Following Cass Sunstein's popular treatment of the concept, echo chambers are often defined as environments which exclude contrary opinions through omission. C. Thi Nguyen contests the popular usage and defines echo chambers in terms of in-group trust and out-group distrust. In this paper, I argue for a more comprehensive treatment. While both exclusion by omission and out-group distrust help sustain echo chambers, neither defines the phenomenon. I develop a social network model of echo chambers which focuses on the role of belief-reinforcing echoes. First, I argue that the model allows us to incorporate Nguyen's main point about distrust without construing other commentators as deeply mistaken about the nature of echo chambers. Second, I use the model to develop an account of collaborative resistance and use it to clarify the role echo chambers play in spreading misinformation.