Randall,

In order to clear my thinking, I put together an Excel spreadsheet which 
describes an experiment with the VolksMeter in which it is placed on a 
horizontal shaker table, oscillating with constant amplitude of 0.01mm, 
swept over a frequency range of 0.002Hz to 10Hz.  The pendulum parameters 
and sensitivity numbers were obtained from the VM User Manual and its 
response was plotted in several ways.

The zipped file is at   http://bnordgren.org/seismo/VolksMeter.zip   (Note 
that upper/lower case counts on my server).  This unzips to 
'VolksMeter.xls' which has a worksheet with the calculations and three 
charts to display the results.

Please let me know if/where I may have gotten my sums wrong,

Many thanks,
Brett Nordgren



At 10:49 AM 2/3/2008 -0500, you wrote:
>Brett,
>      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 
>upper
>right pair of graphs shown on John Lahr's page at
>http://jclahr.com/science/psn/response/plots.jpg
>     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 
>seismometer
>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'.
>     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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