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There are sealed ports in the case and the sand/dust test conditions were 3 hours at up to 59 feet/second. I appreciate this protection since there are times when dirt is blowing around where I work.
If you are blowing through the woods on an ATV then you are likely subjecting your iPad to some heavy vibration. This test was conducted over 18 hours at 20-2000 Hz and the silicone piece helps absorb these vibrations.
The Griffin Technology Survivor case also has four pieces; front with integrated screen protector, back frame piece, silicone outer piece, and a small stand piece that secures to the case.