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Title: RCF 21" Sub Build Pics and Impressions
Post by: Loren Jones on October 31, 2010, 09:48:18 pm
Hi folks,

Some may remember that I was asking for advice here a couple months ago on building a couple standard front loaded subs.  I ended building a pair of subs using RCF 21LFN451 drivers  http://www.rcf.it/en_US/products/precision-transducers/neody mium-low-frequency-transducers/lf21n451

The general shape of the enclosure is similar to a TH-118 or to a JTR Orbit shifter (see pics).  Around 32" tall and 22" wide or so with wheels on the back corner for tilt-back rolling.  I have been attempting to get some measurements of them but don't have gear or experience to do that properly.  I was trying to get some measurements with a demo copy of Smaart today and couldn't manage to do it, so sorry for that.  All I have is pics and listening impressions.  I do know that Fb came out at 32hz.

I have used them on really only one occasion which didn't require any significant output (music playback and some acoustic acts).  Overall I think I would describe the sound as characterized by deep extension, tight rather than boomy, and quite musical.  I did power them today off of a bridged Crown CE4000 using the DSP recommended by the LABster who designed the enclosures for me.  They handled music program with the clip lights on the CE4000 blinking lightly and there was no audible sign of distress that I could hear.

I was also going to add that the total cost to build a pair came in around $1400 including 3 sheets of 18mm baltic birch, handles, feet and wheels, duratex roll on speaker paint, screws, t-nuts, speakons etc.  I am very happy with the final product overall.  That $1400 number does take into account the fact that I got this pair of unused drivers from another LABster for a reasonable discount off of retail new price ($800 for the pair).

Anyway here is one pic and more in the posts to follow.

Thanks to all who offered advice in helping me decide what to build and for the design itself.

edit:  added paragraph about cost

Loren Jonesindex.php/fa/33455/0/
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Loren Jones on October 31, 2010, 09:49:03 pm
More picsindex.php/fa/33456/0/
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Loren Jones on October 31, 2010, 09:49:35 pm
more picsindex.php/fa/33457/0/
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Loren Jones on October 31, 2010, 09:55:07 pm
more picsindex.php/fa/33458/0/
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Loren Jones on October 31, 2010, 09:55:46 pm
index.php/fa/33459/0/
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Loren Jones on October 31, 2010, 09:56:31 pm
and the drivers with an appropriate item for size comparisonindex.php/fa/33460/0/
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Steve Hurt on October 31, 2010, 11:20:38 pm
Nice work!
They need pole sockets on the top, handles on the bottom.  
I like the way  the grilles look.  Good job.
How much do they weigh?

Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Benjamin D. Hill on October 31, 2010, 11:41:48 pm
i bet that there will be people willing to measure them at the mid west lab get together early next year. just a though.
I like the design can you post that too.
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Evan Kirkendall on November 01, 2010, 12:44:55 am
Is there any internal bracing inside of them?

I have another modified version of those plans that I'm planning to build sometime soon when I have some time... If you're willing to travel, I can easily measure them, and see what they can really do!



Evan
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Loren Jones on November 01, 2010, 07:19:47 am
Yeah, pole sockets will be added. Don't know why I forgot those initially. Theatre handles should have been on the bottom. Dumb choice on my part. They can't be all the way at the bottom though because they would be inside that big beautiful port I went to so much trouble building. But those handles way at the top aren't that useful.

Thanks.
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Loren Jones on November 01, 2010, 07:33:22 am
Evan,

They do have some internal bracing between the side walls and the top and back of the cabinet.  I wouldn't say they are extensively braced compared to what I have done for some home subs in the past.  I could always add more bracing if I needed to.

As far as building a modified version of these, I am sure you would be happy with them (a dozen of them that is Smile ).  Building the flared port was the only mildly difficult part.

As far as measuring them I may take you up on that.  I will PM you.  Would love to hear your new Mackie HD's as well.  I have been wishing our church would buy a pair of those 1531's to go on top of these for the youth services.

Thanks,
Loren
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Loren Jones on November 01, 2010, 08:20:37 am
I dunno about the weight for sure.  I can pick one of them up by those top handles and put it in my minivan.  I would guess they weigh 100lbs or so but that is just a guess.
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Loren Jones on November 01, 2010, 08:27:16 am
Quote:

I like the design can you post that too.


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
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Jeff Knorr - Cobra Sound on November 01, 2010, 08:44:22 am
Hi Loren,

Congratulations on the subwoofer build!  I'm glad that the drivers found a good home.

