well, the glue up of the 6" and 3" thick 20" diameter blanks was a lot
easier than I thought. Building a solid cylinder instead of trying to do
fancier segmented bowl style glue-ups is the way to go.

Error #1:
I was planning to use a coping saw to round the blanks. I forgot that you
can't use a coping saw on something 6" thick, and even a  3" thick hunk is
tough - there isn;t enough room in the frame to get a good stroke,and the
blade clogs up. So, I ended up a slew of cutting tools instead. Draw knife,
spokeshaves, and a couple of plains where the grain was being difficult.
Took a couple hours, but got both blanks round enough to toss on the lathe.

Smart thing #1:
using a brace and hex adapter to drive the 1 1/2" screws to hold the blank
onto the faceplate. I love it when I can use a brace to drive screws. So
easy.....

I manged to get the exponential curve turned today, and it works pretty
well. This was out of the 6" thick blank, so the grain ran across the face
(flat surface) of the blank. That means that it wasn't as smooth as I would
have liked - using cheapo dimension 2 by X stock makes it hard to smooth
the surface without getting SOME grain ripples. Confirmed my idea to set it
up for end grain when I make the real one (still thinking Beech...).
In any case, a pool ball does a nice round orbit that decays with a very
satisfying wub wub wub wubwubwubwubwus as it moves down the throat of the
funnel. I was surprised how much noise it made just rolling across the
higher parts of the curve, but overall still completely satisfied (also
assuming it will be a bit quieter without the grain ripples).
A marble (about 3/4") does OK, but doesn't have the mass or size to deal
with the surface defects, so it decays a LOT faster, and doesn't exhibit a
round orbit. Still makes a nice wub wub as it goes down though.

I'm supposed to get a batch of 1" stainless steel ball bearing in a couple
of weeks. I expected the proof of concept to take longer, and I didn't want
to take anything from the client until I was more certain it would work.
Sometime in the next few days I'll turn the 3" thick blank, and see if I
can get a curve in that that works. I'm guessing no - I had to use most of
the 6" in the current one to get a decent orbit and decay, but we'll see.
the 3" disc is also set up with end grain on the face, so, assuming the
tear out isn't too horrible (I don;t really like turning softwood) I can
see how much difference it makes to the sound....

-James