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Thread: Help with 4312's

  1. #16
    Tom Loizeaux
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    At first glance it looks as if the low frequency response doesn't change that much when you closed the port! I'm surprised at this.

    I'll be watching for other comments and developments.

    Tom

  2. #17
    Senior Seņor boputnam's Avatar
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    With that test gear set-up, why are you working on 4312's...?

    Seriously, the LF curve looks very strange. That extreme roll-off starting at ~200Hz is just wrong. And, as Tom sez, there is no material difference between plugged and ported.

    What is the source data - Pink Noise?

    Can you feed your Pink Noise signal output directly into your measurement set-up, bypassing the mic, and the 4312? I'd like to see that trace...
    bo

    "Indeed, not!!"

  3. #18
    RIP 2011 Zilch's Avatar
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    As Tom observes, the fact that there's no difference in measured response vented vs. closed is telling. Something's up.

    My take on it: you can't measure LF response using the method described. Nearfield, ground plane, and in-box are options. Nearfield, you have to sum with the port output when measuring vented.

    Also, you've got to get the woofer up off the floor to measure it in that range, even nearfield; 1/2 the ceiling height's optimum.

    What's "nearfield" in this instance? Mic within one driver diameter. Some advocate at like 1/4" out from the center of the dome. Try that, and see if you don't get more rational results....

  4. #19
    Senior Seņor boputnam's Avatar
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    Quote Originally Posted by Zilch
    Some advocate at like 1/4" out from the center of the dome.
    Yea, when Widget is using CLIO he gets his measurement mic right up close.
    bo

    "Indeed, not!!"

  5. #20
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    Hello everyone!
    Mr. Putnam, the input signals I'm using are software calibrated sweep tones.The green lines in the graphs are the actual freq. response curves of the tones that are sent to the power amp. The small deviations in the green line are sound card freq. response deviations.
    I forgot to mention that the speakers are 3 ft. off the floor-sorry about that!
    Included here are more curves.
    Zilch is on to something, something's definately up,I just don't know what to make of the results.
    These tests were run in the room, all speakers tested in the same location for comparison. The up close mic'ed positions are centered on the 2213H and E120 ,rather than the 4312's x/y axis. The mic is 2 inches from the dust caps.
    Since the original published freq. response curves were taken 1 meter away, on axis, I don't know how to interpret these results.
    I will repeat these tests outdoors again this weekend, and I'll capture the results and post them here. I'll capture 1 meter x/y axis, and 2 inches away centered on 2213H's. Due to mic/mic holder dimensions, 2 inches is as close as I can get to the dust cap.
    The last graph is mislabled. It should read:JBL E120 utility cab(approx.1.75 cubic ft.),mic'ed 2 inches away from dust cap,centered on dust cap,3 inch port hole, no duct, well padded.
    Here they are!
    Thanks everyone!
    Attached Images Attached Images     

  6. #21
    RIP 2011 Zilch's Avatar
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    Well, there you go.

    You can now even see the difference between closed and vented behavior.

    There's nothing wrong with the woofers, it doesn't look like.

    E120 in 1.75 cuft. w/ 3" dia x 0.75" port:
    Attached Images Attached Images  

  7. #22
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    Hello Zilch!
    If I get similar results outside, what will that mean?
    I definately remember seeing that dip in response outside as well.
    There is an overhang above the speaker where I tested it. Could this cause the same problem? I can move it over to be flush with the chimney next time.
    More confused than ever!
    I would like to thank everyone for everybody's help.
    The lightbulb's got to light up eventually!

  8. #23
    Tom Loizeaux
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    Isn't it true that if you position your mic close to the woofer, you will get a low end roll-off reading when the signal starts to reach cabinet tuning. At that point the port is contributing most of the SPL and the cone is doing the least.
    To get a true performance curve for this speaker, wouldn't you need to make the mic placement closer to equal-distant between the cone and port?

    Just my thoughts...

