Murray, 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. Regards, Sean-Thomas __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>