Please keep us posted if you get any measurements of the cabinets.

Thank you,

Jeff
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Franz Francis on November 01, 2010, 09:51:54 am
Not trying to hijack your thread,
Eighteen Sound is offering a dual twenty one inch design on their website, the specs don’t look too efficient, wondering if anyone made an attempt to built this box.
http://www.eighteensound.com/staticContent/applications/kits /kit21_dual.htm

From what I heard Adamson’s huge T21 is clone of their design, just bigger and heavier.

http://www.adamsonsystems.com/products/yaxis/t21/t21.php


Franz
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Bennett Prescott on November 01, 2010, 01:16:34 pm
Loren,

I'm jealous! I hope I'll have the opportunity to hear them sometime, "that LABster" talks a lot of shit, I'm curious to hear how the math worked out. Twisted Evil

P.S. Beautiful carpentry.
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Ned Ward on November 01, 2010, 01:48:59 pm
Bennett Prescott wrote on Mon, 01 November 2010 10:16

Loren,

P.S. Beautiful carpentry.

+1 on that - love all the pocket screws. Norm Abrams from the New Yankee Workshop would be proud.
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Loren Jones on November 01, 2010, 03:17:07 pm
Thanks Jeff for the good deal on the drivers.  I will definitely post some measurements if/when I get them.
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Loren Jones on November 01, 2010, 03:29:24 pm
I believe that the RCF spec for sensitivity of the driver is 98.5 db but I didn't see on their website the test conditions for that.  So that would be pretty similar sensitivity to or perhaps a little higher than an average single 18".  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.

My $.02 on the efficiency question.  I don't know about those commercial offerings.
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Loren Jones on November 01, 2010, 03:36:55 pm
Well Phillip errr I mean that Labster's excellent sketchup drawing of the design made it super easy to build.  Everything was measured down to the millimeter on the drawing.  I just had to set the ripfence and start cutting.

Thanks for the kind words on the carpentry.
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Loren Jones on November 01, 2010, 03:38:43 pm
I was gonna do biscuits or put cleats in the corners for assembly, but a friend of mine suggested going with the pocket screws.  I am really glad he did.  It made assembly easy and precise.  I think there are about 250 pocket screws between the pair of them.
Title: Re: RCF 21--Designer comments
Post by: Phillip_Graham on November 01, 2010, 06:51:39 pm
Loren Jones wrote on Mon, 01 November 2010 08:27

Quote:

I like the design can you post that too.


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


Since it appears I have been "outed" on this design, I will offer a few thoughts:

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.  I should say Loren and I have never met, and that I am incredibly impressed with the cabinetry of the finished item!

1. This design came out of a previous LAB thread where I disagreed at length with Art Welter about the principles behind high performance vented box design.

2. In that thread I mentioned the RCF 21" because I realized that it was a very affordable driver for what it offers.  I became aware of this driver as a result of unrelated conversations with fellow LABster Jeff Knorr.  There are better 21s (e.g. B&C 21SW115), but I'd never seen a driver with better specifications for the money at retail.

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.

4. I presented Evan with a modified version of the design to further test the accuracy of the tuning behavior of the model.  To the best of my knowledge he has not built the modified enclosure yet.  The modified design could end up only useful for firewood, model accuracy depending.

5. There are many more detailed measurements that need to be made of this cabinet before I would consider releasing a public design.  Measuring a speaker system with SMAART for system tuning at a gig is much different than the detailed measurements one should take of a professionally designed loudspeaker.

6. Any sort of plans would also include a completed collection of measurements far in excess of what one normally gets from datasheets.  Those measurements would provide unambiguous performance behavior, good/bad/ugly.

7. 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 Very Happy

8. The box design requires 2 bands of in-band parametric eq and a highpass, so any builder would need access to DSP processing.  Also, the box really needs an amplifier capable of 180+ V_peak (360+ V_p_p) to get everything the driver is capable of. For those with small amps, there are probably better ways to go.

9. 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" (what a dual 18 will really produce output-wise, not what the datasheet says).  Those expecting to move mountains and cause earthquakes have read too many manufacturers spec sheets.

10. While the coupling efficiency of a 21" cone is superior to an 18" cone, one should not expect the efficiency of a single motor structure on a 21" cone to match that of two driven 18" motor structures for a given v_input.  I don't expect the "1w" at 1 meter specs to be as high as a dual 18 or tapped horn, even if maximum outputs would be comparable to the dual 18".

