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BBV2: a small sub/sat system

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  • BBV2: a small sub/sat system

    FINALLY got my BoseBuster sequel done and written up!! I've shortened the name to BBV2 so I don't get any hassle from Bose. My Soma Sonus site is now hosting my projects, and will have the most up-to-date version. Here is copy of that page:



    BBV2

    A small format sub/satellite system


    The goal for this project was a small, high quality speaker that was shallow enough to be mounted on wall next to the thin LCD TV’s that are now common. The Fountek FR88EX was perfect for this project, with its low distortion and ability to fit in a truly tiny box (you can get the cones really dancing with little change in character). After several tweeters, I finally settled on the SB Acoustics SB26STCN-C000-4. This tweeter not only measures well, but is subjectively a very clear and open sounding tweeter. Crossover point is 2.8khz @ LR4. Here is the on-axis frequency response:



    With reverse polarity to check phase integration:


    I did considerable testing of both the crossover point and notching out the breakup around 12khz of the mids. I found that I didn’t really need to do anything too special for the breakup of the mids, a nominal LR4 network was good enough. The tweeter sounded most well behaved at 2.8khz versus pushing much lower.
    The frequency response measurements consist of a far field measurement with a 6.5ms gate (no smoothing!!), merged to a near field measurement at 300hz, resulting in a quasi-anechoic response. The low frequency ripple is the noise floor of my shop.

    Impedance:



    Here is the crossover schematic:


    Inductors are 18g on the woofer and 20g on the tweeter. The small resistance value shown with each inductor is the resistance of the inductor itself, not a separate resistor.

    Baffle layout:


    The following is how I initially integrated the sub and satellites. This is intended to be a quick way to get the system up and running. Obviously there is always a lot more one can do to get the smoothest bass and mid-bass response, and this is only one simple method.

    First, I use two subs, one roughly at the foot of each main channel. Multiple subs in different locations is one of the best ways to get smooth bass response. In this case I tried REW as the measurement software since it’s free and easy to use. I use a mic from Cross Spectrum Labs. I didn’t bother with all the calibration of the measurement setup since we just want to see real-time in-room response for integration. Given that, it is pretty straightforward to setup: just open the RTA window, change the scale to go up to 2khz, 1/6 octave smoothing, select FFT Length 32768, Blackman-Harris 7 window, 8 averages, and that’s it. You might have to open the preferences and set the soundcard output if you get no sound, or the input if the mic doesn’t read anything. I aimed for a 175hz crossover, I’d like to experiment with 200-250hz in the future.

    1) First set the levels of each channel (without subs on), I use a Radio Shack meter on A weighted Fast

    2) in the AVR set the sub to “LFE+main” (this is on a Marantz, other brands should be similar)

    3) set the sub crossover to 250hz

    4) set the mains to small and crossover of 80hz

    5) starting with just the left channel outputting pink noise (sub off), adjust the crossover until your -6dB point is at roughly 175hz:


    My bass doesn’t appear to drop off quick, but that is due to the noise floor of my apartment being fairly high. Raising the signal will correct this, but I didn’t want to get my neighbors bent out shape!

    6) Now running only the left sub, adjust its level to match the main. I like a little boost in my home theater bass though

    7) Adjust the sub crossover until its -6dB is also 175hz:


    8) Now turn on both the main and sub, and check integration:


    9) Looks good so I moved on to the right channel

    I was lucky and integration was very good right away. After integration I turned on both subs and the dip around 50hz filled in nicely and all I was left with was the hump at 40hz. A simple notch EQ here took care of that. Anyone who measures their setup knows how badly the front wall (often in combination with the floor and side wall) can really mess up the response in the 100-500hz area. One of the nice things about a sub/sat system crossed around 200hz is that you now have drivers with different crossover slopes/points, and in two different positions in the room, to smooth out the response. At this point I switched 1/24 octave smoothing to make sure I wasn’t smoothing over the ugly parts.

    If one did not have good integration yet, you can adjust the phase on the sub, or adjust the delay setting in the AVR, or move the sub around, to get the proper phase match. Sometimes I find the response improvement to be ambiguous as I change settings. When that happens I like to run the main in reverse polarity, switch to 1/24 octave smoothing, and adjust delay/phase and sub placement to get the deepest null at the crossover. When you find it, you then switch the polarity back to normal and you should have excellent phase integration.
    Last edited by augerpro; 08-03-2014, 06:45 PM.
    ~Brandon

    Soma Sonus
    DriverVault

  • #2
    Re: BBV2: a small sub/sat system

    The Dayton SD215PR is only good for up to 250g of mass. Are you sure you did not exceed the suspension's ability to support the mass here?
    Later,
    Wolf
    "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
    "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
    "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
    "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

    *InDIYana event website*

    Photobucket pages:
    http://photobucket.com/Wolf-Speakers_and_more

    My blog/writeups/thoughts here at PE:
    http://techtalk.parts-express.com/blog.php?u=4102

    Comment


    • #3
      Re: BBV2: a small sub/sat system

      Originally posted by Wolf View Post
      The Dayton SD215PR is only good for up to 250g of mass. Are you sure you did not exceed the suspension's ability to support the mass here?
      Later,
      Wolf
      I think total mass was in the 230/240 range. I'd have to look at the Sim, it's been a couple years since I built the sub's. But yeah,this is close to the limit.
      ~Brandon

      Soma Sonus
      DriverVault

      Comment


      • #4
        Re: BBV2: a small sub/sat system

        I like the look of that little MTM. Cool.

        Comment


        • #5
          Re: BBV2: a small sub/sat system

          Wolf> checked out my sim, and I had Mmp of 201g, of which 75g for the passive radiator diaphragm itself. I did verify with a impedance measurement back then, and it was basically as predicted, which was an 18.3L boxed tuned to 33hz (not 30hz as I had above). The response isn't terribly great actually, but the whole thing was predicated on two things: I had amps with boost around 30-35hz (and wanted the Fb to align with this frequency), and I wanted to push how small the box could be. If a person wanted to push out box size it would definitely open up other driver choices and amps with no boost.
          ~Brandon

          Soma Sonus
          DriverVault

          Comment


          • #6
            Re: BBV2: a small sub/sat system

            Edited the first post after I reran the sub integration and included some new plots.
            ~Brandon

            Soma Sonus
            DriverVault

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