ar-3 stuffing

[Guest John Faulkner]

Ok I am rebuilding an pair of AR-3s and a pair of AR-3as. My wife threw out the bag of damping material from one of the AR-3s - I think it was rock woll. I have one old cloth surround woofer, and 3 70's ceramic magnet woofers. What weight of fiberglass should I use for these woofers - I was thinking maybe the original rock wool I still have in one cabinet for the cloth woof, and then 20 oz of fiberglass for the newer ones? I also have a pair of AR-2as, so I could maybe steal the old stuffing from them - it is also an off shite material, the same as the stuff I thought was rock wool, and then restuff them with fiberglass - how much? One of them has original cloth surround woofer, the other has been re-coned - not just new surround - but I have a cloth woof coming.

thanks for any help.

[Carlspeak]

You need to match the old style woofer and new style woofers with the appropriate woofer coils in your cabinets.

The old style cloth surround woofer is most likely an Alnico type which must be installed in a cabinet with a #7 coil and 28 oz. FG.

The newer foam surround types with ceramic magnets should be matched with the #9 coil and use 20 oz. FG.

[JKent]

Ok I am rebuilding an pair of AR-3s and a pair of AR-3as. My wife threw out the bag of damping material from one of the AR-3s - I think it was rock woll. I have one old cloth surround woofer, and 3 70's ceramic magnet woofers. What weight of fiberglass should I use for these woofers - I was thinking maybe the original rock wool I still have in one cabinet for the cloth woof, and then 20 oz of fiberglass for the newer ones? I also have a pair of AR-2as, so I could maybe steal the old stuffing from them - it is also an off shite material, the same as the stuff I thought was rock wool, and then restuff them with fiberglass - how much? One of them has original cloth surround woofer, the other has been re-coned - not just new surround - but I have a cloth woof coming.

thanks for any help.

Hi

First--download and read the excellent AR3a Restoration booklet pinned to the top of this section. It will answer ALL your questions.

Don't use rock wool--it is corrosive.

Use plain ol' fiberglass building insulation. You can get smallish bags of unfaced FG at Home Depot if you don't want a full roll

Good luck

Kent

[Guest John Faulkner]

You need to match the old style woofer and new style woofers with the appropriate woofer coils in your cabinets.

The old style cloth surround woofer is most likely an Alnico type which must be installed in a cabinet with a #7 coil and 28 oz. FG.

The newer foam surround types with ceramic magnets should be matched with the #9 coil and use 20 oz. FG.

I understand the #9 coil for the AR-3a woofer and AR-3a Mid and #7 coil for AR-3 woofer and AR- 3 mid, but what about a ceramic magnet woofer with an AR-3 mid?

[Carlspeak]

With regard to the stuffing, woofers and associated coils that match, the midrange is of no consequence. It has it's own coil and in no way impacts the stuffing type or amount used. JUST THE WOOFER interacts with the stuffing.

[soundminded]

Ok I am rebuilding an pair of AR-3s and a pair of AR-3as. My wife threw out the bag of damping material from one of the AR-3s - I think it was rock woll. I have one old cloth surround woofer, and 3 70's ceramic magnet woofers. What weight of fiberglass should I use for these woofers - I was thinking maybe the original rock wool I still have in one cabinet for the cloth woof, and then 20 oz of fiberglass for the newer ones? I also have a pair of AR-2as, so I could maybe steal the old stuffing from them - it is also an off shite material, the same as the stuff I thought was rock wool, and then restuff them with fiberglass - how much? One of them has original cloth surround woofer, the other has been re-coned - not just new surround - but I have a cloth woof coming.

thanks for any help.

Unfortunately, the quantity and nature of the stuffing is a critical element in tuning an acoustic suspension woofer. It is tuned in a similar way and according to the same equation as an automobile suspension. These relate the mass of the moving parts of the woofer, the springiness of the trapped air, and the aerodynamic drag of the air moving between the fibers. AR3 and AR3a woofer/enclosure combinations are tuned for critical damping Q=0.707 at a resonant frequency of 42 hz. The stuffing and box size affect both parameters in at least two different ways. The space taken up by the fibers displace air. The remaining air contols the springiness, the less air the tighter the spring. The number, geometry, spacing and surface of the fibers control the aerodynamic frictional drag created by pushing and pulling the trapped air between them. The original design formula was undoubtedly determined by exhaustive trial and error using the materials at hand. Substituting a different type of stuffing will change the tuning even if he quantity is the same because the parameters of the stuffing will be different. I think in the case of both speakers it's about a pound and a half. It's in the library an probaby in the restoration guide. Unfortunately there is no way short of measuring the finished product in an anechoic chamber to determine whether or not you have come close to the original performance specification, you cannot tell by ear. But don't feel too badly, the odds are that most of the original units no longer perform exactly the way they once did anyway, the changes to the stuffing over the decades has probably altered their performance as well.

It's been reported here that the invariably seen corrosion of the tweeter and midrange potentiometer electrical elements which has plagued these speakers is the result of sulfites in the mineral wool stuffing reacting with them over time, probably the result of small amounts of sulfuric acid fumes they'd be expected to create. They can be cleaned and restored but the process will only start all over again unless they are hermetically sealed from the air inside the cabinet if you use the same stuffing. The quantity of what is proably the same stuffing in an AR2 type speaker is not enough for an AR3 series unit.

[Guest John Faulkner]

I wish I knew exactly what the stuffing material my wife threw out was - it was a dirty slightly yellowish white in maybe 4x6x1.5 inch sort of blocks. I did handle it with bare hands and didn't find it irritating. Also there was remarkably little corrosion on the old style pots in the AR-3 - the ones that do not have a wire bail holding them together but just three metal bands. I think the AR-2as have the same material - I believe they are about 1962 vintage, likely about the same for the AR-3s. Of course for the AR-3s I need neither #7 or #9 inductor, but rather they coil for the 1000 hz crossover in the AR-3. I only have one original AR-3 mid, but have 2 1970 AR-3a mids and two AR-11 mids - I know these can go lower than 1000 hz but will any of them work well at the 7000 hz crossover to the hard AR-3 tweeter? From ARs graphs in their old ads it doesn't look like the AR-3a mid goes effectively to 7000 hz. I would really like to have a pair of AR-3 and a pair of AR-3a. but have not been able to locate another working AR-3 mid. I don't know what my old AR-3 replacement mid (maybe replaced in late 70's or early 80's) is, have to get that speaker out from behind some boxes and pull the mid and see what it looks like.