1962 AR3s with 2 lbs, 7.5 oz rock wool?!

[Lucky Pierre]

Hi all,

I am getting back into my 1962 AR3s to replace the pots and the rock wool with fiberglass.  I weighed the rock wool from each cabinet and it came out to 2 lbs., 7-7.5 oz per cabinet!  All of the references I have read, here and elsewhere, states that there should be roughly 28 oz (1 lb, 7.5 oz) per cabinet.  Any ideas on this?  I am the first person that opened up the cabinets when I did the crossovers a few years ago, based on the condition of the screws and woofer sealer goo on the woofer hole.

I currently have 2 b, 7.5 oz fiberglass in cabinet one.  I'll out 1 lb, 7.5 oz in cabinet two, just to hear the difference.

Peter

[Lucky Pierre]

[genek]

You might want to post a picture of the actual stuffing.

Rock wool and fiberglass do not have the same properties, which may be why your speakers have different weights compared to the recommended amount for fiberglass.

[Lucky Pierre]

12 minutes ago, genek said:

You might want to post a picture of the actual stuffing.

Rock wool and fiberglass do not have the same properties, which may be why your speakers have different weights compared to the recommended amount for fiberglass.

 

[genek]

That does appear to be rockwool. If you want to replace it with fiberglass,  28-30 oz is the recommended amount for the alnico woofers from the 3 and early 3a.

[Lucky Pierre]

10 minutes ago, genek said:

That does appear to be rockwool. If you want to replace it with fiberglass,  28-30 oz is the recommended amount for the alnico woofers from the 3 and early 3a.

Thanks, genek.  It most certainly is mineral wool.

I understand the proper weight for replacement with fiberglass.  I am wondering why there was so much of it in the first place.

[RoyC]

3 hours ago, Lucky Pierre said:

Hi all,

I am getting back into my 1962 AR3s to replace the pots and the rock wool with fiberglass.  I weighed the rock wool from each cabinet and it came out to 2 lbs., 7-7.5 oz per cabinet!  All of the references I have read, here and elsewhere, states that there should be roughly 28 oz (1 lb, 7.5 oz) per cabinet.  Any ideas on this?  I am the first person that opened up the cabinets when I did the crossovers a few years ago, based on the condition of the screws and woofer sealer goo on the woofer hole.

I currently have 2 b, 7.5 oz fiberglass in cabinet one.  I'll out 1 lb, 7.5 oz in cabinet two, just to hear the difference.

Peter

The stuffing in your 3's is the early type of AR damping material, which was used until 1970 coinciding with the introduction of foam surround woofers. There has been uncertainty as to whether it was a type of rockwool or just an early type of fiberglass (Tom Tyson maintained that the early material was simply early fiberglass.) Regardless of the definition, there was approximately 1/3 more of this dense material in these cabinets compared to later cabinets with fiberglass. When putting together the 3a restoration guide, and after many measurements, we concluded that 20 to 24oz of modern fiberglass was optimal for any model using any iteration of the AR 12 inch woofer in cabinets of this volume. Carl Richards independently later confirmed these conclusions. Additionally, Tom T. commented that Villchur thought that cabinets containing the early material were somewhat "overdamped". 

One way or the other the early material is very nasty to handle, so take precautions.

Roy

[Lucky Pierre]

Thanks Roy!

This is what I was referencing earlier about the correct amount of fiberglass.  (there was approximately 1/3 more of this dense material in these cabinets compared to later cabinets with fiberglass.)  This makes sense.

I have worked a lot with both rock wool and fiberglass.  While I did take precautions (sleeves, gloves, mask), as always when working with this stuff, I did do a touch/feel test with bare hands.  Unless early 60s fiberglass feels significantly different than modern fiberglass, this is certainly rock wool.  It feels nothing like fiberglass.

I think I might give these a listen with one speaker overstuffed, and one with 20-24 oz, just to hear the difference.

 

Peter

[RoyC]

5 minutes ago, Lucky Pierre said:

Thanks Roy!

This is what I was referencing earlier about the correct amount of fiberglass.  (there was approximately 1/3 more of this dense material in these cabinets compared to later cabinets with fiberglass.)  This makes sense.

