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
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