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  1. #1
    Senior Member Baron030's Avatar
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    My Drew Daniel's inspired DIY project.

    Well, I have finally gotten very close to finishing the construction phase of my little DIY project. So, I decided to take a little break and take some pictures to share.

    As you can see, it just a plain brown box. Why, I didn't even try to make it look like any existing JBL product. So, this enclosure is a total sleeper...

    Baron030
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  2. #2
    Senior Member Baron030's Avatar
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    Additional Pictures

    Check out this view of the back, nothing looks unusual here, right?
    Baron030
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    Senior Member Baron030's Avatar
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    Additional Pictures

    Well, the system is quad amplified, and I am feeding it with just a wee-bit of power.
    An Ashly XR4001 4-way electronic crossover feeds to a K2, K1, and 3 D-75a amplifiers.
    So, in my small apartment, clipping is never a problem.
    Baron030
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    Senior Member Baron030's Avatar
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    More Pictures

    Here is another view of the back, and with something that you don't see every day.
    I will give you a little hint, it's not a port.
    Baron030
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    Senior Member Baron030's Avatar
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    More Pictures

    OK, this really does give it all a way...It's a lift handle.
    These boxes started out weighting in at 75 lbs. each, with just the JBL drivers alone.
    So, lift handles really are a helpful addition.
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  6. #6
    Senior Member Baron030's Avatar
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    Additonal Pictures

    Here is an inside view from the back.
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    Junior Member Turnitup's Avatar
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    Awesome Craftmanship! I wish I was your neighbor, so I could hear'em
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  8. #8
    Senior Member Baron030's Avatar
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    A Major Port Tuning Surprise

    The plan that I had in mind when I started my project was have to the 2226 drivers enclosed in a box with net internal volume of 5 cubic foot and have the bass reflex ports tuned to 40 Hz. This would follow the recommendations on the JBL 2226H spec sheet.

    Using the WinISD program, I calculated that I would need to have 3 ports with an inside diameter of 3.94” and a length of 9.4 to reach my target tuning of 40 Hz. Well, being a little unsure of the WinISD programs accuracy, I decided that I would initially leave the ports a little long at 10.5 inches. And then I could do some tests and trim the ports shorter as needed.

    When I started testing my enclosures the 10.5” ports, I expected the tuning to be around a WinISD program predicted frequency of 38 Hz. But, my test results clearly showed that the tuning to be a about 31 Hz.

    The only explanation that I have for this error is that I forgot to take into account on the effect of using R-11 fiberglass insulation on all of the enclosures inside surfaces.
    The fiberglass has clearly increased effective acoustic volume of the enclosure dramatically. So, the really big question is this. What to about this virtual volume issue?

    One possible solution would be to remove the R-11 fiberglass and then replace it with a thinner layer fiberglass. This would hopefully shrink the virtual volume back down to match it’s actual volume.

    Or I could leave the R-11 fiberglass alone and accept that the enclosure is now “virtually” much larger and possibly come up with a slightly different enclosure tuning.

    Lately, I have been experimenting with different ports lengths and I am starting to see a pattern emerging. After, running tests with 9.25” and 8.0” port lengths, it appears that my test results are very closely matching the WinISD predicted results for an enclosure volume of about 7.5 cubic feet. The 9.25” port tuning came in at about 33Hz. and the 8.0” results are posted below, which are coming just in a little below 35Hz.

    And from just listening to my system, I have found that the 8” port tuning sounds a lot better than the other port lengths that I have tried. So, this would suggest that I am getting a little closer finding the correct enclosure tuning.

    But, I am not sure what I do next. Should I pull out some fiberglass or shorten the port tubes further?

    So, any advice here would be more than welcome.
    Thanks, Baron030
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  9. #9
    RIP 2011 Zilch's Avatar
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    BB6P says 3 x 3.938" vents at 8.065" long will give 40 Hz tuning with typical (all sides but the front baffle) fill.

    "Heavy" fill lowers it to 37.95 Hz.

