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Thread: Help with 4312's

  1. #1
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    Help with 4312's

    Hello! Anyone here with intimate knowledge of early '80's 4312's?
    I have a strange low freq. response problem.
    My ears have heard it from the beginning, but I finally have the tools to really see it in great detail.
    Both speakers now have a sharp 20db dip at 125Hz, and a sharp peak at about 270Hz.I've religiously checked for air leaks and loose joints-none found.The 125/270Hz relationship made me suspicious of this.
    For years, I wrote this off as room boundary issues, yet every room I put them in, regardless of room dimensions,I ended up with the same result. I recently tested these 4312's outdoors, one at a time,flush against a large brick wall, a reference mic 3.3 ft. away, the closest north/south hard surface at least 250 ft. away (the only way that I can mimic the original JBL test conditions), and got approximately the same results. SPL level was 80.
    The room they live in is treated, with massive bass traps in the rear to reduce room boundary effects.
    I found a test graph I made in the '80's with a Goldline RTA in an untreated room, and the dip at 125Hz was 10db down.
    Today, in a treated room or outdoors, it's 20 db down.
    The testing software I'm using is ETF 5.XXX (demo version).
    This software also reveals the 250-270 Hz box resonance that's always bothered me from day one.
    Has anyone tested these monitors this extensively?
    Has anyone seen this before?
    Thanks for everybody's help!

  2. #2
    Senior Señor boputnam's Avatar
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    All your "issues" appear to lie within the range of the 2213H. There is no Lo-pass filter. The 2213H runs wide-open, allowing it's natural roll-off.

    The MF has a 1.5KHz Hi-pass - everything you mention is far below that.

    I wonder if a recone would be in-order...?
    bo

    "Indeed, not!!"

  3. #3
    Tom Loizeaux
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    Your attention to your 4312s is impressive. I don't mean to question your sense that the irregularities are getting worse, because you may actually be experiencing a problem ... but it's possible you are simply hearing more clearly the limitations of that design.
    I like my 4312As, but I suspect the 4412s, with their true 3-way crossover and foam surround woofer would prove to be flatter and be a more accurate performer.
    Please keep us posted as you learn more about this.

    Thanks,

    Tom
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    Hello everyone!
    Thank you for sharing your ideas.
    Since I first noticed this problem to a minor degree right out of the box, I'm not sure if a recone would help,unless of course the magnets need to be recharged...that was JBL's last suggestion. All previous suggestions by the JBL crew have not solved this problem. The recharge/recone would be an expensive gamble to take. I've looked at the 25 watt 40Hz output from the speakers on a scope, to check for major suspension problems,and the waveforms are clean and symetrical.
    I wish I had tested these speakers outdoors back around '88 or '89 when I first tested them indoors with an RTA.
    Like Mr. Loizeaux, I've wondered if this is just a design flaw.
    I was hoping for a response curve like the one Mr. Loizeaux posted, but I've never measured a low freq. response like that. If you start with that curve,then draw a dip starting at about 92Hz,with a maximum attenuation of 20 db at 125Hz, then back up at about 164Hz, and a peak of 8-10 db at approximately 270Hz, then you have something like what I measured in a treated room and outside.
    Since the original literature regarding freq. response testing raises lots of questions, I was just wondering if anyone else had tested their 4312's to see how their set really measured up.
    Mr. Loizeaux's statement about the low end response of 4412's possibly being more accurate than the 4312's is true in my case. There is quite a low end difference between my one of my 4312's and a 4412 side by side.
    The cabs of those models are very close in specs, and I've been tempted to try and find a good used 128H-1 and drop it in and see what happens.
    Unless I'm mistaken, it's either the cabinets or the 2213H's.
    If it's the 2213's then the big question is are both actually defective?
    If I ever figure it out, I'll post my findings.
    All insights and suggestions are appreciated!
    Thanks everyone!

  5. #5
    Senior Señor boputnam's Avatar
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    Quote Originally Posted by old_4312
    ...I've been tempted to try and find a good used 128H-1 and drop it in and see what happens.
    I've done that, myself. The 2213's went over the transom...

    I don't advocate swapping elements, but I ran some models in WinISD and BoxPro comparing the 2213H vs the 128H in the 4312 cabinet - the swap was reasonable and the resulting low frequency response superior (acoustically - I never measured it). Ideally, I suppose the network should be re-worked to include a Low pass filter.
    bo

    "Indeed, not!!"

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    Thank you Mr. Putnam!
    I believe I see some light at the end of this tunnel....
    Did you by chance,happen to notice a whopper of a dip in modeled response around 125 Hz with the 2213H's?
    I would guess modeling programs would allow you to tweak the flux density to see what field variations do to the response?
    That might explain a few things.
    I'll Google these programs to see if there's a consumer demo version available.
    I'd like to experiment with the numbers on a program like that.
    I might actually learn something!
    Thank you Mr. Putnam for mentioning these!
    With everyone's help, I feel like I'm finally getting somewhere with this.
    Thanks everybody!

  7. #7
    Senior Señor boputnam's Avatar
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    WinISD is shareware, although they appreciate donations!

    http://www.linearteam.dk/

    The models do not show a dip at 125Hz - they show a gentle curve and in general are used to predict the (extreme) LF response expected from the input box dimensions and Thiele-Small parameters.

    I regret I never measured the 4312's - before and after. The acoustic difference was quite dramatic (as I recall - the 4312's are boxed away right now) and so I was not inclined to do any measuring. Also, at that period, I did not use an EQ in the signal path so any measuring would have only served as observation not as guidance...
    bo

    "Indeed, not!!"

