In the commercial realm, 20dB of quieting is not good enough. All of the signals have to be near "full quieting" or they are annoying! Certainly for hams 20dB of quieting should be good enough for communications without a lot of repeats, but it is not considered a "good" signal, it would be "adequate". The term "full quieting" is often misused. I have heard stations through a repeater that were maybe 20dB quieted, and other stations will tell them they are "full quieting". The ones I like are the guys who say, "You're full quieting, only about 10% noise."