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.