  8. #8
    Senior Member duaneage's Avatar
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    The 2213 is a High Qts speaker and as a rule should be in a sealed box. Venting a speaker with a Qts above .38 almost always results in a peak in the response. Some like the peak and tolerate it, I don't like the transient response that it gives, too punchy for me.

    The 128H is an excellent 12, probably one of the best JBL made, and it should go ruler flat to 40 Hz in 1.7 cu ft without much trouble. I know mine did.

    If you run an impedance curve on the 2213 in the box you will see how different the 128H is, provided you had a 128h to swap out with.

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    Hello everyone!
    I think I've learned 2 things already:
    JBL should have used the 128H's in the 4312's, and obviously I have some kind of funky problem with my pair of 4312's, that is making mine even worse.
    I think it comes down to the 2213H's or the cabs having a bizarre resonance.
    Since it's nowhere near as expensive to build a MDF test cab, compared to having a 2213 overhauled, I think I'll try that first. I can always build a test speaker out of it later....
    I'll try to build one using WinISD as a guide, and see if I get better results.
    I'm also going to rerun all previous tests on one of the old girls sometime this weekend,with the port plugged, to see what that changes in the real world.
    (Thanks for the tip Duaneage!).
    If I find something interesting, I'll let you know.
    Thanks everyone!

  10. #10
    RIP 2011 Zilch's Avatar
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    So, the first (and easiest) step toward "fixing" L100 bass would be blocking the port?


    [We KNOW how to do that.... ]

    That gives Qtc 0.874, F3 43.92 (black).

    Then upgrading the damping to "heavy" yields Qtc 0.779, F3 45.72 Hz (red).

    [Not sure of the net internal box volume, so "first approximation"....]
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    Senior Señor boputnam's Avatar
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    Sméagol, is that you...?

    Quote Originally Posted by Zilch
    [We know how to do that.... ]
    Gollum...? Hello??
    bo

    "Indeed, not!!"

  12. #12
    RIP 2011 Zilch's Avatar
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    Quote Originally Posted by boputnam
    Gollum...? Hello??
    This forum knows the local hardware plumbing department well by now.

    Well, me and CLIO do, anyways.

    Need I illustrate (again)?

    If I do that, I'll have to take some measurements.

    [And NOBODY wants to see that happen, certainly.... ]

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    jpg's and more jpg's

    Hello!
    Here are some response measurements from the old girls.
    Since there's little difference in response 3.3 ft away inside or outside, I decided not to scare the neighbors anymore than I have to.
    The measurements were taken inside the treated room.
    The jpg's are in pairs, one pair is 20HZ-200Hz, the other pair are 100Hz-1000Hz.
    Two are with the port open, the other two with the port sealed.
    The green line is the calibrated response curve of the sweep tones I'm sending to the amp/speaker, and the blue line is what I'm getting back.
    Let me know what you think.
    The limited demo version of the software I'm using for this is free. If anyone wants info on it, just let me know.
    Thanks everybody!
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  14. #14
    Senior Señor boputnam's Avatar
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    Dunno...

    Explain (again...?) the test set-up?

    What is the mic (brand and model)?
    Where is the mic in relation to the woofer? You say 3.3-ft away - why?
    Describe the "treated room", again?
    bo

    "Indeed, not!!"

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    Hello Mr. Putnam!
    Here's the skinny: The mic is a Groove Tubes MD5SM fet microphone, that has a razor flat response from 22HZ to about 5Khz. From there to 18Khz, it's tested and guaranteed not to deviate more than 1.5 db.
    The 48 volt phantom p.s. is a Newmann.
    The preamp is a custom built ultra low noise unit,built around a Burr-Brown INA103 op-amp.The soundcard is a 20 bit Echo Layla, the power amp is a Yamaha P2250, and the speaker cables are 12 gauge Monster Cables.
    I mention that because JBL once told me the problem was occuring because I was using 15 gauge standard cables, which could cause excessive capacitance and excessive resistance, reducing the damping factor of the amp.
    A valid argument I guess, but resulted in no change in my situation.
    The mic is placed dead center of the speaker's x/y axis, 3.3 ft away.
    The reason for this, is because this is the intial measurement point JBL uses for their freq. response measurements.
    According to the supplied literature and techs at the factory, they test the speaker flush against a large baffle centered in a near anechoic enviroment, with the mic centered on the speaker 1 meter away(3.3 ft).
    If necessary, they repeat these tests, swinging the mic in an arc, to publish response variations due to listening angles.
    I'm just trying to stick with the program.
    Concerned room boundaries was still wearing me out, I tested them outside, flush against a brick wall, to simulate JBL's setup--with no real difference below 300Hz. Above 300Hz, it smoothed out some...
    The room, well that's a novel....but here goes!
    Structural dimensions are: 15' 3.6" long, 11' 3" wide, and 8' tall.
    The north wall (where the 4312's live), is modified into a octagonal shape to reduce standing waves and head distortion.50% of this wall is covered with Auralex--couldn't afford to put wide band traps everywhere.The front of the east and west walls are covered with pyramid Auralex from 2ft. up.Only the first 9ft. are covered.
    In the center of the rear wall there's a open 5 ft wide x 30" deep closet that's packed to the ceiling with bats of R13, with somewhere between 7-10 inches of dead space behind all that.
    On each side of the closest are home built (and not too pretty) blue cloth covered open frame bass absorbers that's comprised of 8 layers of plastic wrapped R-13 each, packed down to 10" deep units.These are 5ft tall, and are on stands that lift them to the ceiling. According to my calculations, they yield .99 absortion down to about 60Hz, falling to about .50 at 40Hz in their coverage area. I may be wrong about that, but if I walk to the back of the room, I don't hear bass booming anywhere.
    I think that about covers it.Hope this post wasn't too long in length!
    Thanks everyone!

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