Moving mass (MMD and MMS) Calculation - JBL 122A-1 recone

[C Hart]

Hi AKers. I have a simple question for driver mavens: How much of the surround and spider weight should be included in calculating moving mass?

Background: I am doing my first recone on a buzzing JBL 122A-1 woofer from an L166. The last service on this driver made a mess of it and I am restoring it hopefully to JBL specs and appearance.

To get to its Fs of 17 hz, this driver's MMS per JBL is 100g +-5%. It gets to that weight (on my particular example at least) by using both a mass ring (10g) behind the dome, and a coating of Aquaplas.

The parts from The Speaker Exchange include a cone (numbered "MWJBL128H" - Midwest Speakers? 128H cone for sure) with a preassembled foam surround, and a loose dust cap, spider, voice coil, and leads.

Separately, I got some Aquaplas from eBay which seems like the right stuff.

Collectively the new parts weigh 55g. So I have to add weight using the reused mass ring and some Aquaplas to get it to spec. The specific amount of Aquaplas is the root of my question.

I cut out the moving parts intact with ~2/3 of the surround intact and all but ~1/4" of the spider. That assembly (cone, voice coil, dust cap, and most of the spider and surround) weighs 104g. Given about ~10g for the surround and spider, that gave me a net weight of 94g for the cone, Aquaplas, voice coil, dust cap, and mass ring. The weight of the air is <1g and is therefore somewhat negligible.

Many thanks to @Pete B for his excellent info and the suggestion to make it a post.

I am grateful for any info and apologies for the long read. If there is any interest in a rookie reconing process I am taking photos, weighing and measuring and can post more on that also.

 

[Pete B]

You might want to add "JBL 122A-1 Recone" to the title if you're able to edit it.

 

[C Hart]

Thanks Pete. Done.

 

[C Hart]

OK I am going to try to shill this thread a little.

@GordonW @tarior @edgewound - I saw a thread in The Lansing Heritage in which there was some debate about the mass ring in the 122A. I suspect you all have figured this out already but in case you need verification, here is the mass ring from my 122A-1 (which, as apparently they do, also had Aquaplas on the back). 10 grams. It also appears to need in the ballpark of 40 grams of Aquaplas.

The color and feel for the eBay Aquaplas seem quite authentic.

 

[EarlK]

Hi,

Good Effort!!

If you do a bit of digging, you'll eventually find out that the "air-load" for most typical 12" woofers >> is in the area of 8-9 grams.

For instance, study the info at the following link; Loudspeaker Database ( I've already narrowed most of the important parameters )

I wouldn't get hung-up on the fact that air doesn't weigh 8 grams for this amount of Vd >> it's best to consider that some of the woofers compliance ( or resistance to movement ) gets converted into weight ( expressed as grams ) and is then included within the final "air-load" terminology.

Also, you might want to tie final MMS to your woofers BL product to zero in on making a woofer with similar performance to the original ( at least in sensitivity ).

BL is a measurement that needs to be derived by obtaining the woofers TS parameters .
- IOW, if your aftermarket kit has less BL ( meaning, less voice-coil windings within the gap ) >> then you'd adjust the MMS downwards somewhat to maintain the original spec for sensitivity.

This ( BL vs MMS ) is just one of a few "trade-offs" that you'll need to decide-on if the aftermarket Voice-Coil doesn't have as many windings as the original JBL part.

 

[C Hart]

Hi @EarlK and thanks for the great advice and info. When I did the math I was surprised to find how heavy air is. And compliance is a bit of black magic alas.

Great database! I played with it for a few minutes and it is impressive but only replacement woofers are in there. I have the T-S parameters from JBL but some of the extra info is missing. That database seems very complete for the drivers it covers.

I compared the windings and they are as close as my eye can discern to the same on old and new. Since the true weight involves some guesswork, plan A is to use the same weight as what was in there. My 104g measurement included most of the surround (full weight 8g) and the spider (2g).

RE is probably unrelated but it's 4.7 ohms on the new and 5.5 old, both within spec.

Now adding the air (8g), 1/2 the surround (4g) and 1/2 the spider (1g) are a total of 13g.

That means the OEM cone, VC, mass ring, and dust cap actual weight (estimated 6g for the surround and 2g for the spider) would be closer to 89g, which is too light per specs of 100g +-5%.

So that is why I'm trying to figure out how much surround and spider weight to include. Without that extra 8g the weight is at the low end of spec. With it the weight is well under the spec. You've introduced the compliance issue, which I had not thought of. So I will try to make new parts weigh the same as the old and call it a day.

Anyway I finally got the cone to weigh the same as the old at 71g. using 4.5 successively heavier coats of Aquaplas with some swearing in between coats.

Really appreciate the comments! Still wondering about the answer to the OP question but it sounds like it is super complex and/or a SWAG and/or impossible to determine before assembly.

 

[EarlK]

@C Hart

FWIW, some of your numbers ( methodologies of inclusion & exclusions ) seem off to me.

Assuming your scale is accurate, an ( all-in ) static measurement of around 104grams ( which excludes air-load ) means that old cone-kit ( that included the 10gram mass ring ) would have a working MMS of a bit over 110 grams.
- There's always the possibility that JBL's numbers are off ( & didn't include the Mass-Ring ) >> since there are other examples of their published numbers being wrong//off.