11. This new class of super-woofers stretches what we can do with vented boxes.  My modified port geometry seeks to address some of the limitations that now come into play with drivers that have so much Vd.  The goal was to reduce the limitations on box peformance that result as a consequence of turbulent effects in the port while still producing a design with acceptable ease of construction.

-Phil
Title: Re: RCF 21-Designer Comments on output
Post by: Phillip_Graham on November 01, 2010, 07:27:56 pm
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.



The true low frequency sensitivity of this driver should compare very, very well with other single driver vented box systems with similar -3dB points.  The large cone and high BL product are helpful here.  The T/S model gives the driver sensitivity as 97dB.

Also, without knowing the port Q, the box excursion behavior is, for the moment, based on my experience with real-world port Q.  I'd have to have a measurement of the port behavior to check my estimates for port Q.

As for the long term output, a couple points of clarification:

1. 128dB continuous represents the output at V_max =200V_peak at the worst case frequency, excursion-wise, above the box Fb after considering BOTH voice coil heating and reduced magnet flux from a hot driver magnet structure.  This number should be considered a realistic, long-term value after a hard days use.  I have no interest in presenting inflated output numbers.  Peak output before driver damage would be a little more than 6dB above this point.

2. With a cold fresh driver, and the same V_max= 200V_peak, the long term output would be about 1.5dB better.  X_lim is now a touch under 6dB above the improved output value.

If I were to make a spec sheet from the model, I would state the long term SPL at 129dB at 1m, and the SPL peak as 135dB.

Finally, just because the driver's AES power rating is 1500W, that doesn't make the driver happy, or sound good, after being hammered with that much integrated input.  A realistic, long term continuous input power level would be 300-400W in amplifiers that can provide such power limiting (e.g. iTech, powersoft, lab.gruppen PLM).  It should be mentioned that 400W continuous power input is quite high for the vast majority of music signals with a peak level of 200V_peak into to 6 ohms.


-Phil
Title: Re: RCF 21--Designer comments
Post by: Loren Jones on November 01, 2010, 07:52:54 pm
Quote:

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.


This is absolutely true.  Phillip didn't ask me to post the build and certainly didn't ask me to mention or promote him as the designer.  He very kindly provided me with a design that clearly took a fair bit of thought and effort on his part.  He did ask me not to share the details publicly which is very reasonable given the effort and knowledge he put into coming up with it.  

Quote:

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.


It was a smaller risk than I would have taken if I had just tried to make up a design myself using inadequate knowledge and poor modelling software.  While the Fb was a few hz lower than designed I think it speaks pretty well for Phillip's model given that it isn't just a straight rectangular or round port of a certain length.  I frankly am happy to have a little extra extension in exchange for possibly exceeding xmax a little sooner in the 45 to 50 hz range (this is where the box is apparently most excursion limited above Fb).  In our application we will never be pushing these as hard as they can go so the extra extension is a good tradeoff.

Quote:

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.


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.

Quote:

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


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.



Title: Re: RCF 21--Designer comments
Post by: Phillip_Graham on November 01, 2010, 08:01:39 pm
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.


While Evan's offer is gracious and useful, I don't know that he knows how to perform all the measurements I would like to see made.  For instance, I don't know if he has the facilities and experience to do impedance testing at different input powers.

That said, Evan is certainly fully competent on general SMAART measurements, and on comparing performance with the 728.  Certainly he could measure the boxes, compare relative sensitivities, etc.

I look forward to seeing how that comparison turns out.
Title: Re: RCF 21--Designer comments
Post by: Phillip_Graham on November 03, 2010, 10:43:21 am
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.


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.
Title: Re: RCF 21--Designer comments
Post by: Phillip_Graham on November 03, 2010, 02:48:21 pm
Doh, hit the wrong button, double posted.
Title: Re: RCF 21--Designer comments
Post by: Tim McCulloch on November 03, 2010, 08:09:07 pm
Phillip Graham wrote on Wed, 03 November 2010 09:43

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.


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.


Hi Phill-

You nicely illustrate the trade-offs.  It takes more voltage swing for the 21", but the box is smaller and probably lighter.  Amp output is cheaper than it's ever been (the original Peavey CS800 was the same 'cost per Watt' as an ITech 8000).  An amp has much less cubic volume that the 30% sub space.

Thanks for the additional info.

