Jbl 1200fe-8

[Dr.db]

I just bought a couple of these 12" woofers which I am going to use in some bookshelf speakers.

I want to follow an existing JBL enclosure-design so I´m going to build some test-boxes first. So far I´m aware of the 4428, 4429 with aprox. 1,8 cubic ft3 and the S5800 with aprox. 2,25 cubic ft3.
Is there another system using the 1200FE with a different enclosure?
The published specs from JBL actually do confuse me a little. The smaller 4428 is specd with -6db at 40hz while the bigger S5800 is specd with -6db at 50hz...

What are your thoughts on these enclosures?
Is it worth building an even larger enclosure than 2,25 cubic ft3, or is the 1200FE specifically designed for small enclosures?

Best regards,
Olaf

[RMC]

Hi Olaf,

I don't think the 1200fe was made for use with larger box volumes. With more volume than you mentioned the driver shows signs of being stretched. I tried 3 cu.ft, Fb 37hz, F3 36 hz, to try to make it flat and overstretching is evident between the low to mid bass range with a quite noticeable drop of 1.3 db or so. I think its designed for smaller volumes in mind. I used Winspeakerz to model these, Ql 7 as usual.

I've also modeled 2.25 cu.ft, Fb 40 hz, F3 is at 38 hz, response shows some drop between the low to mid bass region.

I've done 1.8 cu.ft., Fb 40 hz, F3 is at 42 hz, response drop of 0.5 db below 125 hz. Using same volume but Fb 42 hz leads to F3 42 hz but response is flatter within a small fraction of a db.

So with my three quick modelings the best bet appears to be the last one considering the only slight differences in F3 and the flatter response curve obtained: 1.8 cu.ft, Fb 42 hz, F3 42 hz and response remaining within about a third of a db. Regards,

Richard

[Dr.db]

Hi Richard,

thanks a lot for your effort.
I had simulated the FE1200 with WinISD too and came to the same conclusion, but I have to do some real life testing to prove it next.

Do you know of any JBL-system containing the FE1200 using a larger enclosure than the S5800 ?

[grumpy]

Maybe nicely ask TiDome to measure the boxes Greg had built.
They sound quite nice and are real-life examples.

[RMC]

Hi Olaf,

I don't know if there is any JBL system using a larger enclosure than those you mentioned for the 1200FE. My impression is I doubt very much there would be one. Simply because of the modeling results I got in Winspeakerz showing driver being stretched as box volume increases above the volumes you gave (quite noticeable response drop between the low and mid bass range). Typical response of a drivers made for small volumes when used in larger box. One has to tune the box higher to get somewhat acceptable response, however the low to mid bass response drop remains...

My 12" 2214H are better behaved on this aspect since they have almost double the Vas number of the 1200FE (224 L. VS 125 L.). The box size ratio (alpha) is Vas/Vb.

Seems Bill Mc Fadden has provided one of the best simplified explanations I have yet seen in relation to Fs, Qts, Vas:

"2. Small Signal Parameters

The three parameters that primarily determine the frequency response of a loudspeaker are compliance, free-air resonance, and Q.

The compliance, Vas, is a measure of the overall stiffness of the cone, surround (the part the attaches to front of the cone), and spider (the part that attaches to the rear of the cone). It is specified as the volume of air having the same compliance as the driver. A small number corresponds to a small volume of air, which is stiffer than a larger volume of air. Thus, compliance and stiffness are inversely proportional. Optimum enclosure volume is proportional to Vas.

Free-air resonance, Fs, is the resonant frequency of the driver's voice coil impedance with the driver suspended in free air (no enclosure). The -3 dB frequency (F3) of an enclosure is proportional to Fs.

The Q, Qts, is a measure of the sharpness of the driver's free-air resonance. It is defined as (Fh-Fl)/Fs, where Fh and Fl are the upper and lower -3 dB points of the driver's voice coil impedance in free air. Optimum enclosure volume is related to Qts but is not directly proportional. It is accurate to say that the volume gets larger as Qts gets larger. Likewise, F3 gets smaller as Qts gets larger, and for the sealed box enclosure, F3 is inversely proportional to Qts." (RMC: and optimum volume gets smaller as Qts gets smaller, other things being comparable). (Bill Mc Fadden, Loudspeaker Primer, From his Web page, rdrop.com/users/billmc/).

Your driver's Qts is pretty normal at 0.26 so this one doesn't seem to be the issue. Instead, it looks more like the rather low Vas number of 125 L. is responsible. And since "Optimum enclosure volume is proportional to Vas", well low Vas number means smaller optimum box size here. The woofer's Design Engineer (Jerry Moro) may well have chosen a stiffer suspension system for the purpose of good performance in smaller box volumes.

BTW, I have not seen in Moro's 6/11/03 specifications data a number for Xmax which surprised me. Maybe I'm blind. In any case when modeling the driver with Winspeakerz the software assigned a 6mm Xmax number, calculated number or ballpark figure?, since I didn't see a number to enter from Moro's specs. 6 mm MAY be a reasonable number here VS driver's stiffer suspension. The 2214H is at 6.6 mm but its a much older driver than 1200FE and the latter may have more than 6 mm...

