At the previous night's party, Mrs. Weston tells Emma:

I do not think Mr. Knightley would be much disturbed by Miss Bates. Little things do not irritate him. She might talk on; and if he wanted to say any thing himself, he would only talk louder, and drown her voice. (Chapter 26)

And in this scene he does exactly that:

So began Miss Bates; and Mr. Knightley seemed determined to be heard in his turn, for most resolutely and commandingly did he say,

"How is your niece, Miss Bates..."