Build your own 'spyder' demo model

[harvey]

Ric wanted one of the variable crown models for experiments.

The original fork on a board demo was to illustrate the effect of the
"spyder" (tension resonator) on Mason & Hamlin's. Yes, it was point-of-sale
gadget, a simple bit often effective one. In case anyone has never seen one
of these models, after the following setup remarks is the idea in words...

As we know, Dampp-Chaser is in a constant state of research to improve
their climate control products. During a conversation between Bob Mair and
LaRoy Edwards, the simple(?) discussion came up about the effects on a
soundboard in varying climate situations... what happens to the board under
'x' conditions -- in other words, a cause and effect dialogue. To
illustrate one of his points, LaRoy sent Bob the model. 

Enter me. Prior to a chapter meeting, Bob and I were playing with the demo
model, so my memory of the device is still pretty good. This particular
model, while professionally built, did not look like the original's I've
seen, so I don't know whether it was authentic or a clone.

Now let's build it.

Overall dimensions: 5-5/8"W x 12-3/8"L x 3-3/8"H (minus tuning fork)
Panel (soundboard) thickness: .165" (supporting rationale below)
[Thanks to Bob Mair for confirming these dimensions]

- Build up a small soundboard. I don't think drying is requisite except for
stability of edge gluing, and maybe to prevent cracks or pressure ridges,
because you're gonna adjust 'crown' on the fly, and because DC doesn't make
a system this small ! <grin> The panel thickness needs to be less than a
real soundboard, due to the diminutive size, and to provide easy and
repeated flexing.

- Prepare the tuning fork. The 'Kitching'-style aluminum forks are ideal,
because of their long ring time, their light weight (doesn't load the
board), and because aluminum makes for easy tooling. Thread the (already
round) stem of the fork to whatever common size nut/washers works with the
stem of the fork, and long enough to go through the panel along with two
flat washers, a lock washer, and a nut. TIP: A "nylox" nut might be
superior to the lock washer/nut here, although the originals didn't have 'em;

- Drill a hole dead center in the panel to accommodate the threaded stem of
the tuning fork;

- Mount (glue) each edge of the panel to the "rim" or "feet". Visually, you
now have a small picnic table or shop bench, with solid sides instead of legs;

- The crowning mechanism on the original model had a sliding steel (or
aluminum, depending on vintage) bar, approx. 1/8" thick, to represent a leg
of the spyder. One end of the bar had a hole in it, which was loosely
pinned to permit pivoting on one of the legs of the base. The other side of
the bar had notches in the lower edge that attached to a pin through the
opposite base. The notches, 3 or 4 of 'em, approximately 1/8" apart,
provided the adjustable radius on the panel. The resulting tension (hope
I'm using it right here) on the metal bar prevents the bar from buzzing and
rattling in its otherwise loose mounting arrangement.

- TIP: To avoid the tooling required for the above paragraph, consider an
all-thread and turnbuckle arrangement instead -- it provides a vernier
method of calibration instead of the fixed notches in the bar. For that
matter, it more closely approximates a real tension resonator;

- Mount the fork -- washer above on threaded fork stem, to spread out
surface area and keep the fork stem from punching through the panel, washer
below, lock washer, and nut;

- Make a striking mallet for the fork -- a [piano] hammer and upright
shank. The originals had a little shaped dowel handle where the hammer butt
would be.

That's it... hope I didn't leave out something important!

Jim Harvey, RPT


Ric said...
>----------
>Shucks! I would be glad to put it to the  test you suggest, but I don't have 
>that
>demonstration model, or I would put a diff freq fork on it. Or one of those 
>forks
>with sliding weights or glue on a loud speaker (little one) and hook it up to
>Tunelab and test every frequency..