SeisMac Frequently Asked Questions

Q: How accurate is SeisMac?
A: It's as accurate as your Sudden Motion Sensor (SMS), which in absolute terms isn't incredibly accurate. The best test is to sit your laptop absolutely level and see what the graphs does; any deviation from the center line is due to inaccuracy. Note that it actually is a pretty poor true seismograph; seismographs measure absolute displacement (and can sense signals down to 1/10 or even 1/100 Hz), while the SMS measures acceleration (and can't sense much below 1Hz).
Q: What is the resolution of the SMS?
That depends on what model computer you're using. On G4 laptops, the SMS only resolves about 50 values between zero gravities and one gravity (making the graph look "chunky"). On the MacBook line, however, the SMS resolves 256 values between zero gravities and one gravity. In either case, the range is about plus or minus two gravities.
Q: How many samples a second does SeisMac record?
A: The default sample rate is 200 Hz, or 200 samples per second. Using the Preferences you can set SeisMac to sample anywhere from 10Hz to 500Hz, although slower machines may not be able to sample at the highest rates.
Q: Even though my laptop is flat on a level table, the graphs aren't centered.
A: The SMS isn't a high-precision device, and in particular may have a fair amount of offset. You can improve this by calibrating your SMS using SeisMaCalibrate.
Q: My G4 Mac laptop has SMS, but SeisMac suddenly stops working on it.
A: On some models of G4 laptop, the access method SeisMac uses may fail suddenly, not working again until the computer is rebooted. Since Apple doesn't support access to the SMS by third parties (like me), and I don't own such a laptop, it would be difficult for me to fix this. Sorry.
Q: I set SeisMac to store 200 hours of data. But, when I started it today, I couldn't see yesterday's data. What's up?
A: SeisMac only stores its data in memory; it does not save it to disk. When you quit SeisMac, all the data is discarded. But, as long as you leave SeisMac running (and your computer doesn't go to sleep) you'll have access out to the end of the buffer.
Q: Is there any way of saving SeisMac's data into a file?
A: Currently not, although this is on my To Do list. Do note that my SMSLib comes with a command-line utility called smsutil that captures acceleration data.
Q: What is the hardware Apple uses for the SMS?
A: The original MacBook Pro 15" uses a Kionix KXM52-1050 three-axis accelerometer chip, range +/- 2g, analog output, bandwidth up to 1.5KHz, located just under the up-arrow key. The data sheet can be found at http://kionix.com/Product-Index/product-index.htm.
Q: The "Save..." command saves the graph as a TIFF file. How can I save as a PDF file?
A: Choose Print from the File menu, and when the Print dialog comes up click the "PDF/Save as PDF..." button.
Q: May I have the source code for SeisMac?
No, but the underlying SMS access library, SMSLib, is now open-source.
Q: Did you figure out how to access the Sudden Motion Sensor all on your own?
A: Although the code is all my own, I used techniques discovered by a number of other people. For details, see the Credits and Links page.
Q: I've got another question that you haven't answered here.
A: You can contact me at griscom@suitable.com.