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Author Topic: RCF 21" Sub Build Pics and Impressions  (Read 22702 times)

Steve Hurt

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Re: RCF 21--Designer comments - Quick side question-2268H
« Reply #40 on: November 04, 2010, 05:18:02 pm »

<edit - should have quoted - the post I answered said that the output would have been lower if JBL's were measured in full space>

Efficiency in half space is higher, not lower isn't it?

(I'm not an expert on this stuff by any means.  Wouldn't be the 1st time I was wrong!)
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Steve Hurt

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Re: RCF 21--Designer comments - Quick side question-2268H
« Reply #41 on: November 04, 2010, 05:20:20 pm »

My belief is that Phil's numbers are showing the increase that efficiency/output that half space loading provides over full space.
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Art Welter

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Re: RCF 21--Designer comments - Quick side question-2268H
« Reply #42 on: November 04, 2010, 05:25:16 pm »

Phil Lewandowski wrote on Thu, 04 November 2010 14:18

Phillip Graham wrote on Wed, 03 November 2010 10:43



I found the JBL 2268H on the Altec Heritage website:
      http://www.audioheritage.org/vbulletin/showthread.php?11346- 2268H



Quick question on the 2268H driver.  On that spec sheet, I see that with a 8 cubic ft. sealed box in 1/2 space and 2.83v it averages around 95dB under 100hz.

Now when you take the SRX718 outside in 1/2 space and measure it with 2.83v you get closer to around 100dB.

Would this raise in efficiency is attributed to the porting?  Because the SRX718 is still about 8 cubic feet.  I don't remember porting having an effect like that, but I could have missed it.

Just curious because the SRX718 spec sheet shows similar to the 2268H sensitivity, but when I have measured the SRX718 both at 28.3v at 10M and 2.83v at 1M, it averages about 99-101dB under 100hz.  (SRX728 between 102-104dB with 2v)
Thanks,
Phil


A port will raise the output of the box around 3-6 dB around fB, then the output will drop at a nominal 24 dB per octave.

Sealed, the cabinet will drop off at around 12 dB per octave.

Lowering fB without a large enough box will also make the response “droop” down low, the lower left shot is the SRX 728, Phil G. estimates using a 35 Hz Fb.
Note how the 728 (also around 8 cubic feet per driver) response is a bit between the 718 and the sealed box.

The question of half space or full space is hard to assess, looking at the 718 (we assumed to be full space, looks like not), which you found to be  40 Hz fB  (upper left), compared to the 2268 eight cubic foot “half space” response, it appears the nominal levels are the same above 100 Hz, and the difference between the two are what one would expect, port gains about 3 dB at fB, then the sealed unit has about 6 dB more output an octave below.

index.php/fa/33549/0/

Your reading 6 dB high kind of looks like a sound level meter difference now.

Of course, I can't get my 3 meters to agree either  Sad .

Art Welter
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Phillip_Graham

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Re: RCF 21--Designer comments - Quick side question-2268H
« Reply #43 on: November 04, 2010, 05:26:39 pm »

Steve Hurt wrote on Thu, 04 November 2010 17:18

<edit - should have quoted - the post I answered said that the output would have been lower if JBL's were measured in full space>

Efficiency in half space is higher, not lower isn't it?

(I'm not an expert on this stuff by any means.  Wouldn't be the 1st time I was wrong!)


I decided to tone down my unfortunately rather testy response to your assertion...  My apologies for being curt!

The efficiencies calculated earlier from the T/S parameters assumed half space loading for both drivers, match JBLs measured behavior, and reflect the industry standard for measuring LF sensitivity.

It is exactly moments like this that highlight just how difficult taking accurate measurements is, and how miscalibrations are easy when you don't have reference standards.
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Steve Hurt

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Re: RCF 21--Designer comments - Quick side question-2268H
« Reply #44 on: November 04, 2010, 05:30:33 pm »

No need to apologize to me!  I appreciate your posts and have learned quite a bit from you. (I need to learn a LOT more, still "tip of the iceberg" on this stuff!
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Phil Lewandowski

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Re: RCF 21--Designer comments - Quick side question-2268H
« Reply #45 on: November 04, 2010, 08:37:40 pm »

I gotcha Phil,

Ya the reason I am curious is because I have done the test several times with my SRX718 and my JTR Growlers, both using the same test equipment, and the JTR Growler ended up being almost exact to what Jeff Permanian's published response is.  While the SRX718 measured a bit higher than what JBL publishes.


How I did this was very simplistic, so of course nothing official from it.  Just by using straight test tones straight through.  First calibrating with a volt meter (Actually 2, mine and my fathers.) so that I saw 2.83v at the output of the amp, using 60hz just in case the meters weren't as accurate at other frequencies.


Then just used a RatShack SPL meter placed on the ground either 1M or 10M in front of the subs.  And would then play the different tones without touching anything and took down the SPL.  (All was done at least 125ft. from the single building in the area.)


