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Aperiodic GRS 12" subs done

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  • Aperiodic GRS 12" subs done

    The path to finishing the aperiodically loaded GRS 12" subs took a few twists and turns.

    After getting the cabinets glued-up I decided I wanted to improve my (spray) painting capabilities. I use a large airbrush to finish many of my projects but I had a relatively noisy air compressor that tended to get on our nerves. Seeing as the wife and I have some projects coming up where we're going to want to use our spray booth more, I decided to look around at compressor options.

    There is an outfit named Silentaire that makes super high-end airbrush compressors that use the same type of hermetically sealed pump you'd find on a large refrigerator. They are about as noisy as a commercial refrigerator (in other words, super quiet). But they're $800 - $1000.

    Just as I was researching these compressors, a used Manoir Air-Force AF33-4 came up on eBay. Made in the USA, about 25 years old, it was in rough shape. But it was identical in style/construction to the Silentaire 50-TC. And about $110 shipped, so I pulled the trigger.

    When it arrived I found that the oil in it was pretty old, and likely the incorrect type (these types of pumps need 10W). Quite a bit of polyurethane glue had dripped on it, the pressure switch cover was broken in two places, the tank and regulator gauges were wonky, and the plug's ground was wobbly. So the wife and I embarked on a journey to clean it up. We replaced the oil, removed all the glue, installed a new cover for the pressure switch, installed new gauges and even polished the handle. We were nearly done (still wanting to clean up some paint drips on the sticker) and pretty impressed with how nearly brand-new it was looking, when we picked up a replacement plug and I went to install it. I found the insulation on the inner conductors on the SO cord were all badly cracking. So the 6' of cord to the pressure switch was shot, and the 1/2' of cord from the pressure switch to the pump was shot. Luckily, the wiring internal to the pump and to the pump's terminal block was a different type and in perfect shape.

    It took me nearly a week to get that all cleaned-up (I'm a bit of a nut when it comes to this sort of thing). I ended up upgrading the 18-gauge SO to 16-gauge SJOOW, and then we collected everything we needed to sort of plumb this thing over to the area where we paint, adding a separator and regulator in our painting work area.

    With THAT done, I was able to turn sights back to the subs. I did the painting last week and this week I assembled the aperiodic ports (testing/adjusting the stuffing amounts), and installed the cabinet stuffing and the drivers. Today she helped me bring them up from the basement and we swapped them with the old 10" units.

    We've only listened a little, we need to run them in a bit and then do some more testing and see if we need to adjust the aperiodic ports.

    I'll just say initial listening tests indicate I'm hitting solid in-room to 30-Hz, the bass is very even and not peaky at all, and I think these integrate into the room (sound-wise) better than my 10" GRS subs did. But that is VERY initial.

    Pics of one of the subs (there is a 2nd under the end table on the other side of the loveseat), and the completed air compressor. Love that compressor.

    One of the sub pics shows my aperiodic ports. We can remove the eight screws and adjust the amount of stuffing very easily. I say we especially here because as I added more stuffing, it became a two person job to put it all back together, to squeeze it all in there.

    Glad we did the aperiodic, I had always wanted to try one and this driver really sort of needs one if you want a sane enclosure volume. And now I have something else I can tweak.
    Last edited by philthien; 02-16-2019, 11:32 AM.

  • #2
    the paint on the cabinets looks excellent. It looks satin in the pics, really even tone too. I'm guessing you did that with the kind of airbrush that has a small container on it? I hadn't heard of anyone using an airbrush for this kind of stuff, but it makes sense since most speaker projects are a lot smaller than a car. I'm thinking clean up would be fairly easy too. The bare wood variovent setup looks good on there as well. Sounds like a lot of things worked out well on this deal!

    *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
    *Part 2 *Gluing multiple curved laminations of HDF *Cello's Speaker Project Page

    *Building the "Micro-B 2.1 Plate Amplifier -- Part 1 * Part 2 * Part 3 * Part 4 * * Part 5 'Review' * -- Assembly Instructions PDF


    • #3
      Thanks for the nice words, Tom.

      Yeah the airbrush I mostly use for this sort of thing is a Badger 250, it uses a 4-oz jar and can put down a lot of material quickly. It is also super easy to clean. Another benefit: It uses a jar size perfectly compatible with 4oz. "Mason" jars, so I can keep mixed materials in jars and just slap them on when I'm ready to paint.

      The bare wood variovent is hardboard. I'll likely make replacements and paint them. I honestly didn't know how these would sound and figured I'd button them up and test/listen before spending any more time on appearances.


      • #4
        That compressor looks sweet. Sorry but it is upstaging your sub build...
        John H

        Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower


        • #5
          Originally posted by jhollander View Post
          That compressor looks sweet. Sorry but it is upstaging your sub build...
          I completely understand, I'm not sure which I'm more pleased with.


          • #6
            Nice job on the compressor, it looks brand new. That paint on the sub cabinet looks great also!
            Last edited by Billet; 02-17-2019, 09:47 AM.


            • #7
              Originally posted by Billet View Post
              Nice job on the compressor, it looks brand new. That paint on the sub cabinet looks great also!
              The paint job on the compressor is amazing, like something from a show car. I was very happy that the polyurethane glue came off the paint w/o any damage.


              • #8
                Well I've had some time for some extended listening and I'm pretty thrilled my WinISD simulations weren't entirely accurate.

                The Parks Express page indicated a 1.5 ft^3 box would yield an F3 of 37-Hz. I thought that was pretty decent and suggested these as an Xmas gift my wife could wrap and put under the tree.

