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I like the design can you post that too. |
Bennett Prescott wrote on Mon, 01 November 2010 10:16 |
Loren, P.S. Beautiful carpentry. |
Loren Jones wrote on Mon, 01 November 2010 08:27 | ||
I would love to post the design, however it isn't mine to post. I can put you in touch with the LABster who designed them if you would like to get the design from him. He may have some slight modifications that he has made since my Fb came out a little lower than the design called for. I don't know about that. Perhaps he can chime in here if he cares to regarding his comments on the design or other thoughts. Take care, Loren Jones |
Loren Jones wrote on Mon, 01 November 2010 15:29 |
The model of this design predicted 128db max continuous output at 49hz. The driver is rated for 1500 watts continuous based on thermal capability. So it will take some power to exploit it fully, but I don't think it is any less efficient than other single driver reflex loaded cabinets. |
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0. I, in NO WAY, put Loren up to this. I didn't even see the thread until today, apparently a day old. I am glad Loren is enjoying his new baby. |
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3. Loren took a risk on building this design. I say took a risk because this came straight from a model. Even with the more advanced lumped parameter model, the port tuning is rarely absolutely correct. There was no guarantee the tuning frequency would be correct for this tapered port geometry. Loren went into the project with eyes wide open about the potential of the box Fb to be wrong. |
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At the end of the day, people should be realistic that this is still a vented box. While I feel confident that this box exhibits real extension to 35Hz, and output on par with a high-quality dual 18", those expecting to move mountains and cause earthquakes have read too many manufacturers spec sheets. |
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Should all those measurements ever get taken (by me or a vetted 3rd party), then I would figure out what to do with the design. More than likely I would sell it, not so much to generate revenue, but to discourage "non-serious" builders from approaching the project. I am not interested in becoming "tech support" for a vented box design |
Loren Jones wrote on Mon, 01 November 2010 19:52 |
I will do all I can to get the box either up to Evan or down to Phillip for proper measurements. |
Loren Jones wrote on Mon, 01 November 2010 19:52 |
I currently have a borrowed SRX-728 and the aforementioned CE4000 amps both loaned to me by another exceedingly kind LABster, Mark Phillips. Hopefully we will be able to get at least some meaningful comparisons to the 728 taken under the same conditions. The tests won't be lab quality but hopefully it will give some idea of how this design compares to what is generally regarded as the default standard mid-level dual 18" vented sub. |
Phillip Graham wrote on Wed, 03 November 2010 09:43 | ||
I found the JBL 2268H on the Altec Heritage website: http://www.audioheritage.org/vbulletin/showthread.php?11346- 2268H On pp. 5 of the datasheet, JBL shows the Klippel Bl(x) curves. The industry standard for defining Xmax for low frequency drivers from Klippel measurements is to the excursion where Bl is 70% of Bl_max. By this standard definition, the JBL 2268H has a one-way excursion of 9mm. Plugging the JBL driver into a box model,using the box volume calculated from the JBL 728 datasheet, and assuming the JBL engineers would pick an intelligent alignment, I made a quick comparison of the single 21 vs the dual 18". It appears that the JBL dual 18 will have similar output at 100Hz, +2dB output at 50Hz, and -2dB at 35Hz relative to the single 21" when both systems reach their respective Xmax. This is very similar performance--certainly room response error and driver batch variability would have more measurable influence. The 21 requires additional input voltage to reach the same output level, obviously. Since we often push loudpeakers beyond Xmax, it will be interesting to see which of these designs holds their subjective composure better under additional stress. My gut is that the dual 18 will fare a little better here. The single is 21 approx. 70% the size of an SRX 728, so if there is any take home story message here, it is that 21" based designs are probably here to stay. |
Josh Ricci wrote on Thu, 04 November 2010 10:10 |
Unfortunately my tune came in much lower than I had anticipated. I had aimed for 30hz (yes I know. The application is a little different from normal) and hit 25hz instead due to loading of the port against the rear wall and proximity of some internal bracing. |
Phillip Graham wrote on Thu, 04 November 2010 09:47 |
Cabinet impedance testing at different values of large signal V_in is the most straightforward way, in my mind, to see the port compression effects, and well as shifts in port tuning frequency due to air behavior in the port with increasing Reynolds number. |
Art Welter wrote on Thu, 04 November 2010 12:19 | ||
Phillip, Could you clarify a few points: |
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How do you define "port compression effects" ? |
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My understanding is the port tunes the enclosure fB (box frequency), when you say "shifts in port tuning frequency" are you referring to a fB shift or something else? |
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Does that frequency shift go up, or down with increased turbulence (higher Reynolds numbers) ? |
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 |
Phil Lewandowski wrote on Thu, 04 November 2010 16:18 |
Hey Phil, 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) |
Ned Ward wrote on Mon, 01 November 2010 13:48 | ||
+1 on that - love all the pocket screws. Norm Abrams from the New Yankee Workshop would be proud. |
Steve Hurt wrote on Thu, 04 November 2010 16:55 |
I thought JBL specs were full space. |
Steve Hurt wrote on Thu, 04 November 2010 17:08 |
It would make sense, but things don't always make sense! I'm pretty sure the 718 and 728 are full space numbers. |
Phil Lewandowski wrote on Thu, 04 November 2010 14:18 | ||
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 |
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!) |
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. |
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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... |
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! |
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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. |
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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. |
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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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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. |
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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 |
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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. |
Steve Hurt wrote on Thu, 04 November 2010 20:39 |
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! |
Steve Hurt wrote on Sat, 06 November 2010 00:16 |
LOL! If you turn the Orbit shifters all the way up in a club I work, the owner will kick you into orbit before the 1st verse is sung. Speaking of PRX... Here's something from the PRX518 spec sheet: Maximum Peak output measured with IEC pink noise at 1 meter in front of speaker baffle under free space conditions. Measurement instrument set to peak hold. Speaker muted and released at full power, recording maximum peak level. Free space = full space isn't it? |
Phillip Graham wrote on Sat, 06 November 2010 10:29 |
Phil, Free field is a non standard term, and potentially chosen to obfuscate. |
Loren Jones wrote on Mon, 01 November 2010 17:52 |
I currently have a borrowed SRX-728 and the aforementioned CE4000 amps both loaned to me by another exceedingly kind LABster, Mark Phillips. Hopefully we will be able to get at least some meaningful comparisons to the 728 taken under the same conditions. The tests won't be lab quality but hopefully it will give some idea of how this design compares to what is generally regarded as the default standard mid-level dual 18" vented sub. Hopefully we can get those comparisons done this weekend perhaps. I will do all I can to get the box either up to Evan or down to Phillip for proper measurements. I want to do this out of curiosity to see what the real performance is of "my baby". Also I think that if good measurements help refine the design so that a really professionally designed and documented reflex loaded sub can be made available for the DIY types, then that is a good thing. Thanks again for Phillip in kindly providing me with this design and lots of handholding along the way. Thanks also to Jeff for the good deal on the drivers and to Mark Phillips for loaning me some gear to compare them to. |
Phillip Graham wrote on Sat, 06 November 2010 09:29 |
Go back to the 2268H datasheet linked above, where the driver was measure on the roof of one of the jbl/harman buildings. Id wager that this is SOP for all the lf devices here. |