Differences in the AR 12-Inch Woofer

[tysontom]

Brian D posted a message about the AR 12-inch woofer as posted on eBay. Part of his message was as follows:

“I see the 12" woofer from AR on ebay every now and again...

There semes to be some confusion by the sellers as to which

speakers they fit/were used in.

For instance, right now, item number 3011354753 says that it

is for the 11, 10 and 58, but also states that it will fit the

3a.”

There is some confusion regarding the venerable 12-inch AR woofer used in the AR-1, AR-3, AR-3a, AR-LST, AR-10Pi, AR-11, AR-9, 9Ls, 9Lsi, AR-98Ls, AR-91, AR-98LSi, AR-78Ls, AR-58s, AR-58B, AR-58BX, AR-58BXi and possibly other AR designs through the years. This woofer in its many iterations is characterized by the flat-side or “truncated” frame, but each is very similar in performance.

All AR-built 12-inch AR woofers -- Alnico or ferrite magnet -- with the flat-edge (truncated) frame have basically the same electro-mechanical properties; i.e., all have a 2-inch voice coil, 1/2-inch overhang in the gap, approximately 1-inch long coil with 1/2-inch top plate and 9-lb. magnet structure. The ferrite magnet structure is about 1-pound heavier than the Alnico due to the shape of the ferrite magnet structure (flat and wide). All of these 12-inch AR woofers have the same free-air resonance somewhere between 14 and 18 Hz, depending on the vintage. All of these mounted AR woofers can move ½-inch in a linear fashion, and about 1-inch before bottoming (the Alnico version doesn’t really bottom, but the suspension can hit the top of the magnet). But there are other significant differences as the 12-inch woofer evolved through the years, and many of the changes through the years were subtle differences involving the compliance of the suspension system and different cone materials, etc.

The first version was the original AR-1 woofer (the prototype was based on a Western Electric 700-series woofer), a 12-inch Alnico-magnet, cast-aluminum, flat-sided-frame woofer (part #3700) with a treated-cloth, pleated surround and a straight-side cone cross section. This acoustic-suspension system was conceived 100% by Edgar Villchur (his patent), but about 75% of the mechanical driver and enclosure design was executed by the late AR co-founder Henry Kloss. This AR cone was relatively heavy (approx. 80+ gm.) with a 2-inch voice coil mounted on a bronze former or "bobbin." The AR-1 woofer is characterized by the orange-yellow cloth surround color, and no ribs or foam damping material on the center or edges of the cone. The earliest hand-made 1954 versions had the pleated surround, and also had a thicker cast frame, 1/4-inch thick rather than the 3/16-inch thick frame later adopted for mass production. The half-round surround -- shown in the original patent by Ed Villchur -- was probably the first-ever application of this now-common surround shape, was adopted after the initial-run of AR-1’s in 1954, and the pleated surround was never used after that. The half-surround simply had less distortion than the pleated surround, so Villchur decided to standardize on that design. By 1958 with the introduction of the AR-3, the company updated this woofer by adding lamp-black mixture to the butyl-rubber surround treatment to darken the surround so it would not show through the grill cloth, and added stiffening ribs to the cone cross section along with the foam damping rings. It is here that one can detect the influence of Ed Villchur and Roy Allison over the mechanical design of the system Villchur originally designed. Both were results-driven, quantitative engineers, and they realized that the woofer’s upper-end response – above 800 Hz. or so – was getting rough, so ribs were added to the cone for stiffness and foam rings were added to the center and the edge of the cone to damp resonances. It effectively eliminated the roughness for the AR-3’s 1000-Hz. crossover.

AR introduced the AR-3a in October 1967, and the same woofer that was used in the AR-3 (damping rings and ribbed cone) was used in the AR-3a until the introduction in late-1968 or early-1969 of the ferrite-magnet, stamped-frame woofer. This new-style woofer was Roy Allison’s design, and this woofer was an evolution of the original Alnico woofer with newer materials and improved damping and lower distortion. The cone was a new low-vacuum felted-paper material, with better intrinsic damping and smoother response. This woofer also had a urethane-polymer foam surround, and it was coated with a mixture of butyl-rubber for better edge damping. Most of these woofers have long-since had to have the surrounds replaced, as the urethane-foam material oxidizes after about 10-to-20 years, depending on the humidity and other factors. I actually have a NOS AR-3a woofer with the original surround intact after 34 years, but it is slowly decaying. In any event, this early ferrite AR-3a woofer was one of the best ever, with an extremely compliant suspension. This woofer had the 2-inch voice coil as in the earlier woofer, but the voice-coil former was made of Nomex, a paper-like diaelectric material capable of sustaining very high temperatures – a natural application for a woofer voice coil. The AR-3a and the AR-LST shared this outstanding woofer until the mid-1970s. Some of these woofers had round magnets, others had square magnets and others yet had round magnets with indentations in the magnet structure in three or four places. This was apparently due to different parts sources.

