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Thread: JBL Master Reference Monitor

  1. #1606
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    But won't that all be factored in, with respect to the M2? If one measures the REAL M2 and comes up with some frequency using the AC Voltmeter/driver movement method and if it comes out that it is 27Hz. If one measures their clone and tunes it to the same number using the same method, wouldn't that effectively match the two systems? You are working with the same driver and essentially the same volume.

  2. #1607
    Senior Member Ian Mackenzie's Avatar
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    Thatís one way of doing it for sure. But l think Pos was questioning what the real tuning frequency was?

    If you all used a common flared port then it would only require one test to match the impedance curve and confirm minimum cone excursion. Then adjust your ports to the right length.

  3. #1608
    Senior Member pos's Avatar
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    Same result on the clones and the original M2, be it based on an impedance measurement or a simple excursion test, means same tuning is reached.
    That is what I am looking for.

  4. #1609
    Administrator Robh3606's Avatar
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    The impedance measurement is the way to go! Great resolution

    Rob
    "I could be arguing in my spare time"

  5. #1610
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    I just got done accomplishing both the impedance and dynamic method checks on my OEM, M2's, which I repeated three times to make sure of the readings.

    The Impedance method read 26.6Hz as a minimum point. I have an old but competent Fluke meter.

    The Dynamic method read 24.6Hz as a minimum.

    The Dynamic method was measured using a vibrometer, phone-app, pressing the phone against the bass driver dust cover. A 0.1Hz change was clearly measurable using this method.

    I used a Smart Tools, Vibration Meter app for androids that I downloaded into my phone.

  6. #1611
    Senior Member pos's Avatar
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    Thanks a lot for these measurements Peter!

    The excursion/dynamic test should be the most correct one as it describes the behavior of the port.
    The difficulty with this test is normally to find the minimum excursion point by "hand" (literally), hence a loss in precision.
    The fact that you have been using a vibrometer here makes this measurement extremely valuable.
    Can you share the name of the app you used? (iOS? Android?)

    Regarding the difference between the two results, I think the dynamic/excursion one is the correct value to use.
    The difference between the two is normal and can also be seen in simulation softwares (eg winisd), where the minimum excursion point is always exactly locked at the simulated port tuning frequency, whereas the impedance minimum is typically higher in frequency, depending on the box's Ql/Qp/Qa values.

    So when building clones, having both the dynamic and impedance measurements right at the same frequency as what Peter found is a good indication that both the port tuning and box construction close to the original !

  7. #1612
    Senior Member pos's Avatar
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    Now that we know that the intended tuning frequency is 25Hz (probable theoretical target), we can compare a simulation of the M2 box (around 150L and 25Hz tuning) vs the M2 simulation from Harman that was posted by Giskard a few years ago (130L and 27Hz tuning, also attached for reference).

    Name:  M2 vs simulation.jpg
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    These are "raw" 2Pi responses, without EQ applied, and using the 2216nd parameters from the Harman document (albeit the response curves stay the same with the parameters from the EDS).
    As can be seen the two responses are within 0.5dB from each other down to around 25Hz, with a consistent 2dB difference under 20Hz.
    Attached Images Attached Images

  8. #1613
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    So, it would seem that IF you are using the voltmeter/resistor method, you should target your ports at just shy of 27Hz because if one could accurately measure cone excursion, you'd find that the real port tuning is closer to 25Hz.

    Once again, I think, so long as you are using the same method of measuring BOTH speakers, you'd come to the same result. I was able to get to .1Hz resolution using the resistor/AC Voltmeter method. FWIW, I was using my old Heathkit IM-5238 AC Voltmeter (I like vane/needle meters for variable measurements as it is easy to see a change and one need not deal with bobbling displays).

    The other question is, how close is one to 130L or 150L, including displacement caused by box construction/components.

  9. #1614
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    Does someone know what damping material is used inside the genuine M2? And it's thickness? Is it 1" fiberglass or something else? Thank you

  10. #1615
    Senior Member srm51555's Avatar
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    Thomas posted it in #1117

  11. #1616
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    Thank you. Do you know what thickness is used? One or two inches?

  12. #1617
    Senior Member srm51555's Avatar
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    I don't remember if anyone actually measured it, but they usually used 1" so that is what I went with.

  13. #1618
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    I'm quite confused because I've read that the fiber glass wool can emit particles that can enter the voice coil of the woofer. I wonder if an acoustic fabric placed on the fiber glass wool could stop those particles. The simplest and safest way would be to use another material like polyester, but the box alignment will differ from the original M2...

  14. #1619
    Senior Member grumpy's Avatar
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    If worried, Iíd assign more concern to glass bits blowing out the ports into your breathing space.
    The filter(s) on the woofer are there for a reason (keeping junk out of the voice coil area). That said, fiberglass isnít conductive so it would take a looong time for enough microbits of glass to accumulate to where it would be a problem in the voice coil gap. Acoustic fabric or a light coat of hairspray might indeed alleviate your concern (no smoking while spraying). Keep the fibers away from port entry in general.

    Quote Originally Posted by gbadaut View Post
    I'm quite confused because I've read that the fiber glass wool can emit particles that can enter the voice coil of the woofer. I wonder if an acoustic fabric placed on the fiber glass wool could stop those particles. The simplest and safest way would be to use another material like polyester, but the box alignment will differ from the original M2...

  15. #1620
    Senior Member 1audiohack's Avatar
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    Many people seem concerned about fiberglass blowing around in the listening area. I am not and the reason being is that glass is heavy! Strands of fiberglass have the density of, well, glass, and while I get that this is not news, the fact is it does not drift around in the air, it drops like a rock.

    Thatís been my two bits.
    Barry.
    If we knew what the hell we were doing, we wouldn't call it research would we.

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