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Thread: Measuring Fb...

  1. #1
    Senior Señor boputnam's Avatar
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    Measuring Fb...

    Hey out there...

    I'm just finishing some refitted cabinets that mimic the L65, but for LF use the LE14A, 8-ohm, with foam surround mounted to front of cone (well, some LE14's just do that...).

    Much of the info about this little small format project was on the misbehaving Thread that Don quarantined due to it containing near pornographic JBL lore , and it has not yet been restored, so...

    The cabinet is an old Altec ModelNine, gutted, with new baffle fitted for LE14A, LE5-9 and 075 or 077. Approx. 3.22 cuft. Box is completely lined with 1-in batting (not on baffle back).

    I've tested three different ports. The old JBL plans for this size cabinet and the LE14A+LE20 calls for 4-in diam 3-in duct. One of Giskards old posts elsewhere called for a 8-in duct. Considering the "virtual volume" added by the batting might take these little suckers closer to effective 4 cuft - looking at the chart Giskard posted a few days back (not yet restored, same-same) a 4 cuft cabinet calls for a 2.5-in duct.

    Of the three (er four, = no duct):

    1) my wifey likes no duct, but to me that is a bit boomy and I worry about LF phasing/cancelling. Valid concern, or was the original L100 ductless port right...?
    2) 2.5-in sounds good, better than..
    3) 3-in, which sounds better than...
    4) 8-in which seems a bit muted LF.

    ¿How do I test the Fb of the cabinet empirically, removing subjectivity (which will, of course be add-back, later... )?
    Last edited by boputnam; 09-06-2003 at 05:27 PM.
    bo

    "Indeed, not!!"

  2. #2
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    Re: Measuring Fb...

    The JBL Enclosure Information Manual that I posted recently has a section with several different methods for testing Fb.

    Dan Marx built a great webpage describing a very good method.

    http://www.danmarx.org/audioinnovation/impanalyzer.html

    This little formula gets you about as close as you'll ever need to get.

    FB = square root of (FL2 + FH2 - FC2)

  3. #3
    Senior Señor boputnam's Avatar
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    Very cool, Giskard. I forgot to go back and check that manual you generously posted last weekend (I did manage to snag it, but have been too nose-down in the shop to get through everything...)

    As per usual, many, many thanks.

    Full report to follow!
    bo

    "Indeed, not!!"

  4. #4
    Senior Señor boputnam's Avatar
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    Well, back to some well trod ground...

    Today I finally got back to that small cabinet project of mine - some modified L65's for the lads. The baffle layout was discussed here: Baffle Layout - Forum Thread Link

    The problem that brickwalled me was the lack of bass response. The cabinet is barely over 3 ft3, and while that is OK for the LE14A, Fb was persistently too high. So, using a tip Robh posted about BBPro, I pegged the port diamemter at 4 in, and let BBPro suggest Fb and required duct length. The result - 11.52 in duct for Fb of 31 Hz -seemed long compared to all the historic guidelines, but Giskard kindly provided independent verification. So, 11.52 it is (employed a right-angle turn to keep things >3-in away from cabinet walls).

    Bingo! TasteTested today, and the result is excellent. This project smokes any L100 I've ever heard (sorry, had to work that in... ), and may beat the original L65, too.

    So, thanks to Giskard (as per usual..), Robh for the tip, and JBL Dog for the reconed LE14A's - they're beauties!

    Now, back to rehearsal...
    bo

    "Indeed, not!!"

  5. #5
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    Glad it all worked out Bo. I have yet to experience BB6P being "wrong". It has consitently proven to be quite valuable as a tool. To think I used to spend days and days crunching numbers for every little project, what a waste of time! Oh well, at least it causes me to appreciate these silly little software packages that much more

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