How it all started

About 4 years ago I tried changing my 515-8G bass drivers in my VOTT cabinets to 416-8A's, as I had found out that the deep bass with the 515-8G´s was missing. Fs on this driver was 37 Hz, and I wanted to try one of the drivers with Fs of 25 Hz 416-8A which Altec wrote these drivers had.

I bought a couple of 416-8A´s on Ebay, and switched one of the pairs of my 515-8G pair in my VOTT´s. And what a difference. They played deeper, but still I was a little disappointed, I had expected even more.
So I took the 416-8A pair out again and started examining the Fs of the speakers. To my surprise Fs was between 27 and 28 Hz, and I felt the cones were a bit hard to move, absolutely not high compliance.

I looked closely on the surrounds, and found them somewhat stiff, and the amount of coating on the surround varied a lot in thickness, related to the 360 degrees surround.
This was the beginning of what I can present today, you won't believe the difference in sound, as well as the Fs impedance shift in hight, and the great drop in Fs frequency. The speakers are reborn.

The below pics of a 416-8B sn. 39880 document what the speaker looked like before and after the cleaning, and the influence on the swept data from 1 to 1100 Hz, shown before and after the cleaning and recoating.

Per Hasling.