AR 3a Stuffing Question

[Shacky]

I went to post office to weigh the stuffing I took out of my AR 3a's serial # 67,410 + unknown. One weighs 16.5 and the other 17 oz. including the Kempac and plastic tall kitchen bag. I see the resto guide calls for 20 oz. Should I just add to the vintage or use all new fiberglass?

[soundminded]

I went to post office to weigh the stuffing I took out of my AR 3a's serial # 67,410 + unknown. One weighs 16.5 and the other 17 oz. including the Kempac and plastic tall kitchen bag. I see the resto guide calls for 20 oz. Should I just add to the vintage or use all new fiberglass?

There is only one rational way to determine the right amount of stuffing in an acoustic suspension loudspeaker system, trial and error using measurements to determine what is correct. If you don't have an anechoic chamber handy, do what Villchur et al did, dig a hole and burry it...face up in the ground. Then suspend a microphone over the woofer. The mass of the moving components of the woofer, the springiness of the trapped air in the box combined with the restoring force of the speaker's own suspension, and the aerodynamic drag of the fiberglass form a tuned resonance system accurately described by Newton's second law of motion applied to forced oscillation. The goal is to achieve the correct damping factor by adjusting the amount of fiberglass. But the nature of the fiberglass itself is a critical factor. The size, shape, friction, air displacement, and packing of the the individual fibers as well as their amount contribute to how the system will perform. The goal is to achieve critical damping. This gives the lowest extension of bass without FR anomolies. Underdamped and the system resonance frequency will be lower but there will be a FR peak. Overdamped and there will be no peak but the system resonance frequency will be unnecessarily higher with consequently there will be less bass. The correct answer at the time of manufacture was undoubtedly established by trial and error also. Then the best result was codified in a cookbook recipe for manufacturing that particular system. Repeat measurements of production samples both in the factory and inependently satisfied the manufacturers that their results were onsistent and acceptable. The system pushes the resonant frequency up from about 19 HZ for the raw driver to about 42 HZ for the integrated system with a 12 db per octave linear falloff below that frequency. Response at 42 HZ should be down 3 db and is consistent with the mechanical equivalent of a second order Butterworth filter. The Theil Small analysis and parameters are merely a way of presenting the same equation and analysis in a different form that is more understandable to most speaker builders who do not have a background in physics.

Since the character of the fiberglass may have changed over the years due to settling and packing, just having the right amount is no guarantee that you will get the optimal response. And of course the fiberglass you buy today will be different from the type used in the manufacture of the speaker forty years or more ago.

[Shacky]

I picked up a small bag of pink FG at HD. Wieghs a lot less than what others have indicated. I used the scall at post office and it was 7.5 oz. vs. the 11 oz quoted many times. It's the Corning small bag. Either scale is off or Corning is getting cheap ; )

I added 4.4 pink plus the 16.05 oz old to one cab and 3.1 pink + 17 oz old to second. Made sure I didn't put a lot directly behind the woofer. Will post separate pictures and impressions but these are noticably better sounding than my first pair of 3a's which were in better initial shape before recapping and replacing pots.

Plenty of bass and no boominess that I can detect. The drivers must be in better shape. Original tweeters shine. And that dome midrange is to die for!