View Full Version : Measuring Fb...

09-06-2003, 05:25 PM
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 :eek:, 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... ;) )?

09-06-2003, 05:36 PM
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.


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

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

09-06-2003, 05:45 PM
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. :thmbsup:

Full report to follow!

10-12-2003, 08:03 PM
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 (http://www.audioheritage.org/vbulletin/showthread.php?s=&threadid=685&perpage=15&pagenumber=4)

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 :yes: (sorry, had to work that in... ), and may beat the original L65, too. :hmm:

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... :slink:

10-13-2003, 08:05 AM
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 :)