[time-nuts] Helmholtz Resonator and other Maintained Oscillators

[Dana Whitlow]

Use of a smaller opening would be the first strategy for getting higher Q.
Making sure that the walls of the vessel were solid reflectors would be
an important factor, too.

I noted that several of his sustained oscillators were basically either
relaxation or blocking oscillators, neither of which is noted for good
phase noise performance.  Even the pendulum clock mechanism
was interfering severely with the pendulum's motion- if you look
closely you can see that the pendulum bob's position versus time
function was a severely clipped waveform.

The flex hose demonstration was interesting in that different regimes
of swinging speed resulted in oscillation in different modes.  I wonder
why.  But in hearing people learning to play different musical instruments,
mostly wind instruments but also including the violin, I was once
moved to say that playing these devices the art was in making an
oscillator run in a resonator mode other than the "natural" one.

For an  interesting look at oscillating modes and a really oddball
sustained oscillator, view  https://www.youtube.com/watch?v=m6631u7d4E0.
and/or google "mercury beating heart".  The electrochemical effect
makes the blob oscillate between a hunched-up shape and a flattened
shape.

If given time, this hunching oscillation "pumps" a degenerate parametric
oscillation between the two triangular shapes at nominally half the rate
of the original oscillation.  I first saw this demonstrated in high school,
but the demonstrator also could  not get a sustained oscillation.  I
thought about that for a while and decided to try a little external stimulus
in the form of low voltage DC from an external supply.  After a little
optimization it worked beautifully and could run for hours on end with
little of no attention, giving me the luxury of trying a range of different
blob sizes.  With different sizes I could get sustained parametric
oscillation in four different modes: 2-sided, 3-sided (as seen in the
You-Tube clip), 4-sided, and with difficulty even 5-sided.

Of course I didn't really understand what was going on at the time,
and didn't arrive at the parametric oscillation theory until years later.
BTW, I used a baking soda solution instead of a chromium-based
chemistry, and an electrode coming down from the top center, with
a large ring surrounding the mercury blob as the other electrical
connection.

Solution concentration, voltage, electrode tip height, and electrical
polarity were the parameters that had to be adjusted for best
performance.  A mercury blob about one cm across in its resting
state seemed to be a good starting point.  For polarity, use the
one that results in the blob's hunching up when the electrode tip is
gradually lowered into contact with the blob.  If the other stuff
is not too far off, it will quickly take off oscillating at that point.

I've been wondering whether this could be made to work with
Galinstan obtained from modern-day clinical thermometers,
instead of that dreaded mercury.  A worthwhile experiment
to try.

I never did get around to measuring the Alan Variance of one
of these oscillators; indeed, I didn't even hear of the concept
until years later.  It's probably not up to Time-Nuts' standards.

Dana

On Fri, Dec 8, 2017 at 4:59 AM, Hal Murray <hmurray at megapathdsl.net> wrote:

> What's the Q of a Helmholtz Resonator?  What do I do to make a high(er) Q
> version?
>
> With a narrow band filter, it might make a neat demo/toy to pull an audio
> signal out of the noise.  With 2 at different frequencies you could
> demonstrate FSK.
>
> --
> These are my opinions.  I hate spam.
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