echo chamber

A room or other enclosed space that is highly conducive to the production of echoes, particularly one that has been designed and built for this purpose.

(music production) A sound effect that may be applied to live or recorded sounds through a sound editing process, which creates the impression that the sounds originated in an enclosed space which was conducive to producing echoes or echoing.

(by extension) An environment in which a person is exposed to only those beliefs and opinions that agree with their own, so that their existing views are magnified and reinforced while alternative ideas are not expressed or considered.