In 1975 AR introduced the new AR-11 and AR-10Pi as updates of the AR-3a, and these speakers also used the same basic woofer, but with improvements to the cone, inner suspension and the urethane-foam surround material. A less-porous foam material was used on the surrounds, and the edge-damping butyl-rubber coating was eventually discontinued. The inner suspension was “stiffened” somewhat to improve power-handling capability, but without sacrificing efficiency or low distortion. The original AR-3a ferrite woofer did not have the “bumped” back plate, so common to today’s modern woofers. That is, the voice coil – under extreme excursion of >1” – could travel back and strike the back plate with a frightening (and breath-taking) clatter. This usually spelled the end for that woofer. The voice coil could be flattened on the end. Unfortunately, AR never changed the back plate, and the remedy for increased power-handling capability was to increase stiffness of the suspension, especially at excursion extremes.

By the late 1970s AR made additional improvements to the same woofer, and by this time it was being used in the AR-9, and the subsequent tower and bookshelf speakers. Changes included improvements to the surround and cone material, but these changes were very subtle. The suspension was stiffer than the earliest AR-3a, but was designed such that that stiffness did not restrict the woofer’s linear excursion within its ½-inch linear range.

Finally, AR had service-replacement versions of this woofer, interchangeable in all the above-mentioned AR speakers, and these woofers had the following part numbers: 200003-0, 1200003-0, 1210003-0. There might have been other numbers as well, but these woofers were essentially the same. Later, AR began using (through AB Tech Services) the Tonengen woofer, part No. 1210003-2A (also referred to as 12100032), which was the newest and current version of the flat-side 12-inch woofer. This woofer has a slightly higher resonance of about 20 Hz., and is significantly stiffer, but is far more rugged than previous AR woofers. It sounds pretty much the same as the other woofers, but I would think it has higher distortion. It can handle much more power.

--Tom Tyson

[Guest]

> This woofer has a slightly higher resonance of about 20 Hz., and is significantly stiffer, but is far more rugged than previous AR woofers. It sounds pretty much the same as the other woofers, but I would think it has higher distortion. It can handle much more power.<

You have no idea how much I appreciate your taking the time to put all this information down in one place and in order.

To what do you attribute your expectation of higher distortion?

The replacements from ABTech for my 9s are "tight." Maybe after a while they will break-in more, but as I hear them now; they will still growl awfully low, but they do not "thunder" like the originals did.

I suppose another way to put it is that with the replacements I can hear everything I feel. The originals; well, sometimes I thought they were going to bring the ceiling in or at shake everything in my chest cavity loose (wonder if that's why I don't get pneumonia?). From across the room it was a little like hugging a concert bass drum (yes, I have).

Bret

[tysontom]

>> This woofer has a slightly higher resonance of about 20

>Hz., and is significantly stiffer, but is far more rugged than

>previous AR woofers. It sounds pretty much the same as the

>other woofers, but I would think it has higher distortion. It

>can handle much more power.<

>

>You have no idea how much I appreciate your taking the time to

>put all this information down in one place and in order.

>

>To what do you attribute your expectation of higher

>distortion?

>

Bret,

Well, I haven't measured the new woofer vs. the old one, so I don't know for a fact that it has higher distortion. It's pure speculation on my part. I may try to measure both a new one and an old one to see the difference (Ken may have both there at his place and he could give us the data once he does his comparisons). The new woofer has a stiffer suspension by a considerable amount, and the whole idea of acoustic-suspension is to let the trapped air in the enclosure provide the restoring force rather than the mechanical springs. It seems to me that the new woofer's mechanical springs would tend to bind much more readily than the earlier versions. But even if it did have higher distortion, it probably would not be audible unless the speaker was being pushed really hard. That is to say, in the lower-power ranges of the woofer there probably isn't any difference in distortion. In any event, you would have to rattle the windows hard to bring on anything approaching distortion with the AR-9, so it's probably a non-issue. That woofer may also loosen up a bit after several hours of play.

--Tom Tyson

[Guest Brian_D]

Outstanding.

I'm speechless.

Uh....

Thanks!

[Guest OpusX]

Tom,

Have to ditto what others have said.....thanks so much for sharing all that information about the 12 inch woofer. I appreciated the technical backround and find your last paragraph with the specific part numbers to be invaluable. Have been digging through all the posts and technical drawings etc to glean this kind of information for the past several months and you consolidated a lot of it in one post for all of us. With a set of AR-11s, AR-LSTs and now a set of AR-9s added to the collection, any and all information about those wonderful woofers is appreciated.

[ar_pro]

Bret, I'm very much on board with your description of the perceived differences between original-equipment and replacement AR 12" drivers. My son and I are in the process of restoring his pair of AR-9's and have reached the point where we could install the four all-new replacement woofers. We decided to compare just the LF sections of his speakers against my rebuilt 9's, with original, refoamed woofers. Using LF test tones supplied by a signal generator and a CD, we weren't able to notice *any* appreciable difference. When playing back a few music CDs with substantial LF information (the Telarc "Pictures" & "1812", and a Jack DeJohnette live recording), the 9's with the original woofers had a heavier, *fatter* sound at full extension - it was very noticeable!

