I just bought one of these today (the digital ones) and I was looking through the manual.

I don't remember the exact descriptions of the two, but it said that A was for 'natural environment frequencies' or something along those lines.

It said to use c for the calibration of PA's or stereo equipment.

Edit: here is a link to the manual

http://rsk.imageg.net/graphics/uc/rsk/Support/ProductManuals/3302055_PM_EN.pdf

Last edited by terzaghi; 12/05/07 08:07 PM.
-David