    I'd leave the R11 in place and continue trying shorter vents until you achieve the tuning you like best for your room and listening preferences, which may be something other than the target 40 Hz.

    The insulation is doing more for you than just adding volume, i.e., damping.

    R11 is kinda low density, so I'd say you have the equivalent of typical fill in place, and should leave it there.

    It's common for the simulated port lengths not to produce the exact tuning predicted in the built box. There are other variables involved, and the empirical method you're using is almost always a part of the design finalization process.

    Note: Fmin provides an "approximation" of the box tuning only, so don't consider it absolute. There are more precise means of determining the actual Fb. For example, at what frequency is woofer cone movement minimal?

    Clearly, you are close to where you want to be. Just tune to taste, now....

  10. #10
    Senior Member spkrman57's Avatar
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    Shorten the length of the ports.

    And also, you can adjust one of the ports by itself and tune it that way instead of messing with all 3 ports.

    Ron
    JBL Pro for home use!

  11. #11
    Senior Member Rudy Kleimann's Avatar
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    Check yourself, Baron

    Something is wrong with your math here. No other reason than that. Not to worry too much, though, unless your net volume plus damping yields an effective volume that is way off the mark. A little off here and there isn't going to matter much, if at all. Cabinet size is often adjusted from the theoretical to the practical for aesthetic or other reasons, namely size or fit in a particular installation. Your ears will tell you... they are your best test instrument , and the receiver of good vibrations that make the musical experience... never lose sight of this fact while busying yourself with the math and science of it all .

    As Zilch pointed out, actual testing of the finished system is the only way to see what you really, really ended up with. This includes the ear test

    A couple of points:

    Insulation, particularly fiberglass, adds virtual volume by converting the energy in the air from adiabatic (constant heat) to isothermal (constant temperature) which brings about a reduction in the speed of sound. loosely filling a box with insulation can easily increase the effective volume 20%; theoretically it can increase it by 40%. If you were shooting for 5CF, but your tests indicate 7.5CF, something is wrong here, either your physical dimensions are innacurate, or your test method to determine box tuning frequency is wrong. Insulation improves damping, which controls ringing and overshoot when the driver is fed a transient signal. R-11 is fine; I wouldn't use any less than this on the cabinet walls. This should add about 10% virtual volume, or as Drew has commented in the JBL enclosure guide, should about amount to about the same volume displaced by driver(s), bracing, and ports. A little thicker should add all that much more.

    The same size port in a larger box will yield a lower tuned frequency. As Zilch pointed out, the design programs will get you close -if your physical dimensions and volume calculations are accurate. Always start long on the port as long as it is not too close to any obstruction i.e. a cabinet wall or bracing, and cut it to length to hit the actual tuned frequency desired. If your box volume is too big (or you think it is), fill the void with bricks, sandbags, or other solid, inert, incompressible material and give it a listen.

    How exactly are you determining the tuned frequency of your box? You should be able to get close by feeding the woofer a sine wave at 100-200 watts, carefully observing cone movement as you sweep down through the frequencies below 60 Hz. At the box tuned frequency, port output is at maximum (as felt by your hand) while at the same time, the cone movement is at its' minimum (practically stops moving). The accurate method involves an oscilloscope and a resistor (PM me if you have a 'scope and want the setup instructions).
    As you go below the tuned frequency, the cabinet ceases to load the woofer, and cone movement increases rapidly as you continue sweeping down. At this point, STOP! If you continue sweeping down, woofer excursion increases to the point where the voice coil can crash into the rear plate of the magnet and damage the coil. It can also be hard on the spider, surround, or even the cone itself. I've seen cones curl back near the suspension from over-excursion and even develop a circular fatigue line in the paper cone near the dust cap. This quickly leads to driver failure from severe over powering or driving below the ported box frequency. Just because the coil can handle the power doesn't mean you can't break the speaker...

  12. #12
    Senior Member Rudy Kleimann's Avatar
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    Different sized ports???

    Quote Originally Posted by spkrman57
    And also, you can adjust one of the ports by itself and tune it that way instead of messing with all 3 ports.