I have worked a lot with both rock wool and fiberglass.  While I did take precautions (sleeves, gloves, mask), as always when working with this stuff, I did do a touch/feel test with bare hands.  Unless early 60s fiberglass feels significantly different than modern fiberglass, this is certainly rock wool.  It feels nothing like fiberglass.

I think I might give these a listen with one speaker overstuffed, and one with 20-24 oz, just to hear the difference.

 

Peter

...and the old stuff will also "poof" into the air with the slightest disturbance. It is very prudent to handle it outdoors.

Measurements actually showed rather small differences in Fc as material was varied, but subjectively it was noticed in upper bass frequencies. Too little material can become boomy, and too much material reduces bass response. In practice, it is also listening space and preference dependent. I don't think it was a coincidence that AR began using more series inductance in woofer circuits when the amount of stuffing was reduced, as both have an effect on this area of bass response. Fortunately the AR-3 (and early 3a) woofers have a relatively smooth natural roll off into upper frequencies allowing a more forgiving degree of adjustment.

[Lucky Pierre]

Thanks again, Roy.

I think I knocked something loose inside the over-stuffed cabinet.  When I hooked them up for a test run, I had nothing from the woofer.  Opened it back up, removed 1 lb of fiberglass, and tested again.  This time, woofer works, no mid, tweeter works. 

We have another heat wave on deck in the NE, and the shop gets a bit warm, so I won't be able to troubleshoot until later in the week when the temp drops.

[ReliaBill Engineer]

I’m still not understanding.

Why replace the rock wool?

Is yours worn out?

[genek]

Rock wool has a higher moisture absorption rate than fiberglass, and there's a theory that this contributes to level control corrosion. But the biggest reason is probably that all of the testing done to arrive at the optimal amount of stuffing for bass performance in restorations was done with fiberglass.

[newandold]

9 hours ago, genek said:

Rock wool has a higher moisture absorption rate than fiberglass, and there's a theory that this contributes to level control corrosion. But the biggest reason is probably that all of the testing done to arrive at the optimal amount of stuffing for bass performance in restorations was done with fiberglass.

An acoustic suspension loudspeaker that has been the victim of absorbing moisture within the cabinet for whatever reason (and there are a number of them) is a tough subject, indeed, and one from experience that I am delighted that I no longer have to deal with it.

 

When I was first married, I had no choice, but to relegate my listening area into the basement, which was finished, but was suffering from a mildew problem. I had a dehumidifier running that I would kill off during a serious listening session and then back on again, and the bucket brigade continued until I moved out of there.

The moisture wreaked havoc and greatly shortened the life of the foam surrounds and caused the visible chalky corrosion on the crossover networks.
Ever since I moved, where everything with audio happens upstairs in centrally air-conditioning environment. I have no such problems and the surrounds appear to be nearly immortal.

[ReliaBill Engineer]

Unfortunately, most of us won’t be alive when the fiberglass and replacement pots are 50-60 years old, to compare. 

My ‘65 2ax’s had zero corrosion of the pots. My ‘68 4x’s had corroded pots. Rock wool both. My house in Norfolk VA has rock wool insulation throughout, house built in 1851 (fascinating history in that house; hidden rooms, Civil War weapons and cryptic notes, etc!); rock wool installed in 1955. No degradation nor moisture problems. My house here in Alabama has rock wool insulation also; installed in 1962. No degradation or moisture issues, either. 

Based on my experience with rock wool, I left it in my cabinets. I don’t see a benefit in changing to fiber glass.

[genek]

Many classic era ARs suffered symptoms of potentiometer corrosion within 10 years or so.

[Lucky Pierre]

FWIW, I recapped the crossovers in my 3s in late 2018/early 2019.  As part of that project I fully cleaned the pots with a vinegar bath, polished all contacts, and lubed.  Fast forward to last week when I got back into the speakers, the pots were again rather corroded and the metal straps that hold them together were a bit rusty.  These speakers have always been in either my living room, or the insulated music room above the garage.  Was it the rock wool?  Perhaps, but who really knows.  Either way, I replaced with fiberglass out of an abundance of caution.