                After placing the order it became apparent that the recommended sealed box would result in a 4.5-db peak at approx. 60-Hz (very near where one of my room modes seems to live). My current (at the time) 10" GRS subwoofers seemed to accommodate smaller overstuffed sealed boxes better (less peaky), at least according to WinISD.

                Attempting to play with the "Advanced" parameters in WinISD by setting Ql=3 to model an aperiodic enclosure (according to a post by Jeff Bagby's), I was only able to reduce that peak by about 1.5-db, but it shifted a little higher/closer to my room mode.

                So I attempted to play with Qa and make an enclosure that was heavily stuffed AND aperiodic. I had no idea whether this made any practical sense, but was able to eliminate (mostly) the peak and so I proceeded with the cabinet builds.

                I've got 1.5-ft^3 enclosures (two "subs"), about 1.5-pounds of polyester filling in each, and each with an aperiodic port of my own making, filled with two layers of compressed polyester batting. There is no science involved in this, I'm freely admitting that. I thought I should measure one of the subs with the woofer tester but it was getting late and I wanted to give a listen so I did not measure anything.

                I was very careful removing the old 10" subs, thinking there was better than a 50% chance I'd want to replace them because the new subs would be too peaky/awful.

                My very initial impression was that the new woofers were less peaky than the old. I thought I had a bit of a problem with the 10" models as some tracks like "Hotel California" from "Hell Freezes Over" didn't sound quite right. That initial bass was less resolved than I've heard it before. There is this pitch change going on in that bass and the 10" models weren't doing it justice, while the 12" were doing very good.

                After listening quite a bit yesterday and this morning, I've come to the conclusion that I've gained at least 5-Hz of extension, and reducing peaking I had before. The bass is absolutely less colored.

                I guess one of my next steps will be to measure (with the "original" Woofer Tester I have) the 10" models and compare to a new 12" version. The system with the lowest Qtc should be the least peaky, right?

                In modeling the 12" model with Ql=3 and Qa=5, WinISD indicates a Qtc of 1.063 (gasp).

                In modeling the 10" model with Ql=100 and Qa=5, WinISD indicates a Qtc of .839 (yeah, I know that is high, too).

                So the 12" model should be even more peaky than the fairly peaky 10". Have I got that right?

                I'm a neophyte so please don't hesitate to tell me if I'm getting this wrong.

                So when I measure with the Woofer Tester, I measure just like I'm measuring a driver on my workbench?


                • #9
                  0.8 - 1.0 used to not be uncommon for med-large acoustic suspension boxes from the '70s. I never felt my Large ADVE\TS were "boomy". (but that was then)


                  • #10
                    Originally posted by Chris Roemer View Post
                    0.8 - 1.0 used to not be uncommon for med-large acoustic suspension boxes from the '70s. I never felt my Large ADVE\TS were "boomy". (but that was then)
                    Well I agree the Large ADVE\TS my dad had when I was growing up didn't seen boomy.

                    I see Jeff Bagby once said: "With WT2, WT3, or DATS, (if you have one of these) just run the T/S parameter check on the sealed system's impedance and the Qts value will actually be the measured Qtc of the system."

                    So I'm going to give it a shot and see how close my predictions using WinISD actually come.


                    • #11
                      Yes, if you measure that woofer in your sealed/aperiodic enclosure with WT2, WT3, DATS, then Fs will be Fb and Qts will be Qtc.


                      • #12
                        Just got the Woofer Tester out and tested both the old GRS 10" subs, and the new GRS 12" subs.

                        And I'm relieved to know my perceptions were accurate and I'm not suffering from some builder's bias.

                        The GRS 10" completed subs measured Qts (which Jeff Bagby says = Qtc) is 1.0248 and Fs of 58-Hz. My enclosures I think are approx. 1-ft^3. But if I allow for the driver's volume and specify .85-Ft^3 in WinISD, and change Qa to 10 for a heavily stuffed box, WinISD predicts 1.012. So predicted is very close to measured, it seems?

                        The GRS 12" completed subs indicate a Qts (= Qtc) of .6625 and Fs or 40-Hz. My enclosures are approx. 1.6-ft^3. But allowing for the driver's volume and using 1.4-ft^3 in WinISD, and changing Qa to 10 for heavily stuffed, and setting Ql to 3 per Jeff Bagby's post, WinISD predicts a Qtc of 1.187. So I think in this case it is a matter of the Ql value of 3 not matching my home-made aperiodic port? Perhaps if my aperiodic ports were more resistive, I'd have a measured value more in keeping with what WinISD predicts?

                        As I said above my ports are somewhat easily adjustable. So I can absolutely change the port's makeup and see how response changes. OTOH, I really like the way these sound now, I feel it is some of the best bass I've had in my living room, so my incentive to experiment further isn't very great.

                        Anyway, I thought I'd share before embarking on cleaning-up the enormous mess I've created when finishing this pair off. It seems every tool was removed and left out, and there are bags of consumed consumables left everywhere.

                        Please feel free to comment on anything I can do to improve or if I’ve done something incorrectly.


                        • #13
                          Would anyone be able to comment on the shapes of the curves between the two woofers? The 12" with my aperiodic port has slopes that are very shallow compared to the sealed 10".


                          • #14
                            Originally posted by philthien View Post
                            Would anyone be able to comment on the shapes of the curves between the two woofers? The 12" with my aperiodic port has slopes that are very shallow compared to the sealed 10".
                            Replying to my own question here, I googled "aperiodic speaker impedance" and looked at images and guess my shallow slope is expected:


                            • #15
                              The Aperiodic vent should have the effect of reducing the impedance peak, it appears to be be working quite well.