AR3a stuffing

[lscaffi1]

Thanks to everyone contributing to this information. The original restoration document strongly recommends using only 28-30 oz of fiberglass for stuffing the early AR3a cabinet. Is this still to be strictly followed or can an appropriate amount of polyester be used? If so, how much?

[JKent]

Welcome!

I see Roy already answered in the pinned thread but this is a better thread for discussion.

So... Roy's recommendation is to use 20 oz of new FG and not poly. Roy is an author of the Restoration booklet so go with his advice. The booklet's editor, John, did a lot of very scientific research at the university where he works to compare the structure of various stuffing materials. It all gets technical but the bottom line: Use fiberglass.

Keep us posted on your restoration.

-Kent

[Carlspeak]

My impression is the OP doesn't want to deal with FG.

Getting 28 oz of PET into a 3a cabinet could be quite difficult. Even 20 oz. For example, a pound for crimped polyester will take up a lot more space than a pound of FG, each uncompressed.

The specific gravity of PET vs FG is quite different and I don't recall what they are. It can be looked up. I'd suggest reducing the target weight by the ratio of the differences in specific gravity times 20 oz. for starters.

[RoyC]

There was alot of discussion and measuring of poly when we were putting the restoration guide together. One problem is the variation between the types of poly...and then there is Acousta-Stuf (Madisound and PE sells it). Generally speaking, around 11 oz of poly is a good starting point. Acousta-Stuff yielded the most consistent results, and is what I would use as an alternative to fiberglass.

Roy

[genek]

There are several different revisions of the assembly drawing for the 3a's successor the AR-11 in the library. One calls for 20 oz of fiberglass, another for 10 oz of "polyester wadding." Unfortunately, there's no information or specs about the polyester used.

[lscaffi1]

Thanks Carl, RoyC and genek.. I've seen info elsewhere on this site referring to AR11's using polyester, but these 3a's use the earlier cloth surround woofer so I wasn't sure if the weight would be the same. Given the variations in polyesters, I will probably go with fiberglass to avoid any uncertainties.

[der]

I've had my 1970 AR3a (cloth surround woofers) open three times since new. Once to replace the mid-ranges in the 1980's. Another time to replace the L-pads and a third time to recap the crossovers. I have always reinstalled the original stuffing. Should have I have done so or should I have replaced it with a modern equivalent?

der

[genek]

If the original stuffing was fiberglass, then as long as it hasn't been contaminated by moisture or become a nesting medium for pests, probably no reason not to reuse. I reused mine.

If it's rock wool, I'd want to replace it with fiberglass.

[der]

I guess it's fiberglass and being a completely sealed box I have never seen any bugs. Not wet or anything like that. I emptied each speaker's contents of fiber into it's own bag and marked them. Everything went back in. Must be OK. Of course, I resealed everything to make them air tight.

der

[Carlspeak]

There was alot of discussion and measuring of poly when we were putting the restoration guide together. One problem is the variation between the types of poly...and then there is Acousta-Stuf (Madisound and PE sells it). Generally speaking, around 11 oz of poly is a good starting point. Acousta-Stuff yielded the most consistent results, and is what I would use as an alternative to fiberglass.

Roy

After doing some Googling this morning I found the spec. grav. of glass fiber is about 2.5 g/cc and polyester, 1.38 g/cc. The ratio of pet/glass fiber is 0.55. So, times 20 oz = 11 oz, equiv. to Roy's recommendation. Thus, 11 oz. of pet will occupy about the same volume as 20 oz. of fiberglass fiber.

Note though that not all PET fibers are the same as Roy pointed out in their studies. At a minimum, the type of PET fiber used for bulky pillow stuffing is a good place to start. Those fibers are called crimped PET.

[stan461]

I just restored a set of AR3a's.

Cleaned the pots, replaced all the caps which were way out of spec and re-stuffed with 14 oz. of Acousta-Stuf.

Of course it's my room, my preamp, my amp, my ears and "my" everything else in the chain, but they sound fantastic.

I am totally satisfied with the results.

I'll have to listen to the same CD on both my AR3's and AR3a's to discern any difference in the bass.

[Carlspeak]

I just restored a set of AR3a's.

Cleaned the pots, replaced all the caps which were way out of spec and re-stuffed with 14 oz. of Acousta-Stuf.

Of course it's my room, my preamp, my amp, my ears and "my" everything else in the chain, but they sound fantastic.

I am totally satisfied with the results.

I'll have to listen to the same CD on both my AR3's and AR3a's to discern any difference in the bass.

Acousta-stuff is quite bulky. Did you find it somewhat difficult getting all 14 oz. in? The S.G. of nylon is 1.14 g/cc. Applying the same formulary I did with PET, only 9 oz. of Acousta-stuff should equal 20 oz. of FG.

I've written this before here at CSP and I will once again... "Stuffing of acoustic suspension speakers is not an exact science". Trial and error is the best route to optimum performance.

[stan461]

No problem at all getting 14 oz. to fit.

Actually, after I finished stuffing Acousta-Stuf into the cabinets, they both appeared lite on stuffing.

I reckoned this was to be expected since I was using a little over 1/3 the weight.

I measured the Acousta-Stuf carefully with a weight watchers diet scale which I know is good because it precisely measured an unopened 12 oz. package of coffee.

The original stuffing in my AR3a's was really packed in tight and filled the cabinet. (33 & 34 oz.)

So I'm thinking the Acousta-Stuf is not really that bulky.