Re: Pier design and sand

Joe Zeglinski


If the Petrie dish has value, then it is far simpler to attach a mic to
the side of a pier, and plug the mic output into your laptop audio port. The
waveform can be captured and displayed, and displayed at greater convenience
in better detail, in any music editing program - even the rink dink Sound
tools in the Windoze Accessories folder. Using better sound recording
programs, you can measure the amplitudes and frequencies, and even get the
Fourier Transform to identify the dominant frequencies and power. Then one can
apply dampening techniques like sand, or perhaps bolting on step rings at
sections of the pier to change the modes, and perhaps cancel them.

One other side benefit might be to discover if there are any other sources
(traffic, walking on observatory floor, etc) ground vibrations, or stepper
motor frequencies being coupled from an operating mount into the pier, and
being amplified by the steel pier.

The nice thing about the above approach is that these WAV files can be
graphed and saved, to be shared with others in the group in perhaps the FILES
section. Then, any visitor can get the geometries of the pier used, with the
resulting recorded waveform, for download, or even live online playback using
RealPlayer etc., to compare everyone's pier dampening technique.

If you like the Petrie dish approach, capture the mic output and simply
play it back using the RealPlayer or any PC CD playback program - it will show
the same waves as the water in the Petrie dish.


----- Original Message -----
From: "William R. Mattil" <wrmattil@...>
To: <ap-gto@...>
Sent: Wednesday, October 31, 2007 10:49 AM
Subject: Re: [ap-gto] Re: Pier design and sand

Murray Hammick wrote:
Having been following this thread, I could say with any confidence which
side has won the day.

How about a very simple experiment. Does someone have the
equipment/instrumentation to monitor vibrational energy, damping etc in a
simple steel tube and measure it with and without a dry sand or a sand/oil
mix ?

A few years ago I was in New Mexico visiting Andy Homeyer and he
described how he dampened his piers and it made sense. He drilled a hole
near the top and mounted the mount, OTA and what not. Additionally he
then mounted a small petri dish to the side of the pier and filled with
water. Striking the side of the pier would then cause ripples in the
water in said petri dish that could be easily observed. He then timed
the duration of the ripples and started adding sand and retesting until
he reached a minimum. What he did was essentially "tune" the system for
maximum damping. You can all decide for yourselves if it makes any real
difference. But I thought that it was a very well thought out approach
and makes sense.




William R. Mattil :

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