ear response
These curves are similar to the human ear's response to noise. Filtering telephone channel noise through them before measuring the noise output is a better predictor of perceived noise than measuring the channel.
http://ftp.tiaonline.org/tr-41/tr4135/Public/TR41.3.5-ArchivesForContibutionsAndDocuments/TR41.3.5-00-11-050P%20-%20470.110-C%20-%20Acoustics%20Draft%2000%20(103000).pdf
is a document that has these curves on page 20 or so. (Actually the points of the curve in table format.