    Ron
    I don't know about that... I'd plug a port, but I wouldn't have different length ports... seems wrong to me.

  13. #13
    Senior Member Baron030's Avatar
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    Thanks Zilch and Rudy for all of your help.

    Your responses made me really question my test results. So, it forced me to go back to “The Book”. “The Book” in this case is “Testing Loudspeakers” by Joseph D’Appolito.

    The excel chart that I included in my last post was created using a voltage divider technique described in Joseph’s book. For test equipment, I used a freeware computer program called SigJenny and an inexpensive digital multimeter. I have tested accuracy of the SigJenny program, using oscilloscope and I have found it to be very accurate. So, I know that the SigJenny program is not at fault here. Since, I don’t have my computer hooked up to my stereo. I used the SigJenny program to create my own DIY Port Testing CD with each track spaced just 1 hertz apart.

    Getting back to my excel chart, all of the data contained in it is fairly accurate. But, I have completely misunderstood it meaning. After taking some additional impedance measurements, this time with the ports closed, I have now gathered all of the information necessary to properly calculate the enclosure tuning. The missing bit of information is the closed-box resonant frequency of my enclosures, which just happens to be 48 Hertz.

    Fb = box tuning frequency
    Fc = the closed-box resonant frequency
    Fh = the frequency of the higher-frequency impedance peak
    Fl = the frequency of the lower-frequency impedance peak
    Fm = the frequency of the minimum impedance between the two peaks

    Fb = square root of ( (Fh x Fh) + (Fl x Fl) – (Fc x Fc) )
    So, plugging in the data values for my left enclosure into the formula above, the box tuning works out to be:
    38.74 Hz = square root of ((58 x 58) + (21 x 21) – (48 x 48))
    And for the right enclosure tuning works out to be:
    37.23 Hz = square root of ((57 x 57) + (21 x 21) – (48 x 48))

    These numbers come in fairly close to BB6P predicted values for an enclosure with a “heavy” amount of fill. Since, my project has a dog box, there is larger amount of surface area relative to its volume, which may explain why my project comes in with a heavier amount of fill in it.

    Normally, the Fm value is very close to the Fb value. But, in my project, the Fm value appears to be much lower than the Fb value. And that is where I got really confused.

    Over this last week end, I did try to observe at what frequency the cone movement was at its minimum. But, with separate test tones for each frequency, I found this test method next to impossible to judge accurately. So, I think I will stick with the voltage divider method for now.
    Thanks again Zilch and Rudy, you both deserve a cold frosty one.
    Baron030

  14. #14
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    Quote Originally Posted by Baron030 View Post
    So, the really big question is this. What to about this virtual volume issue?
    That's why we prototype...

    At least you've experienced first hand why I've harped on proper enclosure tuning, virtual volume and enclosure losses countless times to nothing more than blank stares in return.

    Nice project!
    Quote Originally Posted by Baron030 View Post
    Your responses made me really question my test results. So, it forced me to go back to “The Book”. “The Book” in this case is “Testing Loudspeakers” by Joseph D’Appolito.

    These numbers come in fairly close to BB6P predicted values for an enclosure with a “heavy” amount of fill. Since, my project has a dog box, there is larger amount of surface area relative to its volume, which may explain why my project comes in with a heavier amount of fill in it.
    Good book to have in the personal library.

    BB6P does a nice job.
    Quote Originally Posted by Baron030 View Post
    Currently, I got the ports trimmed down to 7.5" long. And my last tests showed that the tuning is still a little below 40 Hz. But, at this point, I realize that I am so very close to hitting the bull's eye that I am not sure if I want to trim the ports any shorter or not.
    Maybe, with some better test equipment, I could be convinced that another ½" may have to come off. But, I am not sure that it would make any real difference in the sound.

    The 2226s now have a great punchy sound and I don't feel the least bit bass deprived, yet.

  15. #15
    Junior Member Dualbios's Avatar
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    Love this project really is helping me strive to get mine done

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