Have fun, good luck.

Tim Mc
Title: Re: RCF 21--Designer comments
Post by: Josh Ricci on November 04, 2010, 10:10:07 am
Nice looking cabs Loren. The build quality especially. I think the rcf 21 is one of the better bargains out there.

Philip a lot of what you posted above regarding the performance and limitations of a vented design are spot on with my way of thinking. Good insight there. I completed a pair of vented cabs housing b&c 21sw152's earlier this spring. 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. I underestimated the amount of shift this would cause. I thought it would be marginal at best. Learned a lesson there. They are still very nice theater or music system subs. Anyway I do have a rather large data set for them taken outdoors that should be viewable in a week or two fwiw. Unfortunately I've never seen any comparable data from a respectable double 18 to compare it to. Next spring I'll be building diy horn cabs for the 21's to test out also.
Title: Re: RCF 21--Designer comments
Post by: Phillip_Graham on November 04, 2010, 11:47:07 am
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.


Did you calculate your port lengths using the "classical" port formulae, or by some other method?  I use models of ducted flow of the style one would see in a program like Akabak.

Even still the values don't come out on the money, mostly because of the end effects you allude to.

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.

I'd suggest switching to the B&C 21SW115, it has substantially more Xmax than the SW152.
Title: Re: RCF 21--Designer comments
Post by: Art Welter on November 04, 2010, 12:19:18 pm
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.



Phillip,

Could you clarify a few points:

How do you define "port compression effects" ?

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?

Does that frequency shift go up, or down with increased turbulence (higher Reynolds numbers) ?

Art Welter
Title: Re: RCF 21--Designer comments
Post by: Phillip_Graham on November 04, 2010, 01:07:08 pm
Art Welter wrote on Thu, 04 November 2010 12:19

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.



Phillip,

Could you clarify a few points:


Sure

Quote:


How do you define "port compression effects" ?


It is a collection of effects.  Let me see if can mention a few:

First effect:
Lets call the acoustic impedance of the port is R + j(w)M where R is acoustic resistance, (w) is angular frequency, and M is mass.  

Mass of air in the port resonates in concert with the compliance (1/stiffeness) of the air in the box as a mass/spring system.  The mass of air in the port is the port's reactive component, and R is the lossy component.

As the Reynolds number increases above about 2000, there is a transition range to fully turbulent flow and eventually at very large Reynolds numbers (200,000+) to a turbulent boundary layer, too.  R increases as the Reynolds number (Re) increases.

Second Effect:
The dramatic increase in R in effect one reduces the suppression of the driver excursion.  Ideally the port air mass resonates out of phase with the driver at Fb, suppressing its movement.  If R is high, the effect is damped and the driver moves more.  This leads to the driver itself producing more harmonic and IMD components.

Third Effect:
Flow out of the end of a open tube does not behave the same way as flow into the end of a open tube, so if you feed, say, a sine wave to a driver, the airflow "waveshape" from the port can be distorted asymmetrically on each half of the waveform.

Fourth Effect:
Vortex formation at the edges of the port to cabinet transition, while technically a subset of the first effect, are significant enough to mention separately.  These contribute to increasing the R in effect one.

Fifth Effect:
The port itself makes extraneous noises (e.g. "chuffing" and "whistling") that contaminate the original acoustic output.

Quote:


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?


Yes, the reactive component of R + j(w)M above can change with different Reynolds numbers, and can change the apparent box Fb slightly at different output levels.

Quote:


Does that frequency shift go up, or down with increased turbulence (higher Reynolds numbers) ?



While technically you might be able to make boxes that exhibit either a raising or lowering effect, generally the behavior of highly undersized ports is to have M decrease as a function of Re, which has the effect of raising the box Fb.
Title: Re: RCF 21--Designer comments
Post by: Art Welter on November 04, 2010, 02:50:25 pm
Phil,
Thanks for the detailed explanation.
Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Phil Lewandowski on November 04, 2010, 04:18:21 pm
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





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)



Thanks,
Phil
Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Phillip_Graham on November 04, 2010, 04:52:41 pm
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)



I am inclined to think that your microphone is improperly calibrated, or your voltmeter is showing an incorrect RMS voltage out of your testing amplifier, or that your measured spl are peak values.  Your numbers seem a little too high.

I would check your mic against someone else's mic calibrator, or make sure the calibrator adapter for your mic diameter is inserted properly into your calibrator.  Similar advice for the voltmeter.

The JBL measured datasheet reflects what the T/S models show as the driver efficiency.  If those are your half space numbers, then they appear too high.

The only other alternative is JBL using a different driver wiring on the datasheet than in the released product.

The T/S model predicts the JBL driver to have an no of 2.7% and a 2.83V/1m sensitivity of 98.2dB in half space.

The RCF 21 comes out at no 3.2% and 2.38V/1m of 98.8dB (also in half space).

Also remember that 2.83V sensitivity is a game, especially with the stout modern amplifiers.  Ultimately a few core parameters determine the output of vented boxes.  Output limits are essentially determined by box volume, tuning frequency, driver Sd, driver motor strength, and driver linear excursion-assuming the port behaves linearly enough to let a stronger motored driver use it.

The RCF 21" has 1.9x the Vd of a single 2268H, and a stronger motor (Bl^2/Re of 212 for RCF vs. 87 for JBL).  Similar Vd means similar output, and the stronger motor of the 21 gives the extra output near the low corner frequency for a given box volume.


Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Steve Hurt on November 04, 2010, 04:55:06 pm
I thought JBL specs were full space.
Title: Re: RCF 21" Sub Build Pics and Impressions
Post by: Phil LaDue on November 04, 2010, 04:57:33 pm
Ned Ward wrote on Mon, 01 November 2010 13:48

Bennett Prescott wrote on Mon, 01 November 2010 10:16

Loren,

P.S. Beautiful carpentry.

+1 on that - love all the pocket screws. Norm Abrams from the New Yankee Workshop would be proud.

Kreg makes some very useful tools.
http://www.kregtool.com/default.html
Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Phillip_Graham on November 04, 2010, 05:03:59 pm
Steve Hurt wrote on Thu, 04 November 2010 16:55

I thought JBL specs were full space.


That would make little sense for LF drivers, as it would be unnecessarily difficult to achieve in the test environment.

I have good collection of AES papers written by JBL engineers, and the pictures of every test structure in them for LF devices shows half-space loading.

They may measure their full range products in full space?
Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Steve Hurt on November 04, 2010, 05:08:49 pm
It would make sense, but things don't always make sense!

I'm pretty sure the 718 and 728 are full space numbers.
Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Phillip_Graham on November 04, 2010, 05:13:49 pm
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.



It is highly, highly unlikely.

Consider that all other manufacturers measure the LF enclosures in half space, so JBL would put themselves at a 6dB efficiency penalty on their spec sheets relative to the competition.  This is highly improbable.

Regardless, I am curious to find out, ideally with a well calibrated microphone.
Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Steve Hurt 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!)
Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Steve Hurt 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.
Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Art Welter 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
Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Phillip_Graham 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.
Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Steve Hurt 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!
Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Phil Lewandowski 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...
Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Steve Hurt 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!
Title: Re: RCF 21--Designer comments
Post by: Loren Jones 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
Title: Re: RCF 21--Designer comments
Post by: Josh Ricci 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.
Title: Re: RCF 21--Designer comments
Post by: Phillip_Graham 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.
Title: Re: RCF 21--Designer comments
Post by: Josh Ricci on November 05, 2010, 10:33:22 pm
[quote title=Phillip Graham wrote on Fri, 05 November 2010 13:34]
Quote:

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.


This latest generation of drivers is really starting to hit on all cylinders...

That's 500 cm of port per driver. Tuning I believe did as you suggested and dropped a little at higher outputs as the port compressed. About 700-750cm per driver seems to be what would be ideal but pesky issues like port length, volume, and resonance get problematic.

Quote:

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


Yes the 21sw150.

I'd say that overall I'm quite happy with the drivers and the measured performance even with the lower than intended tune. As far as what people might expect something like this to be able to do based on manufacturer's marketing specs they might be underwhelmed, but real world output will almost never live up to those inflated specs.

Quote:

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.


Yes. I already had the drivers when I saw that test which made me very happy (assuming some family attributes will rub off). The NW100 also did very well.  Very Happy
Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Jeff Wheeler on November 06, 2010, 12:05:07 am
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!

This must be what inspired Jeff to name his new subwoofer product the Orbit Shifter. Wink
Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Steve Hurt on November 06, 2010, 12:16:19 am
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?