    Tom

  9. #24
    RIP 2011 Zilch's Avatar
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    Quote Originally Posted by Tom Loizeaux
    Isn't it true....
    Yes. You're supposed to measure the driver and port separately, and add them, weighted for the differential diameters.

    Nearfield between the port and driver would be a useful approximation, I would think. It's easy to watch what happens on an RTA as you move the mic closer and further.

    There's also a high frequency limit to summing the nearfield port measurement, 1.6 X the tuning frequency, according to Dickason, due to crosstalk between the vent and driver.

    I forget what D'appolito, Keele, and Small each say on this subject; they've all been there.

    If I want to see the real VLF response, I gotta get up close, is what I know....

  10. #25
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    Hello everyone!
    Don't know what it means, but here's woofer+port 2 inches away.
    More curves comin' sometime tommorrow.
    Later!
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  11. #26
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    Hello! I wonder if JBL may have left an important piece of information out about their testing procedures.After I saw the same dip in low freq. response from my E120 loaded utility cab, I started having more questions about what they are actually doing.
    After getting the same deep dip outside @ 1meter, I'm really curious.
    There's no room boundaries out there.
    For all these tests, the speaker was mounted flush against a brick wall(large baffle) for low end support, and the mic on it's x/y axis for the 1 meter measurements.The attachment limit is 5, so we'll skip the dust cap measurement for now...
    The testing conditions are different this time, to eliminate overhang boundary questions.
    The closest n/s boundary is 65 ft. (compared to over 200 ft. previously)
    This is the best I can do to re-create the original testing conditions.
    "The accompanying graph and specifications were compiled from measurements made under standard laboratory test conditions.The loudspeaker system was mounted flush in the center of a large,flat baffle in an anechoic environment;a calibrated condenser microphone was suspended at a known distance from the sound source,sufficiently far to be safely out of the near field;and all electronic equipment was checked and calibrated before tests were run."
    As you can see from the graph that Mr. Loizeaux posted, the "known distance" is 1 meter.
    The only difference I can see, is that I'm using calibrated sweep and MLS signals.I'm certaintly not getting curves like this!
    Something's missing in this equation.
    In a couple of weeks,I'll be able to run 1/3 octave pink noise measurements. If this reveals a different story, I'll post the results.
    For now, here's what I measured outside.
    All thoughts and comments are appreciated!
    Thanks everyone!
    Attached Images Attached Images      

  12. #27
    RIP 2011 Zilch's Avatar
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    Your nearfield measurements seem to be a reasonable approximation of the factory curve; they capture the L100 et. seq. LF "bump" quite nicely.

    As soon as you move away, I think reflection from the ground (floor) is what's causing you difficulty. Lay the speaker on its back, away from walls, etc., and suspend your mic 1M above it; I suspect the "dip" will disappear.

    See P.3, here:

    http://www.jblpro.com/LSR6300/PDF/RMCUserGuide.pdf

  13. #28
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    Holy cow,thats it!!!
    I've been so consumed with wall boundaries, I never stopped to think about floor or ceiling boundaries! I really pulled a on that one!
    That's the one constant that has remained unchanged in over 22 years.
    Now, if I can just fix the problem.....
    I would like to thank everyone for their thoughts and opinions on this, and especially Zilch, for helping me understand where I've been going wrong all this time.
    It's been a real learning experience for me.
    If I solve the problem to a significant degree, I'll post what was done, and the resulting curves afterward.
    Thanks again, everybody!

  14. #29
    Senior Seņor boputnam's Avatar
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    Good one, Zilch!

    I was going to suggest you forgo the esoteric woofer-specific measurements and take some in/at the listening position and see what the overall cabinet reproduction response is. I still think that would be worthwhile - room effects being what they are...
    bo

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  15. #30
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    Hello!
    Mr. Putnam, you are right, and I'll be doing a lot of that for a while.
    I'm hoping a listening position/speaker position compromise will fix the problem.
    I don't even want to think about building more bass traps! Especially traps for the floor?!
    If I find something interesting and helpful for the whole group, I'll post curves and all.
    It may take awhile....
    Thanks again,everyone!

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