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Vas, stuffing, and resurround jobs

Guest Bret

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Just a couple of nights ago I stuck together all the 10pi parts I had been accumulating - including a couple of the woofers that Ken had done tests on.

One of the things I noted in reading his test results was that there is a significant difference in the Fs of different surrounds. Also there seems to be a fair amount of difference in Vas between these drivers even with like-surrounds (which, as I understand it, is the "equivalent air volume of a moving mass suspension" - per Google search).

The two drivers I used have Vas measured at 120 and 134. I have no idea if that amount of variation is typical or not. I also don't know how close that comes to the number Norwood would have looked for. I also don't know how critical Vas is.

Maybe one of you could help me understand how this number might change the amount of stuffing you might use presuming all drivers were mounted in the same cabinet. I used the 20 ounces specified in the materials list for a 10pi.

As you can see here:


the round 12" Foster driver (303) has a lower Vas number than any of the flat-sided woofers and some of these woofers (the 10pi originals) are much, much higher than the Foster.

Which brings me to this observation: According to my buddy with his 10pi's; removing the Tonegen replacement 12" and re-installing his original, but repaired, 10pi drivers has *completely* changed the perceived sound of his speakers. The repaired originals have FAR more deep bass than do the replacements BUT seem to have less "upper" bass almost to the point of affecting what one would perceive as midrange. His overall impression is that while there is more deep bass the rest of the cabinet became brighter and the image changed for the worse - but maybe that's the way it ought to be.

I have not heard HIS yet, but the 10pi's I stuck together would seem to bear-out his observations (as best I remember what his sounded like). My 10pi's seem awfully bright compared to the way I recall 10pi's sounding. I don't find it objectionable because they still have the AR signature high-end. They don't sound like fingernails on a chalk-board, but there does seem to be a lot of top compared to mid-bottom, but then there is more than adequate bottom-bottom.

Is it that the lower efficiency of the original units compared to the Tonegen replacements is to blame? Is this a result of dropping the resonance of the system as a whole? Is the impedance curve of the original substantially different from Tonegen replacements and that somehow affects the crossover?

I really, really wish Ken had-had an unrepaired, unmodified 12" driver to test so that we might see if repair altered these drivers substantially such that neither the Tonegen nor the repaired units are suitable substitutes for an original.

Thoughts? Observations? Criticisms? Wisdom?


BTW - all of this relates to my concern about telling individuals to do their own re-surrounding work. If you buy replacement surrounds that are too stiff, or not stiff enough, then no matter how expertly you install them you won't end-up with the speaker-system you started with; small changes seem to make large differences.

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These are excellent observations, and clearly show the many variables that affect the restoration of these vintage systems.

My feeling is that these three aspects should always be taken into account:

1.) Crossover capacitors shifting from spec (there was an extensive discussion of this in an earlier thread).

2.) Replacement of original drivers with substitutes of much later manufacture. My own feeling is that a vintage replacement should be used whenever possible, as it is much more likely to be closer to original spec than a brand-new driver.

3.) Inexpert refoaming of cone drivers - it has *got* to be the right foam, correctly installed! My experiences with Millersound have shown a uniformity from one job to another. He's doing something right, because six AR-9 systems with re-foamed woofers have sounded (to me) just like an AR-9 with good original foam. I've also noticed performance aberrations similar to Bret's 10pi experience...Tonegen woofers sound *different*, and repaired 9's 3a's and 91's have all shown bass/midrange anomalies after at-home amateur re-foamings.

A question for Tom Tyson: Would a cloth-surround AR-3 woofer go out of spec over time? Might it be the closest extant to the original AR woofer, and therefore be able to serve as a reference for repaired woofers?

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