Re: Calibrating measurement mics.
Assuming your Rat shack is actually right 
I attended a sub get together (home theatre) a couple of years ago and lots of guys brought their SPL meters. All makes and models. My was recently calibrated-so I ''assume'' it was pretty close to a standard.
From the highest reading to the lowest reading the range was about 7-8dB 
SO just because it ''says'' so-does not make it so.
Consider what a ''real'' SPL meter costs, and then look at the cost of the ''cheapies'' and just think about why the other one cost more.
Remember there is a little screwdriver adjust on the side of the meter-so it is very possible to have it drift a bit.
A man with a meter is sure. A man with 2 meters is sure of nothing-unless it is traceable.