To Tom Tyson - thanks for the detailed information on the 12" woofer...your knowledge is much appreciated!

[kkantor]

(note: this was longer, but I took too long posting, timed out, and lost a few tangential paragraphs at the end.)

Tom,

I'm archiving your post...

I don't have an old woofer anymore, but I suspect I can borrow one from NHT. I'll report.

I tested a fair number of vintage 12" woofers during the development of the 303. I fully expected the distortion levels to be lower than the Tonegen unit, due to greater contribution of the air spring to the system compliance (ie- greater "alpha").

What I actually found is generally reflected in the specs at:

http://www.arsenal.net/speakers/ar/ar-303/303v3a.pdf.

(Note that this was an internal document, and I haven't recently compared it to brochure specs.)

In summary, the THD was mildly lower on the newer unit. Not, in my opinion, particularly audible, but consistent. To this day, I have never fully understood the mechanism. My primary theory is that the older drivers could have all drifted towards a slighly asymmetrical rest position, yielding a degree of THD not fully countered by the more linear compliance. Changes in the soft parts over time is another possibility.

When it comes right down to it, precious few drivers or systems have anywhere near the commercial lifetime of the 1-3-9-N models. Perhaps as a result, there is very little research into longer-term aging effects. Thus, modern comparisons are not fully fair. I remember the first time I visited the multi-story Tonegen lab in Osaka. Most impressive, and a turning point in the industry when it ceased operation. When we came to the climate-controlled store room with QC "keep samples" of everything the company ever made, I asked about this. The response was a mixture of confidence and admission that 100% of the sample testing was done in response to customer rejections. As such, there was never any testing of drivers that had passed their commercial lifespan. Further, small signal testing has evolved in both method and interpretation, and it is difficult to correlate historical data much beyond Fs.

When I "learned" speaker design in Bose's class, we actually cut apart drivers to measure mass. Put them in evacuted bell jars to measure resonance. Used contraptions made from triple-beam balances, etc. Ah, the good old days...

[tysontom]

Ken,

Thanks very much for your input on this subject -- I'm glad you were able to join in, as we all need your guidance with respect to what is really happening with each of these woofers. Perhaps it is the Q, rather than distortion, that is affected with the use of the Tonegen 12-inch woofer. The Tonegen system might be slightly overdamped vs. the original AR-built woofers, but once again this is conjecture. There definitely seems to be an subtle difference in low-frequency extension, but then I doubt that anyone has done a double-blind test using the old AR woofers vs. the Tonegen design. In the end, some objective testing should be done to differentiate the new and old AR woofers.

I know that you were working on gathering several vintages of the 12-inch woofer to make comparisons, and so forth. What do you now have now? I say that because I can provide you with most vintages of the 12-inch AR woofer from the original Alnico, cast-aluminum version all the way to the most-recent Tonegen 12-inch woofer. I hope I can help.

--Tom Tyson

[tysontom]

>

>I tested a fair number of vintage 12" woofers during the

>development of the 303. I fully expected the distortion

>levels to be lower than the Tonegen unit, due to greater

>contribution of the air spring to the system compliance (ie-

>greater "alpha").

>

>What I actually found is generally reflected in the specs at:

>

>http://www.arsenal.net/speakers/ar/ar-303/303v3a.pdf.

>

>(Note that this was an internal document, and I haven't

>recently compared it to brochure specs.)

>

>In summary, the THD was mildly lower on the newer unit.

Ken,

In reviewing your message, it occurs to me that perhaps you are referring to the AR-303A 12-inch woofer rather than the flat-side Tonegen AR-3a replacement woofer to which I refer in a previous message. There is no doubt about the 303 woofer -- it appears to be superior to the AR-3a woofer in nearly every respect, as reported to me from Bill Bush. Even Julian Hirsch noted subtle differences in favor of the AR-303 vs. the AR-3a, and lower distortion was one of them. There is little doubt that the 303 woofer was a more consistent design with greater precision and better voice-coil alignment and so forth. My contention with the flat-side Tonegen OEM 12-inch woofer was the relative stiffness of the suspension, and its possible effect on overall performance. It is also apparent that the Tonegen flat-side 12-inch woofer was much more consistently built than the earlier AR-built woofers, despite the fact that early-on AR did performance testing (to assure consistency) on most production speakers before shipping units.

[kkantor]

Tom,

Sorry, yes that was my confusion. Of course I've tested many of the flatted Tonegen 12" too, since they were in use while I designing at AR. A different animal; wish I had more hard data to contribute here.

I did collect a pretty nice bunch of woofers over time. Alas, I have none now, save for what is in my 303a's and what Bret recently sent me. Some Active 9's that are in storage back East. My collection rightfully stayed with NHT/AR when I left. I assume NHT has most of it now. I'll get a list of what they can loan me, then discuss with you what else we should put into a modern re-test initiatve.