Re: Small hammer-like actuator ideas

Hi Clifford,

On 8/25/2011 3:38 PM, Clifford Heath wrote:
On 08/26/11 02:05, Don Y wrote:
The suspension is on 3/16" diameter machine screws an inch long, with
two nuts on each - one locking the head to the support and one locked
against the threaded hole in the tube. That way each support has about
3/4" of screw to isolate the vibrations from the mounting.
So, the supports are *rigid*! Yet, don't interfere (noticeably) in
the sound?

It's a nodal point, so why not? The tube isn't going to bend significantly,
perhaps a millimeter or two at the anti-nodes, so the actual deflection
angle at the mounts is tiny, and that is taken up as flexion in the 3/4"
free length of the mounting screws. The screws themselves are pretty thin
relative to the moving mass of the tube.

OK, I'll take your word for it ;-) I've already made my phondness
for fyziks known, here... :-/

The exact position of the mounting point is critical. Move it by more than
a couple of millimeters and the tone and Q both drop off hugely.

Some of the material I've read suggests different "character" to
different materials. E.g., imagine making the same device out of
sch 40 PVC...

PVC has very low stiffness and very low Q. It would be like trying
to make a guitar soundboard out of cardboard.

Though you can get a pleasant tone from a struck piece of PVC.
It damps quickly, though.

The "figure of merit" for guitar top plates is the speed of sound
propagation in a wave propagating out from a point on the plate.
If you halve the thickness of the plate, you halve the mass per area,
but the stiffness drops as the cube; so the speed of sound drops as
the square. of the thickness. That's why very low density but very
stiff woods are used, especially spruce, which is really rather
exceptional compared to most other woods.

However, in a guitar, if you use an ultra-stiff synthetic composite,
the losses at very high frequencies are too low, and so the guitar
sounds brittle and harsh. Wood is a very complex nanostructured
composite, and doesn't behave like a uniform material at all scales.

No idea. My bout with musical instruments ended some 40 years
ago...

Understood. I'd love to be able to do it with *glass* but
the tuning would be too "anxious". (but, it would *look*
wicked cool!)

I think glass would work just fine, but you'd need to have an effective
grinding setup to tune it. You would have the "brittle" character I
mentioned, but in a chime, it wouldn't seem harsh.

The problem is exactly the "machining" effort required. You
can't just take out a file and tweek it a bit.

Likewise, drilling for supports becomes a challenge.

(But it would be the most visually striking approach, without a doubt!)

I think wall thickness influences tone a lot. E.g., a woman down
the road has a single chime made from a gas tank (O2, N2, etc.) with
its bottom cut off. It has an incredibly rich tone (and loud!)

What I suspect is happening there is that you have many circumferential
modes of oscillation, because the diameter is so large compared with
the width. It's not really about the wall thickness, it's just that
the longitudinal and lateral modes are in the same frequency range as
the circumferential ones. Whether they're conchordant or dischordant
will depend on a huge number of things, so its pot luck. But that's
the case even with giant orchestral gongs. Despite being so expensive
to make, two gongs that were made as close as possible to identical
have very different character, and one in a hundred just stands out
as being exceptional. Same with guitars from the world's top makers,
by the way; they will only rarely have a complete failure, but can't
explain or repeat their occasional exceptional instrument.

Likewise explaining why you can't just clone a Stradivarius :-/

The *terror* here is that you (I) find *a* chime that looks
promising. Only to discover, later, that it was an "exception"
and the rest of the set that you (I) fabricated sounds like crap!

I think I will have to start with visual characteristics of
the materials, first. Then, purchase some and evaluate the
sound quality at the low, middle and high end of the range
that I plan on addressing. Rinse, lather and repeat.

*Then*, sort out how to handle "striking them" (since the material
may suggest/preclude different options).

(sigh) Far more "empiricism" than I had hoped for but, perhaps,
that's part of what makes it unique?

--don
.

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