In Chapter 16 of To Kill a Mockingbird, Scout observes,
I was beginning to notice a subtle change in my father these days, that came out when he talked with Aunt Alexandra. It was a quiet digging in, never outright irritation.
After Aunt Alexandra's arrival in Chapter 13 the tranquil home of the children has been disturbed by their aunt who takes it upon herself to make Scout into Jean-Louise, a young lady who wears dresses, attends Sunday teas, and is aware of her lineage. Further, she wants her brother to dispense with Calpurnia; however, Atticus adamantly demurs,
"Alexandrs, Calpurnia's not leaving this house until she wants to....She's a faithful member of this family and you'll simply have to accept things the way they are.
He adds that Calpurnia has been strict, yet very motherly to the children, and they all love her.
Later, Jem and Scout begin to squabble and Aunt Alexandra tells Atticus that their quarreling is "just one of the things I've been telling you about." Clearly, she is very dissastisfied with the behaviors of the children and Atticus's child-rearing skills. Consequently, Atticus becomes irritated by her negative criticisms and gratuitous advice as his sister oversteps her place in his home.
Then, in Chapter 16,when Atticus mentions that Mr. Underwood "despises Negroes," Aunt Alexandra chides her brother, "Don't talk like that in front of them," implying Calpurnia's presence. Atticus replies with brittleness and a "quiet digging in,"
"Well, I'm sure Cal know it. Everybody in Maycomb knows it."
For, Atticus considers Calpurnia part of the family and does not whisper when speaking of "Negroes" since he is not saying anything that is pejorative or prejudicial.