Just looked through 1474D but could not find exact definition of what A-weighting means, or most importantly, what it does and how it affects results.

Can someone explain, or link to document explaining it?

You can read this:

http://sound.westhost.com/project17.htm

thanks, just saw it on google search.

I see A-weight is synonymous with A contours or A filters.

For exact weightings at given frequencies see:

http://www.measure.demon.co.uk/Acoustic ... eight.html

good graph of a,b,c filters here:

http://hyperphysics.phy-astr.gsu.edu/hb ... acont.html