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[Art Welter]

The JBL SRX 728 is rated at 98 dB, one watt one meter, +/- 3 dB 33-220 Hz. The 728 dimensions are 23.7 x 42 x 33 inch, 166.5 lb., 19 cubic feet of truck space.

Last night I finally got to do a side by side measurement with one of my dual Eminence Lab 12” sub and the 728 after doing a bunch of system alignment work for a DJ.

For some time I had been curious how the two would compare, as there has been some ambiguity regarding whether the JBL SRX 718 and 728 sensitivity was in half space or whole space.

The published specifications are indeed half space as is common for sub testing.

My 2x12” is 22.5 x 22.5 x 26.5 inch, 100 lb., 7.76 cubic feet of truck space, less than half the size of a 728, more comparable in size and output to the 718.

The measurement was done with the microphone on the ground 5 meters from the subs. Both subs remained in place for the test, the one not in use had the terminals shorted. Speaker cords were swapped at the amplifier output.

A 25 Hz BW HP filter and a LP filter (160 Hz ?) were  used. The results of the measurements correlate quite well to the JBL published chart, with the exception of the filter response, and the test microphone being about -2 dB at 30 Hz.

From a sensitivity standpoint, the 2x12” is lower, a bit over 3 dB down in the lower end, and about 8 dB down in the upper range.

When a pair of  2 x12” cabinets are used, (as shown in the lower graph) the sensitivity goes up by 3 dB, so the low end output is about the same, with less distortion, though the SRX 728  has more upper output at the same power level.



Conclusion: The Lab 12 is a viable choice for good low frequency output in small ported cabinets, comparing favorably to products costing far more.

But now I know the  one watt one meter sensitivity of four front loaded dual Lab 12s is only around 98 dB, I want a pair of DSL TH -218, their sensitivity is 10 dB higher !

Art Welter

[Phil Lewandowski]

Hey Art,

For some reason I remember you having a calibrated testing setup?  Is there any way you are able to use that to measure an SRX728 against a control sub that has a known accurate response chart that we could line the control sub up to, to know that the calibration is correct and such?


Then we could know pretty close to for sure, once and for all, what the SRX718 and SRX728 sensitivities are.


Thanks!
Phil

[Art Welter]

Phil Lewandowski wrote on Wed, 19 January 2011 20:45

Hey Art, For some reason I remember you having a calibrated testing setup?  Is there any way you are able to use that to measure an SRX728 against a control sub that has a known accurate response chart that we could line the control sub up to, to know that the calibration is correct and such? Then we could know pretty close to for sure, once and for all, what the SRX718 and SRX728 sensitivities are. Thanks! Phil

Phil,
I have attempted to calibrate my Smaart readings, but am not sure if they are absolutly on.
I have not attempted to calibrate them with the RTA 420 microphone I was using in the above test.
I have  written several times that I can’t get my 3 meters to agree at more than one frequency..

The 728 is owned by a DJ that I seldom see, so I doubt I’ll get any more test chances for a while.

We discussed the issue of 1/2 vs. whole space  previously (message #594392), but to refresh, there are several charts here:

http://www.audioheritage.org/vbulletin/showthread.php?11346- 2268H

that clearly show the 2268 in an 8 cubic foot sealed box in half space, on the JBL  8400 building roof. The sensitivity above 70 Hz is clearly around 96 dB, just as on the SRX718 charts.

Porting will raise the sensitivity below the sealed box roll-off, dependent on Fb and box size, but in an undersized box (less than 8 cubic feet per cone), the sensitivity would not be more than 95 dB, unless a very high tuning (Fb up at 60 hz or so) was used.

The 728, having 2 cones and about double the box volume, but a slightly lower tuning, has overall 3 dB higher sensitivity than the 718, but the low end starts dropping a little higher, and goes a little lower.

Eminence tests their speakers in a wall baffle at one meter in an anechoic 2700 cubic foot chamber, the Lab 12 sensitivity around 100 Hz is 89.2, so a pair is 92.2 dB. My cabinet actually has a bit of a low end bump up, ( slightly small box tuned higher than Fs) so the sensitivity is more like 93 dB. The microphone I was using  in the test in the post above rolls off a bit down low, so the response looked more flat than it actually is.
Here is the Lab 12 above the 2268H chart:



The above information is conclusive that the JBL SRX sub charts are done in half space.
I trust the JBL and Eminence charted figures as accurate, other than a few "clerical errors" that inevitably may show up.

Believe me, I wish my subs were 6 dB hotter than they are, but the truth is they are only 92 dB one watt one meter or so. The JBL charts are correct, and done in half space.

That said, the Lab 2x12" is flat over it’s intended use range, and it can take full power  (49 volt sine wave) with less than 10% distortion from 30 to 50 Hz, significantly more than the JBL SRX 718, or anything else in a similar size range we tested so far.

Anyway, time to calibrate our dB meters   .

Art Welter

[Phil Lewandowski]

Art Welter wrote on Thu, 20 January 2011 15:20

far. Anyway, time to calibrate our dB meters   . Art Welter

      Yeah no kidding, especially mine I would be very curious how it would stack up, since I figured it was not exactly on, but figured it was with 1dB or so at most frequencies.  But for it to be 5-6dB is a way to much.

The thing that still gets me is how 2 subs with so different of a sensitivity would measure so close in the so many ways I measured them on several occasions.


And I remember the long discussions we have had where we looked at the response you posted of the 2268 and such, it is so hard as there is evidence on both sides, but of course I would be inclined to trust something like that that JBL published.

Still would really like to see a 3rd party test with a calibrated meter and a control sub or 2, to settle it.



~Phil

[Art Welter]

Art Welter wrote on Wed, 19 January 2011 17:00

Conclusion: The Lab 12 is a viable choice for good low frequency output in small ported cabinets, comparing favorably to products costing far more. But now I know the  one watt one meter sensitivity of four front loaded dual Lab 12s is only around 98 dB, I want a pair of DSL TH -218, their sensitivity is 10 dB higher ! Art Welter

Oops..
That should have read "I want a pair of DSL TH-118, their sensitivity is 10 dB higher".
The TH-118 is rated at 108 dB 2.83 v, so the four ohm cabinet is 105 dB one watt one meter, a pair 108 dB one watt one meter.