With regards to test boxes: remember, test boxes have to be identical in quality to the real thing (except for finish) to get meaningful results otherwise its a waste of time to make a quick, cheap box so so rigid, braced and sealed, and to expect truthful or representative results from a "donkey" VS what the originals should/will be... Instead of doing the work twice, I prefer to save time and material by building only one test box, very well made, test it and if its OK you have only one left to do. If not OK then you have only one box to scrap or reuse on something else...

Since you mentioned using Win ISD don't forget to set each time the QL number to the industry standard 7 instead of the default 10 used in Win ISD Pro to get more acceptable and comparable box results with other softwares. I mention this simply because many people forget to change the Win ISD Pro QL number each time they start a new modeling. Sometime ago a fellow member here asked me what software I used to model his driver/box since his results were different than mine. He forgot or didn't know he was using QL 10 instead of the standard QL 7. I do have and use Win ISD Pro 2016 edition also, but Winspeakerz is faster, more flexible, low cost and has a much larger driver database (1,500) that I keep adding to VS Win ISD Pro very limited database. Using QL 10 as a starting point is overly optimistic for most box designs. Using 7 instead of 10 leads to a little larger box to compensate for losses (e.g. air leaks). Regards,

Richard

[Dr.db]

@Richard:
Thanks a lot for this detailed answer, I got a deeper understanding now.
Actually I made the mistake you mentioned with WinISD, thanks for that!
It absolutly makes sense to me the 1200FE is designed for small enclosures, so I will do the same. As these are supposed to become bookshelf speakers for my small room, I actually like the idea of a small enclosure a lot!

I couldn´t find any info´s about the linear xmax as well, propably because these drivers where never meant to be sold seperatly...


@Grumpy:
Thanks for the hint.
But if I remember correctly, Greg Timbers used these with subs below. But in my case I will be using these as the low-mid woofers without sub, so I need full extension....

[grumpy]

You remember correctly.
However, the subs simply augmented what is already an excellent bottom end (having heard them only without the subs, I did not feel there was a missing bottom).
I'm just thinking the measurements might be another useful data point in your assessment. I expect you will enjoy the result, however you arrive!

Best regards.

@Grumpy:
Thanks for the hint.
But if I remember correctly, Greg Timbers used these with subs below. But in my case I will be using these as the low-mid woofers without sub, so I need full extension....

[pos]

Maybe nicely ask TiDome to measure the boxes Greg had built.
They sound quite nice and are real-life examples.

Don't know if these are real clones of GT's arrays, but should be quite close:
http://www.audioheritage.org/vbullet...l=1#post242312

70.8L tuned to 32Hz

[RMC]

Hi Olaf and Pos,

Attached is a picture (sorry for quality) of the modeled response I get with 1200FE/70.8 L or 2.5 cu.ft./Fb 32 hz. Done with Winspeakerz, half-space loading, QL 7. The red dotted line in the graph represents the 6 mm Xmax line done by the software as I didn't input that number. The red solid line at the bottom of graph is excursion at 1 watt.

LF response looks pretty droppy to me starting at about 300 hz and down to - 2.5+ db at 40 hz. Appears like a too large box and/or too low tuning curve to me. Doesn't seem very appealing for a bookshelf stand alone unit. Will sound bass shy vs the rest of the spectrum? Regards,

Richard

[grumpy]

I’m all for educated simulation and device measurement before cutting wood and efforts to understand the difference between a simulation and what is ultimately measured ... and what is heard.

What I can say is the integrated system was not lacking in bass
... at least not to my taste I have no in-room measurements to back it up (maybe next visit )

My recollection is that the same was said about the system while at Greg’s place with subs off.

In both situations, the system was not against a wall but somewhat into the room and on stands (roughly the same position from the wall as a later GT system I -did- have the pleasure of hearing in his LR)
Closer to a wall would put the system nearer to a 1/4 space environment. In any case, both fairly normal listening environments hosting the 1200 driver in Array type systems sounded quite nice. Which was really all I wanted to say. Other design goals and preferences will lead to other places/choices. Viva la difference.

[RMC]

Hi Grumpy,

Was there any form of signal processing involved in the listening test, EQ or other?

I'll tell you why the question after. Regards,

Richard

[pos]

It depends on how you look at it: this dropping response down low can be seen as a rising response in the higher range, and addressed in the electrical LP filter to get the proper acoustical response with a lower final sensitivity.
In an active/digital system you would typically use a shelving filter, and in a passive system you would lower one or two poles of the filter.

I think GT posted the voltage drives of his project array 1200 somewhere, but I cannot find it right now.

[grumpy]

Hi Grumpy,

Was there any form of signal processing involved in the listening test, EQ or other?

I'll tell you why the question after. Regards,

Richard

My recollection is that only the passive crossover was at work. No active signal shaping or tone controls, DSP, electronic delay, etc... between source/pre/amp/speaker.
I'll also look for the crossover schematic (I -should- have it somewhere ... perhaps in a digital photo which I have -far- too many of ... will have to look at some offline storage )

[pos]

schematic:


acoustical response: (LR 24dB/oct @ 700Hz crossover)

[pos]

voltage drive for active biamping, probably quite close to the passive one for the low-pass part: