Final testing and stuffing

[oldguide]

As I get closer to the end I have two final questions for the experts who have done such a great job walking me through this. I hope all my questions have not clogged this board or caused problems for people. I have tried to ask only ones that I cannot find an answer for in searches or the "book."

The first question concerns final testing once the crossover is installed. I have run resistance tests on the pots and on the drivers that will be installed. All test as per the numbers in the rebuild manual. Is there any way to install the speakers and test the complete unit without having to put all the insulation back in just to see that all works? Then if there is an issue, it is just a matter of unscrewing the drivers and trouble-shooting.

The insulation actually does not bother me that much as far as handling it, it is more the issue of the process of reinstalling it, especially the importance of putting the small pieces around the cross-over. It's just that I would not want to do it twice. I've read all the threads regarding health issues with this stuff so no need to add that here, other than for me to say I appreciate the work people put into it--it gave me a needed heads up when I opened up the speakers.

My second question [i wasn't sure whether to make it another thread, but there is lots on this one already] is whether to use what came with the speaker, which seems in pretty good shape as opposed to putting in new fiberglass. I have enough six inch new fiberglass house insulation around to redo both speakers, so do I use this new stuff or the old?

The old looks more like rock wool than fiberglass judging by its density and the picture in the "book," which is all I have to go on having never seen rock wool. The "book" seems to imply I would be better off replacing the rock wool with glass, but the "how much" confuses me. The "book" and the consensus of the various threads about this topic on this board all go with weight, which from a physics point of view makes sense. However, I don't have a scale for measuring other than a bathroom scale which is not very accurate. Anyone have an estimate on the cubic inches of glass that would equal 28-30 oz of fiberglass house insulation mentioned in the threads and the "book?" Unless I missed something I couldn't find this.

Thanks again to everyone.

[johnieo]

Hello Oldguide:

Need to go by weight. I believe the numbers should be included. 30 oz for the older cloth-surrounded Alnico-woofer models; 20 oz for the foam-surrounded ceramic magnet woofers.

Gotta weight it, only way. I have been chucking the rock wool, as it has been know for years to be inferior than fiberglass for making the box volume appear acoustically larger. The reason is that rock wool contains lots of large diameter lumps, whereas fiberglass is of uniform diameter. Thus for equal weight of the two materials, there are more finely spun fibers in the glass than in rock wool. If you have that available, peel it from the paper backing and cut into 4-to-6-inch square chunks and if thick insulation, separate those in to about 2-in thick pieces for stuffing; outside with a dust mask.

One of your friends must have a kitchen scale, or if not, gotta birthday coming up soon? Your anniversary? Ah, Minnesota -- of course, a fish scale.

Cheers,

[Carlspeak]

Here's another suggestion on the fiberglass stuffing.

I have established thru careful measurements that R-13 OC FG weighs 16 ounces for each 5 ft of length with paper backing stripped off. You can spend a few bucks and buy a roll from your local HD and be done with it.

[johnieo]

R-13 OC FG weighs 16 ounces for each 5 ft of length

Fifteen- or twenty-four-inch width?

Cheers,

John

[soundminded]

As I get closer to the end I have two final questions for the experts who have done such a great job walking me through this. I hope all my questions have not clogged this board or caused problems for people. I have tried to ask only ones that I cannot find an answer for in searches or the "book."

The first question concerns final testing once the crossover is installed. I have run resistance tests on the pots and on the drivers that will be installed. All test as per the numbers in the rebuild manual. Is there any way to install the speakers and test the complete unit without having to put all the insulation back in just to see that all works? Then if there is an issue, it is just a matter of unscrewing the drivers and trouble-shooting.

The insulation actually does not bother me that much as far as handling it, it is more the issue of the process of reinstalling it, especially the importance of putting the small pieces around the cross-over. It's just that I would not want to do it twice. I've read all the threads regarding health issues with this stuff so no need to add that here, other than for me to say I appreciate the work people put into it--it gave me a needed heads up when I opened up the speakers.

