sponsor [WBF Founding Member] FrantzM's Avatar
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    Ok let me get this out of the way .. I wasn't ever aware the DBA existed. How do I know more about it?

    I have in the past obtained extremely good results by using what I would call the Geddes method .. Three subs in pseudo-random positions. I am interested in what it takes to implement the DBA. I tend to suppose it can be accomplished rather inexpensively ... If one goes in the direction of IB, pro amps and something like the Behringer DCX2496. A few questions:

    What about the main speakers, should they be crossed-over or could they run in augmentation?
    What should be the crossover points?
    What should be the slopes in an DBA?
    Frantz
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    Quote Originally Posted by randybes View Post
    Are you at liberty to discuss the difference in method between the "BassQ" and the JBL Synthesis approach?
    Hi, well, I was involved in both product's development and did some comparisons, and both approaches seem to work well. The BassQ is a do it yourself, push one button and let 'er rip box. It uses FIR filters and a matrix inversion approach with some enhancements. It will attempt to make the responses at each seat similar and flat (then you can add you own global eq to taste). Synthesis allows for some human intervention, which can be a good thing. It tries to get the seats resposnes similar to each other but allows the installer to have more contol over the global eq. With the SFM algorithm itself, you can do horsetrading. That is you can say I'm willing to trade off a bit of seat to seat consistancy to get some more bass output. I've always liked this aspect.

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    Quote Originally Posted by FrantzM View Post
    Ok let me get this out of the way .. I wasn't ever aware the DBA existed. How do I know more about it?

    I have in the past obtained extremely good results by using what I would call the Geddes method .. Three subs in pseudo-random positions. I am interested in what it takes to implement the DBA. I tend to suppose it can be accomplished rather inexpensively ... If one goes in the direction of IB, pro amps and something like the Behringer DCX2496. A few questions:

    What about the main speakers, should they be crossed-over or could they run in augmentation?
    What should be the crossover points?
    What should be the slopes in an DBA?
    I'm not really sure I could answer your specific qustions, but if you want to find out more about DBA, read

    http://forums.klipsch.com/forums/sto...mt2002_eng.pdf

    The paper does not give much detail however. There is also some discussion and pictures of this technique at

    http://www.avsforum.com/avs-vb/showthread.php?t=837744

    If you want to know more, you can look up papers by Celestinos and Nielsen on the AES website. They call it CABS.

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    Quote Originally Posted by FrantzM View Post
    Ok let me get this out of the way .. I wasn't ever aware the DBA existed. How do I know more about it?

    I have in the past obtained extremely good results by using what I would call the Geddes method ..
    Maybe I will try to do a comparison of SFM to Geddes. But his method is not well documented...

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    Quote Originally Posted by twelti View Post
    Maybe I will try to do a comparison of SFM to Geddes. But his method is not well documented...
    You are right. Earl also has been evolving it form early days, going from no EQ to EQ. Still, any comparison would be appreciated.

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    Quote Originally Posted by amirm View Post
    You are right. Earl also has been evolving it form early days, going from no EQ to EQ. Still, any comparison would be appreciated.
    Hi Amir, if there's a particular thread where he discusses his method, let me know. Still, it is not really documented formally in any way that I know. It is based primarily on his experience. Don't get me wrong, Earl is a smart guy, but it is hard to evaluate somehting that is not rigorously documented.

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    We probably need a one paragraph or so definition of "DBA" and of "SFM" at the start of this thread.
    Links to papers and web-sites would also be good.

  10. #10
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    Quote Originally Posted by Speedskater View Post
    We probably need a one paragraph or so definition of "DBA" and of "SFM" at the start of this thread.
    Links to papers and web-sites would also be good.
    Sorry if it was unclear, but this is a part 2. The "overview" thread has some links and background info.

    http://www.whatsbestforum.com/showth...ement-Overview

    I added a note at the start of this thread to make it clearer. Thanks.

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