Something to ponder >> I just measured the ( all-in ) static weight of a ( NIB ) JBL recone kit ( for a 2235H ).
- It's weight came out to 137-138 grams ( this includes the fully assembled kit as well as the mass ring and dust-cap > no glue of course )

Using the same LoudSpeaker database ( setup for 15" woofers of this type >> MMS weight + sensitivity ) the "Air-Load" ( or MMS minus MMD ) came out to a smidge under 17 grams ( for, 4 examples studied ).

Adding that ( acquired Constant of ) 17 grams to my own 137 grams kit measurement, one gets an MMS of @ 154 grams.

JBL specs the MMS for the 2235H as 155 grams ( meaning my ad-hoc measurement system is statistically right where it needs to be ).

 

[C Hart]

Hi Earl and thanks again for your wisdom. You can see how this could get confusing I think. It is highly possible that my figures are off somewhere.

The "JBL Thiele-Small Low Frequency Driver Parameters and Definitions" reference shows the 122A (no 122A-1 listed) at 100. Also, the 128H, the replacement for the 122A-1, is listed at 90. So I feel somewhat confident of the spec MMS.

The 104 grams includes the old cone, dust cap, voice coil, mass ring, almost the whole spider, and about 3/4 of the surround, but no air. Also a very gloppy glue job on the dust cap.

I weighed an EV 12" surround and it was 8 grams. The replacement spider is 2 grams, the replacement voice coil is 18 grams. So backing into it with a bunch of estimates:

Starting actual value: 104g

Surround: 3/4*8 = 6 (est.)
Most of spider ~2 (est.)
Voice coil 18 (est. based on actual new)
Mass ring 10 (actual old)
Dust cap. 2 (est. based on actual new)
------------------------
Total w/o cone 38g
Cone 66 (deduction: 104-38)

One way to get to ~100:

(0% surround) 0
(0% spider) 0
Voice coil. 18
Mass ring 10
Dust cap. 2
Air 8 (est. incl. suspension compliance)
Cone 62
-----------------------
Target MMS 100

The kit comes with a cone with an installed surround, 29g. So my inference is that I need to get that cone/surround to weigh 70g by adding 41g of Aquaplas.

The Dayton DS315-88 which is a close match for Xmax and SD, has a VD of 366cm^3. Its MMS is 109.2 and MMD is 100.6, or 8.6 grams.

(Apropos of nothing, per some calculators I found, air is .0012929 g/cm^3, which puts the weight of the air displaced at Xmax at 0.473 g.)

So the air and suspension components are the difference between in spec and not. I feel like I might be as close as I can get without assembling and measuring T-S.

Charlie

 

[C Hart]

Well just to close out this thread, in case anyone is interested and for posterity and reference: I am finished now with my first reconing job. It was a learning experience and ultimately I have a much nicer looking (not perfect though) and good-performing 122A.

The Speaker Exchange 122A kit was pretty good overall but had a non-JBL plastic front gasket and the tabs on the voice coil were pretty small. Also I did not love the shim arrangement. And I wish it had the 9.75" cone instead of the 9.5".

There were a few unexpected, ah, issues. Here are a few notes:

I bought a 4" lazy Susan for ~$10 on Amazon to facilitate spinning the frame. It worked really well - highly recommended.

Cleaning the old adhesive was surprisingly difficult and lengthy. I used a wood chisel and it helped a lot.

I used about 3/4 of a jar of Aquaplas, 4.5 coats, weighing the cone after each coat dried, to add 43 grams of mass. The stuff is pretty impressive. It is very light but way way stiffer than the paper. After trying a bunch of methods I ended up using a stencil brush (round with squared off bristles) for most of the area and an artists wide natural bristle flat brush for near the surround.

The cone was smaller than the voice coil and had to be sanded (post-Aquaplas) to get it in. I was unable to wedge the VC so it was immovable with the supplied shim assortment so I actually pushed the VC down into the gap when I tried to mate it to the cone - should have waited for the spider to dry before trying to get it on. It was touch and go on whether any of the epoxy from the spider got into the gap. I was ready to throw the whole thing out the window at one point. But it ultimately worked and there is no rubbing.

The shims were ruined by epoxy and I threw them out.

I replaced the shims with some .010" plastic strips. Not super snug but good enough after I fished out the VC from the gap. The problem also resulted in only about 1/8" of the VC former sticking out through the cone, a trickle gluing job and a position further down the gap for the coil than the original.

There was a danger that the mass ring would short the VC leads so I glued it with rubber cement and put a drip of the glue on the leads at the VC ends.

Fitting the mass ring was tricky, though it looked easy. It got out of round when removed from the old cone/VC and had to be carefully bent back into place.

Gluing the cone was dicey because of the small clearances and also because at this point I suck at it. Next time I will have syringes with hollow needles for glue application. I tried Abatron, a water-thin epoxy, but it didn't set well and soaked into the cone a little, so I went back to the Loctite.

I was overconfident on the dust cap and the glue is kind of wonky. (when the syringes get here I will put a nice bead on it but it still won't be optimal. I used black dyed PVA glue. It was a little thin but otherwise worked well.

I was amazed, relieved, and ecstatic to find the woofer played both tones from 20-500 hz and music with no buzzing and quite loud on the bench with my battery amp. Thanks again @Pete B and @EarlK for the info and encouragement. I am almost ready to attack the other 122A.

Some pics because it DID happen: