I had an interesting experience yesterday.
I have been helping a friend repair some derelict JBL woofers- mostly loose magnets which we've been re-attaching, and then reconing said drivers. I took one of the test subjects- a 2245H- and tested it on the LMS/LEAP system.
Well, as I said, it was VERY interesting. As in, how CLOSE the specs MATCHED original!
Now, keep this in mind when looking at the specs- I DELIBERATELY altered the cone mass of the reconed unit, a bit. I treated the cone with a lacquer stiffening agent, which added about 11 grams to the "stock" aftermarket cone. See how that plays into the specs... really, all it did, was to marginally lower the broadband efficiency about 1/2 dB, while reducing the need for bass EQ on the lowest bottom end... which was what I was intending.
OK, here's the data:
Stock 2245H (from specs on file in various locations), from a broken-in unit:
Fo=20 Hz
Vas = 820 liters = ~ 28.9 cubic feet
Qes= .31
Qms= 2.2
Sd = 1300 sq.cm =~ 201 square inches
Xmax = 9.5mm
Re= 5.8 ohms
Mms= 182.7 grams
Bl= 20.73
Qts= .2717
Sensitivity @ 1w= 95.3 dB
OK, here's the reconed unit data, from LMS:
* Loudspeaker Measurement System
* LMS(TM) 4.5.0.342 May/19/2004
* (C)opyright 1993-2000 by LinearX Systems Inc
* Electro Mechanical Parameters
* Oct 27, 2004 Wed 7:34 pm
* Library=gordon-jbl.lib
* Reference Curve=2245h
* Delta M/C Curve=2245h w/42.0 grams
* Method= Double Curve - Delta Mass
Domain= FreeAir
Model= TSL
Revc= 5.560 Ohm
Fo= 21.915 Hz
Sd= 129.677m M² (= 1296.77 sq. cm)
Krm= 10.286m Ohm
Erm= 0.739
Kxm= 23.029m H
Exm= 0.704
Vas= 23.089 Ft³
Cms= 273.802u M/N
Mmd= 165.773m Kg
Mms= 192.625 g
BL= 20.929 T·M
Qms= 7.179
Qes= 0.337
Qts= 0.322
No= 1.976 %
SPLo= 94.977 dB
Now, the reconed unit is not broken in yet- which handily explains the 10% difference in Fs, the difference in Qms and the 25% or so difference in Vas . But, most tellingly- LOOK at the Bl product, the sensitivity, and the cone mass! Without the mass increase I added, it would have been IDENTICAL TO WITHIN ONE GRAM of stock. And the sensitivity, by definition, would have been IDENTICAL to stock, since the Bl and cone area are EXACTLY the same (the Bl is actually about 1% HIGHER than stock, actualy, and the cone area is completely insignificanly different in any reasonable measurement system).
Also, I verified some other things, during assembly:
1. The new voice coil is indeed EDGE WOUND, and is in the correct (inverse) polarity like the original (+ on red makes the cone go IN).
2. The dimensions of the aftermarket coil, cone, surround and such are IDENTICAL to an original kit (I directly compared them to a stock assembly). As in, the voice coil overhang is 3/8" in each direction... which would result in a 9.5mm X-max, same as stock.
What this means? The aftermarket kit, in a SUBWOOFER application, should SOUND IDENTICAL to the stock unit, once it's broken in.
And the nice thing? This was a kit that is SPECIFIABLE BY UNIT MODEL NUMBER from one of the most prolific aftermarket reconing parts outfits out there. Nice to see 'em get it right, at least once...
Regards,
Gordon.