My second question [i wasn't sure whether to make it another thread, but there is lots on this one already] is whether to use what came with the speaker, which seems in pretty good shape as opposed to putting in new fiberglass. I have enough six inch new fiberglass house insulation around to redo both speakers, so do I use this new stuff or the old?

The old looks more like rock wool than fiberglass judging by its density and the picture in the "book," which is all I have to go on having never seen rock wool. The "book" seems to imply I would be better off replacing the rock wool with glass, but the "how much" confuses me. The "book" and the consensus of the various threads about this topic on this board all go with weight, which from a physics point of view makes sense. However, I don't have a scale for measuring other than a bathroom scale which is not very accurate. Anyone have an estimate on the cubic inches of glass that would equal 28-30 oz of fiberglass house insulation mentioned in the threads and the "book?" Unless I missed something I couldn't find this.

Thanks again to everyone.

In an acoustic suspension speaker system, the stuffing is a critical element for tuning the performance of the system. AFAIAC, the most descriptive model which explains how an acoustic suspension system works (or any other kind of speaker system for that matter) is Newton's second law of motion as applied to forced oscillation. This equation and its approximate solution can be found in any college physics text and any text on mechanics-dynamics. This is where the Theil-Small model is derived from. It relates three variables to frequency response, moving mass (the cone, VC, and bobbin, not the air) the spring which the mass works against, and the damping factor. The stuffing controls both the damping factor and the spring at the same time. The more stuffing in the box, the less room for trapped air remains and the higher the spring constant. The air provides much of the springiness in an AS design. In a pure AS design it would supply all of the spring but most AS woofers have some mechanical restoring force in their suspension. The damping is controlled by the aerodynamic drag the stuffing fibers impose on the drive mechanism forcing air through the spaces between them on compression, dragging it through them on rarifaction. Therefore, the total number of fibers, the density of their packing, their arrangement in the box, and their geometry all play a role. The optimal amount of stuffing was undoubtedly determined experimentally using different types of stuffing, different quantities, and different size boxes to arrive at the optimal final configuration. The problem with using generic or alternate stuffing is that its parameters will be different from the stuffing material supplied with the original.

I don't know which AR speaker you have but for AR single woofer 12" models the in box system design goal Fc (-3db from its horizontal response asymptote at its system resonance frequency) is 42 hz and the system Q (B in Newton's equation) is 0.707. This is critical damping, the lowest Fc without a bump up in the FR before it rolls off. Lower and the FR will show a rise before it falls, higher and it will fall off faster below Fc. The asymptote for an acoustic suspension speaker below resonance is 12 db per octave. BTW, the woofer's out of enclosure resonance frequency is around 16 hz depending on when it was manufactured and if and how it was refoamed. Hope this helps.

[Carlspeak]

Fifteen- or twenty-four-inch width?

Cheers,

John

15 inches width is what I buy. I've posted a little more about this in the Advent area where I upgraded a LA in a few ways including replacing the foam blocks with FG I measured a drop in Fc with the FG vs foam.

[oldguide]

Thanks to everyone on this. Although I am a catch and release guy, I did have a fishing scale around which works fine (I forgot I had it until this thread). Combining that with Carl's method and Soundminded's advice provided a check on the weight. I can see the point about porosity. I am interested now to see how they sound. BTW ended up going with the crinoline advice from the "book" instead of putting the kempac back in.

[dynaco_dan]

Thanks to everyone on this. Although I am a catch and release guy, I did have a fishing scale around which works fine (I forgot I had it until this thread). Combining that with Carl's method and Soundminded's advice provided a check on the weight. I can see the point about porosity. I am interested now to see how they sound. BTW ended up going with the crinoline advice from the "book" instead of putting the kempac back in.

Hi there;

When I was in my teens, I remember a wooden fish measuring ruler, just for fishermen.

It truly was 12" long physically but as the ruler approached the 12" end, it read 24". LOL

If you are referring to the, hanging spring with a hook style scale, I would not bank my insulation weight on such a device.