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johnaec
09-08-2004, 12:32 PM
This is just a general question that I've always been curious about. Why did JBL always use 3" voice coils in their most common 12" studio monitor and home audio drivers, and not 4", (the exception being the 4315)? They obviously had the baskets for 12"/4"VC drivers, and it seems to me they would give a lot more control over cone motion than 3". Is there some overriding technical reason that 3" was used more than 4"?

'Just curious - John

Don McRitchie
09-08-2004, 12:53 PM
I would guess that the main reason had to to with the limited HF extension of the larger coil drivers. The 12" drivers were primarily used in two and three-way bookshelf systems with cone midrange/HF drivers that required higher cross-over points. The smaller coils have less moving mass and therefore greater HF extension. The only JBL systems to use 4" coil 12" drivers were four-way systems (the 4315 monitor and L212) with a midbass driver that could accommodate a lower cross-over frequency to the 12" woofer.

johnaec
09-08-2004, 12:58 PM
Thanks, Don - what you say makes sense.

John

Robh3606
09-08-2004, 06:23 PM
It's not that easy. My E-145 has a 4" coil and a 55grm moving mass. The 12" 123A with 3" coil woofer comes in at 100 grms. D-123 full range has a 3" MM of 45grm. The D-130 A woofer 4" comes in at 70 grms and the 130 at 60grms. The mass is all over the block between the driver use low end extension and VC material copper vs. Aluminum. I could bias this the other way too. I don't think there is any hard and fast rule. Buy the way what exactly is so shabby about a 3" coil in the first place??? Think it is more about power handling than anything else. Most if not all of the 4" 12's are either in MI speakers, pro sound or a monitor.

Rob:)

Don McRitchie
09-08-2004, 08:25 PM
I'm not sure that the power handling issue goes to answer the original question of why the vast majority of JBL 12" woofer systems used 3" coil drivers. I think it is obvious that, all else being equal, large coil drivers have greater power handling and less power compression. However, why would this preclude their use in any system unless there were some other undesirable tradeoffs? The fact is that there are two main tradeoffs in large coil drivers and they are reduced HF extension and increased cost.

As a speaker designer explained it to me, no paper cone bass driver is truly pistonic since they all experience some degree of breakup. One aspect of this beakup is a degree of decoupling of the coil motion from the overall cone motion. For this reason, the coil/dustcap assembly is primarily responsible for reproducing the upper end of a bass driver's bandwidth. Therefore, the coil mass (and not the total cone/coil mass) tends to govern the HF response.

Speaker designers purposely take advantage of this phenomenon to produce extended frequency bass drivers. By using a curvilinear cone instead of a flat sided cone, the decoupling can actually be enhanced, and as a consequence, the HF response is enhanced. This is why extended range JBL's, like the D130 and D123, use curvilinear cones. However, this has its own tradeoff in increased distortion.

The point is that there is some degree of decoupling regardless of the cone geometry, and the smaller and lighter the coil, the more HF response. For example, this is the reason that the standard coil size for the majority of EV bass drivers was kept at 2.5". They were willing to go to great lengths to attempt to engineer out the power restrictions of smaller coils to keep the greater inherent bandwidth.

If you look at the specs for the vast majority of JBL 12" driver based systems, you will see that the cross-over points range from 1khz to 2khz. You cannot get a 4" coil driver to easily extend that high without specialized engineering that further increases the already higher driver cost. Conversely, a 3” coil driver will extend into this range without much problem.

To summarize, my intent of this discussion is not to state that 3” coil 12” drivers are superior to 4” coil 12” drivers. To gain the highest level of acoustic fidelity from a 12” based system, JBL did indeed use the 4” coil 124A/2203. However, doing this required a four-way design with a much larger enclosure than a standard bookshelf form factor. If the bookshelf form factor was a primary design criteria, as it was for the vast majority of JBL designed 12” systems for home use, then the 3” coil drivers were the better choice for that application – recognizing that there was a tradeoff in LF response, power handling, and driver distortion. It all boils down to the intended application.

speakerdave
09-09-2004, 12:12 AM
All very reasonable, but it does occur to me to wonder why the 124/2203 needs a bass/mid driver and the 2235, the direct descendant of the 124's bigger sibling, can operate up to 1000 Hz in the 4430, apparently to good effect, although I have not heard the speaker myself, and the 136/2231A to 800 Hz in the 4333A and its brethren. I think the choice of 3" or 4" voice coil must be primarily a matter of cost, required efficiency and power handling.

David

Don McRitchie
09-09-2004, 05:20 AM
1Khz is the upper limit of the response of the 2235 where this is the bottom of the range used in the 12" driver systems. The L100/4310-12 all used a 1.5khz cross-over and the L88 used a 2khz cross-over.

If I had to pick just one reason for JBL using smaller coil drivers, it would likely be cost. However the bandwidth restriction of the 4" coil bass drivers is real and a factor in system design.

johnaec
09-09-2004, 05:46 AM
I do think JBL went too far with the 2khz crossover in the S99 with the LE14A... :p

John

Robh3606
09-09-2004, 07:25 AM
"Great points! Perhaps we should segment what we are talking about here. The lighter mass twelves designed for the rising response and efficiency and the heavier mass twelves designed for extended LF response."

I would be more concerned over VC inductance as a limiting factor than mass but you can always use a Zobel. Then you have copper vs. aluminum. All the HE extended range drivers had aluminum coils?? E Series 110/120/121/130 LE 8T ect. D123

Look at the TAD and 2441 example same vc size very different bandwidth VC mass may be significant but it certainly is not the long pole in this case.

Interesting subject:D

Rob:)

speakerdave
09-09-2004, 07:26 AM
Originally posted by Giskard
The S21 (LCS version of the L65, and arguably the best version) used the 124/2203 to 800 Hz.

I WOULD like to run across a pair of those!.

David

Don McRitchie
09-09-2004, 09:12 AM
Look at the TAD and 2441 example same vc size very different bandwidth VC mass may be significant but it certainly is not the long pole in this case.

Actually, I think the bandwidths of the 2441 and TAD 4001 are fairly similar, but for reasons unrelated to mass. The 2440 is a different story. However, we're into a completely different kettle of fish when discussing compression drivers compared to direct radiating cone woofers. Compression driver response is a result of phase plug geometry, suspension design (parasitic resonance), diaphragm diameter and mass just to name a few. Voice coil mass does not have the same effect since a small diaphragm driven around its outer edge will not decouple from a coil like a large diaphragm driven at its apex. Overall moving mass is very significant and thus the trend to very light berylium diaphragms.

Maron Horonzakz
09-18-2004, 10:28 AM
I believe the 5 slit phase plug has alot to do with HF extention in the TAD 4001 ......It would be interesting to see plots of be diaphram (TAD) and RADIAN Al side by side.