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Thread: Measuring compression drivers. Questions.

  1. #1
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    Measuring compression drivers. Questions.

    Hi

    This weekend I will be installing a "lightly used" pair of 2441 diaphragms into my 4350 system. Once installed I would like to measure the CD's to ensure the diaphragms and the drivers are good to go.

    Am I able to do this with the CD's hooked up to the passive network? I would not connect any of the drivers, just the 2441's. I understand the network may do something to the FR but as long both sweeps are similar can I take that as an indication that all is well? Is REW the right software to use to do the sweeps?

    The diaphragms have a few tiny dents and one larger crease that was apparently from the previous install. I am hoping that these imperfections don't cause any anomalies. These diaphragms are hard to get and the drivers are quite difficult to remove from the 4350 cabinet. I only want to do this once.

  2. #2
    Administrator Robh3606's Avatar
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    I would run an impedance sweep on them no network and also do a low power sine sweep to listen for any issues. Thy should match up pretty close. Just put a big cap in series to protect against DC. I will try to find the JBL installation instructions and post. It should look something like this. Black curve is a 4" on a waveguide as an example. Figure a couple of peaks on the low end and smooth above.



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  3. #3
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    Just a follow up. I ended measuring the drivers still within the passive networks. I just wanted to confirm that both the diaphragms measured fairly closely. Sweeps were taken within the drivers operating range within the network, 1k-10k.

    I had never opened up my 2440's so I was not aware what diaphragms were installed. I knew they had been cracked open in the 90's. Much to my pleasure I found a mint pair of 375 diaphragms. These speakers had not been used in nearly 20 years so I can assume the 375 diaphragms have had little to no use

    One of the 2441 diaphragms does not look good at all. I switched diaphragms between drivers to rule out the driver itself. The bad diaphragm measured consistent no matter what driver it was in. It appears to have a fairly large dip easily visisble in the sweeps.

    Measurement distances were not perfectly controlled but I took multiple sweeps and the results always looked the same. All sweeps were taken with the drivers attached to the 2311 horn outside the box sitting on a chair. 1/48 smoothing applied to sweeps.

    Any thoughts on the sweeps? Is one of the diaphragms just worn out? They do have a few marks on them but oddly enough the one with more substantial creasing/marks measures the best.

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    Also on the sweeps which show both drivers I used the "Separate" Function in REW. There is not a drop in output between the diaphragms.

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    Administrator Mr. Widget's Avatar
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    Looking at those curves, since the vertical scale is set at 20dB per each delineation, any noticeable difference is actually quite extreme. If the software allows you to run it with 5dB per delineation you will be able to compare them more accurately. Any difference of 1-2 dB between the two diaphragms will be quite audible and will very likely be out of spec.


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    Could the uneven response of the diaphragm be a result of the coil rubbing somewhere?

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    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by kelossus View Post
    Could the uneven response of the diaphragm be a result of the coil rubbing somewhere?
    Could be. It could also be due to improper centering/alignment, or a partially damaged diaphragm due to wear/age.


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    Quote Originally Posted by Mr. Widget View Post
    Could be. It could also be due to improper centering/alignment, or a partially damaged diaphragm due to wear/age.


    Widget
    Is the response of the "good" 2441 diaphragm what you can expect of this driver functioning properly?

    Just checked and I can't change the delineation of the sweeps.

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    Quote Originally Posted by kelossus View Post
    Is the response of the "good" 2441 diaphragm what you can expect of this driver functioning properly?

    Just checked and I can't change the delineation of the sweeps.
    The response of your "good" 2441 diaphragm looks to be normal.

    Its rising response will flatten out somewhat when you put the 2308 ( slant-plate ) in front of the 2311 horn.

    You might want to upload your REW file here so others can look at it for you.

    Simply "Zip" it first so that the forum software allows it as a proper file type to be uploaded.



    PS; There are a couple of places to change the vertical db scaling ( one is within a small icon that has 2 sets of arrows overlaying a rectangular box ).

    PS2; The "bad" response is so off it has me thinking its not a bona-fide JBL diaphragm ( you might want to post a few pics so that others can help with that determination ).

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    Quote Originally Posted by Earl K View Post
    The response of your "good" 2441 diaphragm looks to be normal.

    Its rising response will flatten out somewhat when you put the 2308 ( slant-plate ) in front of the 2311 horn.

    You might want to upload your REW file here so others can look at it for you.

    Simply "Zip" it first so that the forum software allows it as a proper file type to be uploaded.



    PS; There are a couple of places to change the vertical db scaling ( one is within a small icon that has 2 sets of arrows overlaying a rectangular box ).

    PS2; The "bad" response is so off it has me thinking its not a bona-fide JBL diaphragm ( you might want to post a few pics so that others can help with that determination ).
    Thanks for the reply mate.

    I noticed on the 375 diaphragms that came out of the drivers they had shims underneath the plastic ring. Could the lack of shims on the "bad" diaphragm cause this kind of uneven response? Also strange that the 2441 diaphragm that measures well did not require any shims to give a decent sweep.

    Diaphragms are both JBL's. I will attach pictures of them below as well as the REW files compressed as you suggested.

    2441 REW SWEEPS @ https://ufile.io/w1dvztyg


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  11. #11
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    This photo of yours ( of 2x 2441 diaphragms ) shows that each diaphragm indeed has 8 shims evenly spaced around the mounting ring.

    I suppose ( if you want ) you can add additional ( removable shims ) to see if you can improve the response of your one ( poor measuring ) diaphragm ( its far from a sure-thing that you'll get a response improvement ).





    PS; Here's a pic of your 2 drivers responses ( with some proper scaling ) > use the yellow-highlighted box to set scaling in REW.

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    Quote Originally Posted by Earl K View Post

    I suppose ( if you want ) you can add additional ( removable shims ) to see if you can improve the response of your one ( poor measuring ) diaphragm ( its far from a sure-thing that you'll get a response improvement ).
    That is quite upsetting to hear. Do you think the poor response is indicative of the diaphragm being worn out or damaged? I was hoping my lack of knowledge installing the diaphragms was to blame.

    As I mentioned I did not strictly control the microphone distances when measuring the diaphragms so over lapping the two curves may show a difference in output. The microphone remained static but the drivers were placed on a chair pretty closely in the same position.

    Heartbreaking........I managed to buy Speaker Exchanges last 2441 diaphragm which should arrive next week. I don't like the idea of installing just a single new diaphragm. Makes more sense to do it in pairs but if it measures close enough to my "good" diaphragm I don't have a choice.

    Otherwise I buy a pair of Radian diaphragms and sit on the brand new 2441 diaphragm until I can find another. This is turning into a very costly lesson.

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    Also how do you determine if you should add/remove a shim?

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    Quote Originally Posted by kelossus View Post
    That is quite upsetting to hear. Do you think the poor response is indicative of the diaphragm being worn out or damaged? I was hoping my lack of knowledge installing the diaphragms was to blame.

    <<<SNIP>>>
    It's impossible to answer your question since ( IME + I'm in Pro-Audio ) a good 25% of JBL's over-the-counter replacement diaphragms had ( in the past before production was shifted to Mexico ) problems with Quality Control.
    Now that their QC is apparently worse I have to wonder what the percentage is ( for us out in the booney hinter-land of Canada ).

    IMHO, JBL got away with lax QC because very few had the means to test what they had just purchased ( I also think they shipped more of their "dicey" components out-of-country ).

    Therefore, I have no idea whether you purchased a worn-out diaphragm or one that just didn't meet proper Quality Control.

    IM (personal) E, distortion ( with a sine-wave ) has to rise above something like 3% before it's obvious to my ears that there's a real problem.

    The 3rd Harmonic on your bad diaphragm registers as well below 1% ( somewhat implying that there is not a fitment problem > iow; where the VC is rubbing and generating odd numbered harmonics ).

    Using a tone sweep ( again, IME ) was easier to pin-point a bad diaphragm ( assuming one has a good one to compare to ).

    In all fairness to eBay sellers ( or any other ) , few-to-none measure the response of what they sell before putting it on the market ( most simply take a DCR reading and call it good ) .

    I'd recommend ( since you have measuring capability ) that you play around with the diaphragms fitment to see if you can adjust the FR of that diaphragm ( by re-torquing the 8 screws wth various amounts of pressure and changing-up the sequential pattern used for the 8 screws ) .
    - Do you know how to torque down a head-gasket ( or even a car-tire for that matter )?



    PS; You might find that the components dedicated for install into systems ( ie; speaker boxes or monitors ) may be better matched ( iow; hand-picked & tested for QC by JBL ) .

  15. #15
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    Quote Originally Posted by kelossus View Post
    Also how do you determine if you should add/remove a shim?
    By measuring FR response ( before & after ).

    One can make and use paper shims .


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