Prototype number 3 has a 10 mm diameter thick-wall aluminium armtube. Wall thickness is 2mm. No surface treatment of any sort was applied.
Below is the vibrometry chart. I have left the resonance trace of SAEC WE-308L tonearm in light blue, and Prototype 2 with "clean" counterweight is in light green.
And the most noticeable thing in the new chart is our new data acquisition card! Finally, we got rid of weird low frequency artifacts that appeared due to our old DAQ's inability to deal with DC inputs correctly. Hurray!
But back to the Prototype 3. Armtube bending modes are more or less as expected. The thick-wall tube is stiffer and flexes less.
Surprisingly, the counterweight resonances are all but gone. The only change I've done to the counterweight is moving it towards the rear of the arm to re-balance higher mass armtube. This goes against the accepted wisdom of trying to position counterweight as close to the center of mass as possible.
Subjectively, we are more or less in the same territory as Prototype 2. Sibilance and HF harshness are completely gone now. On the negative side Prototype 3 doesn't sound as resolving as Prototype 2. I think it's the fault of higher frequency 2nd and 3rd harmonics. Again, the result is acceptable, but definitely not great.