PSN-L Email List Message

Subject: magnitude display
From: sean@...........
Date: Sun, 20 Aug 2000 15:17:10 -0500 (CDT)


I have sent you some info regarding a visual logarithmic display for
magnitude that I posted on the PSN but didn't send to you.

Here are some more thoughts about using it for public education
that may also be interesting for others.

In case you aren't familiar with the db scale, for voltage ratios
 db = 20*log(A), so if the amplitude ratio A = 1000, it is 60db.
So for a seismic magnitude display, the log (1000) = 3, so a 60db
scale would cover three orders of magnitude.

The display I described is made with the DIP bargraph LEDs connected
directly to the LN3915 log driver, so is quite compact, about 6" high 
for a bi-polar 60 db range. Since it is designed for monitoring a 
broadband instrument, including mass centering, it is DC coupled 
and bi-polar, indicating either positive or negative offsets.

For a public display of magnitude, it doesn't need to be bi-polar,
but would need to drive large separated LEDs or other indicators.
I would cascade three LM3915s for a 90db or log 4.5 range display,
so each of 30 indicators would be 0.15 magnitude units. With additional
circuitry, the indicators could be numerical displays of the magnitude.
Not all would have to be connected, like a scale of 1.3, 1.6, 1.9, etc.
This would allow the building background noise to be shown at the
lowest amplitude, which conventionally is about magnitude 2, since
another convention we use is that it takes a M=3 to be felt in the 
near field. A major show of force would be needed to light the 
+4.5 magnitude indicator (you could label it M=6.5) at the 90 db peak
at about 31623 times the background level. This could result in
a circus atmosphere with kids trying to light the top indicator,
so I would put it on a ground floor (or stop at log +4 or less).

I don't know if you can recover the analog signal from the PS2.
I have evaluated it, but it wasn't mine, so I couldn't take it
apart. But a geophone and an amplifier would work fine to drive 
the magnitude display. I would suggest a 1 hz geophone and a low
pass filter, so the kids would have to jump in unison to make a
large enough coherent amplitude, rather than just a bunch of noise
from random foot stomping. The coherent energy of earthquakes is what
does the most damage, and the magnitude scale is measured at the peak
sustained amplitude of the waveform. Of course, the signal can be 
simultaneously digitized and displayed on a screen. 


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Larry Cochrane <cochrane@..............>