PSN-L Email List Message

Subject: Re: Digest from 02/02/2008 00:00:01
From: Brett Nordgren Brett3mr@.............
Date: Sun, 03 Feb 2008 13:20:48 -0500


I think I might have done better if I had made it clearer that the 
instrument response is distinct from the transducer response.  I completely 
agree that to create the highest performance instrument you have to use a 
good displacement transducer, as the VolksMeter does.

At 10:49 AM 2/3/2008 -0500, you wrote:
>      When you refer to the VolksMeter's response being flat from D.C. to 
> 1 Hz, you
>are correct; however, to say that the velocity response is narrow-band is not.
>The difference between acceleration response (position sensor such as the
>VolksMeter0 and velocity response (most seismometers) is summed up by the 
>right pair of graphs shown on John Lahr's page at

However, the overall instrument response to sinusoidal ground motion 
described by displacement, shown as the blue line in figure P2 of the 
User's Manual, rises from DC at 40db per decade, and levels out at unity at 
about 0.9Hz.  That implies that the instrument response, if the ground 
motion were instead described by its velocity, would rise from DC at 20db 
per decade to a maximum at 0.9Hz at which point it begins to fall at 20db 
per decade.  I had understood that in commercial instruments, that was 
usually described as a narrow-band velocity response.

I would contend that the same instrument can be characterized by stating 
either its response to displacement (flat above 0.9Hz) or to velocity 
(peaking at 0.9Hz), or for that matter to ground acceleration (flat below 
0.9Hz).  Different curves, same device.

>     These illustrate (for perfect electronics if it existed) the difference
>between an 'acceleration' detector (VolksMeter) and a 'jerk' detector
>(conventional instruments that use a Faraday-law--magnet coil- detector) 
>in terms
>of their response to earth's motion.  The only thing that causes any 
>to respond is acceleration (or tilt as a special case therof), and so the
>conventional instrument is measuring the derivative of the acceleration, which
>engineers call the 'jerk'.

Yes.  I was first introduced to the technical use of the term when I was 15 
by my engineer father.  It was, as I recall, in connection with my early 
attempts at driving.

>    For 'perfect' electronics, the acceleration response is superior for 
> sensing
>lower frequencies of earth motion, whereas the jerk response is superior 
>for higer
>frequencies.  The limit of detectability, within the differing constraints of
>their architecture, is the noise introduced by the electronics.  My statement
>about 'superiority' assumes equally effective electronics for the cases.
>   Randall


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