View Full Version : C 53 "Libra" Tuning - (Fb) ?
06-11-2003, 08:22 PM
Does anyone know the official Fb ( box tuning ) for the C-53 enclosure ?
Is it the same tuning as used in the Lancer (S) 99 ?
I have a pair of ancient S99 s . Both enclosures measure slightly different.
One's Fb is @ 44 hz while the other is about 42 hz .
And for those who want to figure out JBLs' reliance on that fiberglass insulation for virtual box volume, here are some S99 dimensions ;
The enclosure has internal measurements of 22" x 12.75" x 9" with a single 4" diameter port that is 7.25" deep . I use a displacement factor for the le14a of .15 cu' ( this is borrowed from the displacement spec given for a 2206H ). I believe the little crossover occupies about .025 cu' of internal space . I'm ignoring the le20 magnet at this time .
regards <> Earl K
06-11-2003, 08:36 PM
"I have a pair of ancient S99 s . Both enclosures measure slightly different.
One's Fb is @ 44 hz while the other is about 42 hz."
Are you using the same transducer to measure both enclosures?
"And for those who want to figure out JBLs' reliance on that fiberglass insulation for virtual box volume"
When "JBL" tunes an enclosure "they" usually start around 30 Hz and adjust accordingly during extensive listening and measuring tests. The expectation with the fiberglass is that it will negate the displacement volume of ports/ducts, drivers, subenclosures, bracing, etc.
06-12-2003, 05:45 AM
posted by Giskard
"Are you using the same transducer to measure both enclosures? "
I'm after JBLs' official target frequency for the port tuning of the stock box. I'm curious whether it was 40 hz or 42 hz, etc. This is purely of academic interest. In fact, right now, both of my enclosures have Fb figures that measure in at @ 42 hz . This is only because I've added extra fiberglass stuffing into one cabinet to increase its' virtual volume .Though listening to a system with different box tunings is fine by me - it's a primitive version of stagger tuning. But a port tuning of 44 or 45 hz, sits a little high for my listening environment - mostly because of the coupling between the systems effective LF response curve and room loading .
regards <. Earl K
06-12-2003, 07:12 AM
The only reason I asked if you used the same transducer to measure both volumes is because if one transducer had an issue with it's compliance (leaks or cracks) it could throw your measurement off. Also, if one enclosure isn't as "solid" (leaks or cracks) as the other it could throw your measurement off.
Fb for the LE14A & LE14C in a 1.5 to 2.0 cubic foot volume in that era was 42 Hz. It was later changed to 32 Hz. (3" port x 8" duct).
06-12-2003, 08:37 AM
I think you know too much about the decriptitude of some of my old le14a woofers LOL ;) and their "refurbished" LansaLoy surrounds .
To answer your question directly (sort of ) ; Over the last 6 months of measurements , it's been pretty obvious that both enclosures have measurably different Fb(s). This is so, even when the same or different le14a woofers drive the ports. All the various permutations & combinations between 3 ( usable le14a ) woofers & 2 cabinets have been tried. Also I went back this morning and measured the same woofer in the 2 boxes ( plus other combinations ). My results have matched ( the theory ) that Fb is a stand alone box measurement when driven by similar diaphragms/motor structures. Now, I have measured "lossy" transducer fits in my larger test box ( @ 2.7 cu' ). This was caused by a poorly made ( a few misplaced T-nut holes ) adapter plate that held a le14h in place of the standard 15". The measurement manifested itself as an inability to "close the ellipse" in the expected Fb area.
I've had little to no success in determining whether or not, room loading effects are measurable as a sort of reflection back into the oscilloscope by way of the diaphragm/tuning duct . Thoughts ?
Thanks for the official Fb info !
regards <> Earl K
06-12-2003, 09:10 AM
What is the trick question?
"My results have matched ( the theory ) that Fb is a stand alone box measurement when driven by similar diaphragms/motor structures."
True, a 5.0 cubic foot enclosure tuned to 30 Hz will have nearly identical Fb when measured with a 2231, 2235, 2215, E130, E140, etc. So long as each of those transducers is structurally intact and free from excessive loss.
"I've had little to no success in determining whether or not, room loading effects are measurable as a sort of reflection back into the oscilloscope by way of the diaphragm/tuning duct. Thoughts ?"
Reflective surfaces would need to be quite close to the port to have any effect. The usual recommendation is all surfaces should be at least one port diameter length away in order not to adversely affect tuning. Some systems violate this and their duct lengths are adjusted accordingly. As for transducers, they are basically big "microphones", so any disturbance in the measurement area can also affect accurate measurment of the tuning frequency.
06-12-2003, 09:49 AM
transducers ... are basically big microphones
Off topic (apologies...) but one of my personal favorites: in a pinch at the soundboard, desperate to get the bands ever-flagging attention, I have used the headphones as a "talk back" mic. Never ceases to blow their feeble minds...
Now I just plug my AudioTechnica ATR3M into the board - a more appropriate solution! ;)
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