Sounds like they wind it up and let it hit one note before the speaker can heat up.  (and then add 6-10 db)
Title: Re: RCF 21--Designer comments
Post by: Peter Morris on November 06, 2010, 02:27:58 am

"That would now make the 21SW152 the top dog :)"


Hi Phil,

Sort of ...  but the 115 and the 152 appears to be designed for slightly different applications. The 152 will go very very low, but if you want a more general purpose sub that does not go quite as low, the 115 is more efficient and will probably give you better results.(from what I can remember when I did all the sims)

In terms of output per cubic foot this new generation of speakers from B&C and RCF (and 18sound) has raised the bar quite a bit.

Peter

Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Phil Lewandowski on November 06, 2010, 10:15:32 am
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?



Yeah, I noticed that too a while back with several of JBL's subs.  So either JBL is making up those full-space responses or they are actually measuring subs in full-space which still doesn't make sense to publish and advertise full-space responses, but who knows.

Actually the VRX918-SP says "free field conditions":

http://www.jblpro.com/catalog/support/getfile.aspx?doctype=3 &docid=581

Looks like both the MRX subs are measured in full as well:

http://www.jblpro.com/catalog/support/getfile.aspx?doctype=3 &docid=599

http://www.jblpro.com/catalog/support/getfile.aspx?doctype=3 &docid=600



Why can't you just measure all the subs in half-space JBL and make it much easier...?



~Phil
Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Phillip_Graham on November 06, 2010, 10:29:41 am
Phil,

I think it is a stretch to assume that free field is Full space.  Free field is a non standard term, and potentially chosen to obfuscate.

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.

Manus generally don't provde detailed info about their test methodologies, otherwise it would be harder to juice the numbers.
Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: Steve Hurt on November 06, 2010, 11:06:49 am
Phillip Graham wrote on Sat, 06 November 2010 10:29

Phil,
 Free field is a non standard term, and potentially chosen to obfuscate.



On the PRX series, I would choose to obfuscate as well!
The crank em up, hit the on switch, and measure the first note thing, says "chosen to obfuscate" to me.


Be great if someone would measure the SRX subs
The numbers seem low if they're half space, but my "measurements" have been with my ears (and my ears have been known to tell big fat lies)

Title: Re: RCF 21--Designer comments
Post by: Phillip_Graham on November 06, 2010, 01:40:35 pm
So the B&C website was down this morning when I went to look up the T/S parameters of the 21 SW152.  So I went to "the Google" and quickly found a thread by Josh Ricci, who is contributing to this thread, on building his 21 SW152 speakers:

 http://www.avsforum.com/avs-vb/showthread.php?t=1240709& page=1

Quickly skimming the thread, it appears that one of the papers on port behavior in my AES library (that I paid good money for!) is available for free on the Harman website:

 http://www.harman.com/EN-US/OurCompany/Technologyleadership/ Documents/Scientific%20Publications/11094.pdf

If you try to access the root directory, though, it appears to be password protected.  It appears Harman is perhaps better at building speakers than playing administrator on Apache...

Regardless, this a long (27 page) paper on ports.  JBL/Harman typically has excellent technical papers, and this is no exception.

Cliff Hendrickson, Doug Button, Mark Ureda, and Don Keele--all JBL/Harman guys--have collectively authored a great library of AES papers on all things loudspeaker design.

As a PS in appreciation of the LAB community, I could not believe how unruly the thread on that "AVS" forum appeared to be.  Thanks for being civil everyone!
Title: Re: RCF 21--Designer comments
Post by: Josh Ricci on November 06, 2010, 06:47:10 pm
Yes...That JBL paper on porting is a great read if you are interested in porting and overload effects and how ports of varying geometries behave under various power levels. Can't remember where i first saw that years ago.

Sorry about all of the off topic banter and forks in the road on the AVS thread if anyone attempts to read through it. (8 pages of bickering over the ultimate port design Mad ) It was a long journey. There is some good stuff in there though. Rolling Eyes
Title: Re: RCF 21/SRX 728 comparisons
Post by: Art Welter on December 21, 2010, 02:30:34 pm
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.



Loren,

Wondering if you got a chance to do any comparisons or measuring of the RCF 21 cabinet ?

If not, any simple dB meter readings, or subjective descriptions?

Art Welter
Title: Re: RCF 21--Designer comments - Quick side question-2268H
Post by: drewgandy on December 21, 2010, 08:50:04 pm
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.



On the roof huh?  I was going to mention that the anechoic room @ JBL's Northridge location was definitely not large enough for "free field" low frequency measurement.  Of course I think it's supposed to be abandoned now so no one has that problem there anymore.?