I wouldn't be as interested, but since the JTR Growler measured right at what Jeff published, I was more interested by what I got with the SRX718.


So it would be interesting to see what the SRX718 and SRX728 would measure with a calibrated rig.



Take Care,
Phil


P.S. Plus to make it interesting, several guys have e-mailed JBL tech, and we have all gotten the response that the entire SRX line is measured in full space, not just the top boxes.  But I guess you have to take what some of them say with a grain of salt...
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Steve Hurt

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Re: RCF 21--Designer comments - Quick side question-2268H
« Reply #46 on: November 04, 2010, 09:39:24 pm »

Phil Lewandowski wrote on Thu, 04 November 2010 20:37

....the reason I am curious is because I have done the test several times with my SRX718 and my JTR Growlers, both using the same test equipment, and the JTR Growler ended up being almost exact to what Jeff Permanian's published response is.  While the SRX718 measured a bit higher than what JBL publishes.


This is the exact reason I looked into full vs half space on JBL SRX subs.  If SRX718s was rated in half space, the Growler should destroy a SRX718 at similar power levels, but instead, they are pretty similar in output even though the 718 is rated 6 db lower.

Quote:

  P.S. Plus to make it interesting, several guys have e-mailed JBL tech, and we have all gotten the response that the entire SRX line is measured in full space, not just the top boxes.  But I guess you have to take what some of them say with a grain of salt...


I do believe the SRX's were rated in full space, however, some of the other lines like the PRX subs for instance, must have been rated in "Outer Space" to get their output ratings!
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Loren Jones

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Re: RCF 21--Designer comments
« Reply #47 on: November 04, 2010, 10:35:41 pm »

Josh,

Thanks for the kind words.  I will be interested to see your data from your reflex loaded subs and it would be very cool to see your horn loaded design for your 21's.

Loren
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Loren Jones

Josh Ricci

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Re: RCF 21--Designer comments
« Reply #48 on: November 05, 2010, 01:44:36 pm »

Phil,
I used hornresponse mostly for the modeling with some investigation in akabak but I am still on the learning curve for akabak. Anyway I used a 10" port with 0.75" roundover at both exits. The proximity to the back wall is the cause of the large shift down over the model. I had left only about 9" clearance if memory serves and the port is also baffled internally as part o fthe bracing. I thought it would drop the tune at most only marginally. Wrong!

Unfortunately I did not do impedance measurement at varying power levels but I did do power compression tests using sine waves of long duration and I did get a base impedance curve. Port compression and some shift was evident at the highest drive levels.

I am curious why would I want to downgrade slightly to the lighter duty sw115.? (If you could possibly call it that in seriousness) The 21sw152 is their top of the line with the 15mm xmax rating and 6" split wind coil. Btw a 18sw115 was tested in vc mag and did very well. Id expect similar or better with the 21's.
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Phillip_Graham

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Re: RCF 21--Designer comments
« Reply #49 on: November 05, 2010, 02:34:09 pm »

Josh Ricci wrote on Fri, 05 November 2010 13:44

Phil,
I used hornresponse mostly for the modeling with some investigation in akabak but I am still on the learning curve for akabak. Anyway I used a 10" port with 0.75" roundover at both exits. The proximity to the back wall is the cause of the large shift down over the model. I had left only about 9" clearance if memory serves and the port is also baffled internally as part o fthe bracing. I thought it would drop the tune at most only marginally. Wrong!


I use Akabak very infrequently, fantastic manual for high level learning, though!  Hornresponse is powerful and fairly useful, though it rarely seems to nail port tunings.  It is FAST for just playing around.

Quote:


Unfortunately I did not do impedance measurement at varying power levels but I did do power compression tests using sine waves of long duration and I did get a base impedance curve. Port compression and some shift was evident at the highest drive levels.


I am curious which way the tuning shifted.  If I had to guess, based on the state port area of 500 cm^2 (per driver?) is that it first went up, and then back down slightly at very high power levels?

The Le(x) curve variation for these high power professional drivers is usually so small that the impedance data, even at large excursions, will give you more information about the port Q and tuning frequency than almost any other measurement.

Quote:


I am curious why would I want to downgrade slightly to the lighter duty sw115.? (If you could possibly call it that in seriousness) The 21sw152 is their top of the line with the 15mm xmax rating and 6" split wind coil.


Oh, my mistake, I was under the impression that the 6" VC 21 did not have the variable "split" vc winding, but it appears that it does.  Thus it should should have the excellent Bl(x) behavior of the SW115 with more power handling.  Oops!  Apparently there was a discontinued 21" driver that did not have the split coil.

That would now make the 21SW152 the top dog Smile

Quote:


Btw a 18sw115 was tested in vc mag and did very well. Id expect similar or better with the 21's.


The test in VC is how I became aware of the split winding drives.  Fantastically even and symmetrical Bl(x) and kms(x).  Good Le(x), too.
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