Comment: Is there any basis for saying that a 20 decibel difference is good enough? How do I explain to our customers when they ask why it requires a 20 decibel difference? I think "just because WCAG 2.0 states" is not a sufficient answer.
Add a note or any kind of explanation which states the reason why a 20 decibel difference has been chosen.
There are a number of studies that this is based on. One is the Report done for the Access Board in 1999. In that study, 75% of the subjects needed a S/N of 18 dB to be minimally acceptable. http://www.hearingresearch.org/Pubs/Access_Bd_Final_Report.pdf
The most recent is Interference in Hearing Aids from Digital Wireless
Telephones by Levitt, Kozma-Spytek, and Harkins in Seminars in Hearing/Volume 26, Number 2, 2005. It found that a signal to noise ratio of 32 to 28 db was needed for 90% to find phones highly usable, 24 to 20 db for 90% to for minor limitation on use and 15 to 12 db for Major Limitation of use.