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Dual 21" Down-firing-Port Subwoofer Build

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  • Dual 21" Down-firing-Port Subwoofer Build

    This 21" down-firing-port subwoofer build(2x B&C 21SW152-4), was previously discussed HERE in regards the port design suggestion, but I decided for an 8"dia x 33" long at the end.

    This 21" subs will replace the current single Ultimax UM18-22 in a 5-FT^3 sealed enclosure, wanted some lows, strong midbass and low distortion on my 2CH music setup.

    Final project specs:
    Material: 6x 5/8" MDF-Light 4'x8' sheets.
    Measures: 42" H x 24" W x 27" D, (46" H with feet), nominal.
    Walls: ~1.25"(dual 5/8") thick walls, ~1.9"(triple 5/8") baffle with braces.
    Finish: Black-mate laminate.
    Volume: ~9.5 FT^3 Net. after port/driver/bracing.
    Weight: ~222 lbs (~180 lbs without driver)

    Initial subwoofer testing/setup:
    DSP: DBX DriveRack PA2
    High-pass xover: 16Hz BW4
    Low-pass xover: 60Hz LR4
    PEQ's: Off
    Delay: Off
    Subharmonic Synth: Off

    My "subjective" listening impressions:
    After getting them in place and connected, I've played some EDM and Pop music and I was a bit skeptical at first, then I remembered that I had to invert the subs phase, after inverting the phase resulted in an instant jaw-drop followed by an ear-to-ear smile, one box just blown away the sealed UM18-22 and I've disable the PEQ's previously boosting the 20~28Hz range on the sealed sub.

    Notes worth mentioning:
    The B&C 21SW152-4 drivers were installed without any breaking-time, so the subs will significantly dig lower overtime as expected.

    The room measurements are 10'-9"W x 9'-2"D x 8'-4"H, just small and almost square and so far away from optimal.

    The enclosures were tuned to ~19Hz regarding WinISD, however since the 8" port clearance from top wall is 7.75", port clearance from floor is 4" plus 2 large pillows, the final tuning dropped to around~17 or 18Hz, while I don't had at hand DATSv3 nor a diy impedance jig, I've tested with an adhesive tape and cranked up the amplifier and used a tone generator, so far when the tone generator was set to 17Hz, the woofer drop excursion dramatically and everything in the room was insanely shaking and I feel the high pressure, I didn't hear any port noise/compression when testing so far.

    Part Express part list:
    B&C 21SW152-4 Driver = Part # 294-689
    Speakon NL4MPR Connector = Part # 092-054
    Penn-Elcom 9106 Cabinet Foot = Part # 260-772
    1/4"-20 Deluxe 6-Prong T-Nuts = Part # 081-1090
    Belden 5T00UP 10 AWG Cable = Part # 102-1190


    Complete step by step build log HERE

    Few build images:

    Gluing started next to a 16oz glue bottle for size reference, I've used my favorite glue TB3.
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    The braces/port support in place.
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    Interior view of the port supporters embedded on the braces.
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    Spreading a generous amount of glue per brace and top/bottom walls.
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  • #2
    Few build images continuation:

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    • #3
      Few build images continuation:

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      • #4
        Few build images continuation:
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        • #5
          Few build images continuation:

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          • #6
            Few build images continuation:

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            Really sorry for the room mess, repairs and setup is still unfinished.
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            Complete step by step build log HERE

            Regards

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            • #7
              Looks brutal in that sized room . . . But I think I like it.


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              • #8
                Hi Steve, yeah they looks rather overkill for such a small room, but the bass is completely articulate and immersive with just a tad of power.

                I will post the previous quick measurements for reference.

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                • #9
                  Here are some Quick measurement I've made the past morning.

                  Since taking on-axis measurements is not so useful on subwoofers for a quick sub/room response representation, this time I've took them at the listening position and looks like I have great room gain in the low end despite the room being almost square and cluttered with modals, hopefully the measurement may look ok.

                  WinISD RAW prediction(21SW152-4, ~9.5FT^3, ~19Hz tune):
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                  Quick measurement on the LP without and with light PEQ(0~250Hz sweep):
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                  Quick measurement on the LP with light PEQ and smoothed(0~250Hz sweep):
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                  The subs are crossed at 60Hz LR4, with an high-pass of 16Hz BW4.

                  Regards

                  EDIT:
                  Here are the graphs with 120dB(10dB/div) scale for a much better reading:

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                  Last edited by JRivera; 12-31-2021, 07:21 AM.

                  Comment


                  • #10
                    Nice work. I like the legs (feet?).

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                    • #11
                      Originally posted by djg View Post
                      Nice work. I like the legs (feet?).
                      Hi, thanks for the input, the legs are made of 2-3/4" diameter kiln dried white ASH dowels they are sturdy and durable.

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                      • #12
                        I would be so sad to have such tiny woofers in my space.


                        J/K... total overkill!!! Very well executed and documented, thanks for sharing!
                        Electronics engineer, woofer enthusiast, and musician.
                        Wogg Music
                        Published projects: PPA100 Bass Guitar Amp, ISO El-Cheapo Sub, Indy 8 2.1 powered sub, MicroSat, SuperNova Minimus

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                        • #13
                          Originally posted by wogg View Post
                          I would be so sad to have such tiny woofers in my space.


                          J/K... total overkill!!! Very well executed and documented, thanks for sharing!

                          Hi, thanks for the kind words, I will leave a front view of the subwoofer next to the main speaker for better size reference.

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