The Flex-12, expandable, adjustable subwoofer/mid bass design using the MBM-12 kit

Discussion in 'DIY Speakers and Subwoofers' started by Matt Grant, Oct 20, 2020.

  1. #1 Matt Grant, Oct 20, 2020
    Last edited: Dec 10, 2020
    Trying to copy things over from the now long gone AVS thread. I may need to re-upload images if they break. Let me know if everything displays correctly.

    Using the MBM-12 kit as a subwoofer / mid bass with adjustable tuning (17hz, 24hz, or 34hz) in an easy to build 3cuft enclosure (14.5"w x 28"h x 18.75"d). Designed for use with the iNuke 1000DSP or 3000DSP with downloadable iNuke DSP settings for each tuning mode for optimal output and sound quality.


    With a footprint of only 14.5" x 18.75" it is easily placed in most locations where a larger sub would not pass, but don't let it's size fool you even a single cabinet is capable of some very respectable output. With multiples able to be run off a single amp, up to 3 off a 1000dsp or 12!!! off a 3000dsp you can keep adding them until your output desires are filled or your house starts to fall apart.

    This design uses a custom 12" woofer (named Magnum-12 by Erich) designed to excel as a mid bass driver but I found it to be quite excellent as a subwoofer driver in ported enclosures. It very good sensitivity in the upper bass and the strong motor allows it to maintain decent sensitivity down towards tuning in a moderate sized low tuned ported enclosure as well. The sensitivity of the driver >50hz combined with the fairly strong motor strength give it excellent efficiency in the midbass, 10x as efficient as the Ultimax-12 in that range in converting electrical power to sound. This minimizes power compression giving it great dynamics and detail at high volumes very suitable for use with high sensitivity speaker designs.

    This design uses two of the 3" precision ports for which the center tube needs to be cut to 10" in order to give the correct tuning. This makes them 16" long and when placed in the enclosure the proximity of the rear port flare to the back of the enclosure allows increases the effective port length to around 19-20" which gives it the 24hz tuning. Plugging one port lowers tuning to 17hz. Taking out the center port extension tubes (leaving just the flares) gives the 34hz tuning. You can tape the ports together using electrical tape which allows you to take them apart if you wanted to experiment with the lower tunings vs. the 34hz tune.

    I have designed DSP setting for the iNuke amps that maximize performance for each tuning mode. In the 24hz mode response is +-3dB to 21hz but in room you can expect it to reach to the upper teens. In the 17hz mode response is +-3dB also to 21hz though below that is much flatter and with good room gain usable response can extend down to mid-low teens.

    Link to Flex-12 iNuke DSP Files
    - Note change the file extension to .zip after downloading, I had to upload as it as PDF because zip files are not allowed.

    Frequency response of the 17Hz, 24Hz and 34hz modes (outdoors 2m ground plane):


    Rough idea of max output for the three different tunings of a single enclosure powered by an iNuke 3000DSP:

    Measured impedance of the 17-24-34hz tunings:

    Enclosure design, this was designed to be simple to build using the 14.5" x 28" MBM-12 baffle.
    There are only three main panel sizes to be cut:
    2x 14.5" x 28", These are the same size as the included baffle and make up the rear panel and second layer on the double baffle (the included baffle being the front).
    2x 16.5" x 28", These are the two sides.
    2x 16.5" x 13", The top and bottom


    The bracing I used is just 2" wide strips of left over MDF. Three running side to side, two with strips running front to back and the one behind the driver going from the front to the center. There is also a brace going from top to the bottom made up of several smaller pieces that fit between the others. Looking in at the bracing and damping:



    Attached Files:

    cranks, EarlK, Brinkman and 1 other person like this.
  2. Matt, I was planning on building 4 VBSS subs until I stumbled across the Flex-12. Can you explain the critical differences between the 2 designs? I like the idea of smaller boxes, but the VBSS goes lower. Is the driver still available?

    I'm in a townhome with a shared wall, and even though my HT room has great soundproofing (double wall, double 5/8" drywall, Green Glue, hat-channel/clip isolated ceilings), I know it's impossible to trap really low frequencies. So my thinking is to use a BOSS setup for a "simulated" low end/TR response and rely on the MBM approach to reduce ULF transmission to my neighbor. This is of course all speculative because I have no real-world experience in this area - just a theory about how it "might" work.

  3. Matt, I am playing with the idea of laying a pair of Flex-12s down on their sides to act as stands for a pair of (2020 model) HTM-12. The iNukes have been discontinued, and prices on the replacement NX series are higher, which has also driven the prices of the old inukes upwards. I am already using a minidsp nanodigi 2x8 and I have a 2 channel power amp that puts out 385w into 8 ohm into each channel.

    Minidsp does not have dynamic EQ but I imagine that's mostly "nice to have" and not critical to Flex-12 operation. The compression setting for the 17hz tuning seems minimal, and I don't intend to push high SPLs. It also does not have a way to apply the LS12 low shelf I see on the 17hz tuning, but I think that can be replicated easily with a Q=2 PEQ filter at 20hz.

    Would you have any concerns about such a minidsp implementation given these two caveats - 1) 17hz tuning, 2) moderate SPL no more than 105db?

    Lastly, what would you recommend for a crossover point for the HTM-12, given its close proximity to the Flex-12? Is there mid-bass advantage in crossing over much higher, despite the 2020 HTM-12's improved woofer?
  4. @Matt Grant , I see MBM-12 baffle is no longer available on the DIYSG site but the Magnum-12 woofer is in stock. Is this still a recommended sub to build over the Marty line of subs like the MartyCube or the Impact M4-15i? I'm looking for a smaller-size sub to pair with my recently build HT-8 LCR and I'm hoping to buy a woofer plus baffle or flat pack currently in stock (narrows my choices). Thanks.
  5. @Matt Grant Is there any way to build these today? I see the Mag 12 is in stock but not sure about the ports, baffle, etc.

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