## PSN-L Email List Message

Subject: RE: New Lehman on line (almost)
From: Jack Ivey ivey@..........
Date: Tue, 1 Oct 2002 08:08:44 -0400

```In defense of the underdamped proposition, a slight amount of underdamping
(that produces a second peak about, say, 10% the size of the first peak)
will
produce only a mild peak in the frequency response, and in fact will extend
the
low frequency response of the system slightly.  The main advantage of this
arrangement is that it is very easy to tell exactly how much damping you
have.
If you adjust for critical or overdamped, you can only guess, and lots of
people
will end up with a massively overdamped system, reducing their low frequency

response unnecessarily.

It is important to use realistic deflections when measuring damping, I have
found
that if you use large (1/8") deflections, you sometimes get very different
(greater)
damping than you do with micrometer deflections.  Bob Barns' calibrator (see

PSN site) is an excellent way to produce these small test signals.

Jack

-----Original Message-----
From: ACole65464@....... [mailto:ACole65464@........
Sent: Monday, September 30, 2002 10:05 PM
To: psn-l@..............
Subject: Re: New Lehman on line (almost)

In a message dated 10/01/2002 12:14:06 AM !!!First Boot!!!,
ChrisAtUpw@....... writes:

In a message dated 30/09/02, shammon1@............. writes:

The standard rule is to pull the boom back a few inches and let it go. The
boom
should loose 30% of its motion on each swing past center and come to rest
in 3 1/2 swings.

Hi Steve,

I am puzzled as to where this *standard rule* is supposed to come
from? But using it will give you a quite seriously underdamped system! A
critically damped system experiences no oscillation at all. This is inherent
in the maths.
This is important if you apply post processing to the recorded signal
with the assumption that it was critically damped to start with. It will
also give problems with the amplitudes and frequencies calculated in FFT
displays and may 'smear' P and S wave recordings.
A procedure to get critical damping could involve deflecting the beam
a very small amount (microns) and recording the amplifier output. You
progressively increase the damping until the arm just returns to the balance
position without having crossed the zero line. If you increase the damping
further, the arm will simply take longer to get back to zero. If you use
huge deflections like a few inches, you are likely to encounter non linear
effects which do not apply to the tiny (hopefully!) signals that we normally
record.
It is helpful if the recording displays just what the earth is doing.
It is really not helpful if the system adds an oscillating tail to every
transient.

Regards,

Chris Chapman

Steve,

In support of what Chris has stated, please go to:
http://www.seismo.com/msop/msop79/inst/inst4.html#aa250  Go to section 4.5
for a text description, and then click on figure 4.5.1a to see how pendulums
are supposed to be damped. About Critical is the response you should obtain.
I hope this helps a little, the diagrams may not make much sense at first
but it shows how professional instruments (electromagnetic, aka Lehman

Regards,

Allan Coleman

In
defense of the underdamped proposition, a slight amount of
underdamping
(that
produces a second peak about, say, 10% the size of the first peak)
will
produce only a mild peak in the frequency response, and in fact will
extend the
low frequency response of the system
slightly.  The main advantage of this

arrangement is
that it is very easy to tell exactly how much damping you have.

If you adjust for critical or
overdamped, you can only guess, and lots of people
will
end up with a massively overdamped system, reducing their low frequency

response unnecessarily.

It is
important to use realistic deflections when measuring damping, I have
found
that
if you use large (1/8") deflections, you sometimes get very different (greater)

damping than you do with micrometer deflections.
Bob Barns' calibrator (see
PSN
site) is an excellent way to produce
these small test signals.

Jack

-----Original Message-----From: ACole65464@.......
[mailto:ACole65464@........Sent: Monday, September 30, 2002 10:05
PMTo: psn-l@..............Subject: Re: New Lehman on
line (almost)In a
message dated 10/01/2002 12:14:06 AM !!!First Boot!!!, ChrisAtUpw@.......
writes:
In a message dated 30/09/02, shammon1@............. writes:

The standard rule is to pull the boom back a few inches and
let it go. The boom should loose 30% of its motion on each swing past
center and come to rest in 3 1/2 swings.Hi Steve,       I am
puzzled as to where this *standard rule* is supposed to come from? But using
it will give you a quite seriously underdamped system! A
critically damped system experiences no oscillation at all.
This is inherent in the maths.       This is
important if you apply post processing to the recorded signal with the
assumption that it was critically damped to start with. It will also give
problems with the amplitudes and frequencies calculated in FFT displays and
may 'smear' P and S wave recordings.       A
procedure to get critical damping could involve deflecting the beam a
very small amount (microns) and recording the amplifier
output. You progressively increase the damping until the arm just
returns to the balance position without having crossed the zero line. If you
increase the damping further, the arm will simply take longer to get back to
zero. If you use huge deflections like a few inches, you are likely to
encounter non linear effects which do not apply to the tiny (hopefully!)
signals that we normally record.
It is helpful if the recording displays
just what the earth is doing. It is really not helpful if the system adds an
oscillating tail to every transient.
Regards,       Chris Chapman Steve, In support of what Chris has stated,