cedarhurst asks ” is there a difference between one erva and another?” Actually, yes. and you can see this from the two “ervas’ that are discussed here- kol isha and uncovered hair.
The gemoro in megillah talks about different women and a man’s reaction to their names. It explicitly differentiates between these different women, saying that yes, things are relative when it comes to ‘soft’ ervas. The Aruch Hashulchan bemoans the habit (in his day) of women going with uncvoered hair, yet alllows krias shema to be said between it has become so common (uncovered hair) it does not rise to the din of erva.(by the way- he was criticized for this psak by the chassidim…). Basically, real ervas that leave no room for interpretation consist of uncovered parts of the body that should be covered. So- called ‘soft ervas” consist of singing, hair and the like that can be considered ‘subjective’ rather than “objective” (uncovered body parts mainly)