Well, this is different.
The 0-900 Hz baseline is a lot higher than with all the metal arm tubes. I actually thought it's some kind of error, and used a different channel of the signal conditioner
for a second measurement. Nope, it's all correct.
My hypothesis is that it's composed of the usual resonances (you can see the tiny "hills" at about 300 and 600 Hz if you look closely). They are quite pronounced (at least as significant as Prototype 4
's), but they have extremely low Q and thus they blend into each other.