When used literally, the term echo chamber
refers to a hollow enclosure used to produce echoing sounds, usually for recording purposes. For example, the producers of a television
program might wish to produce the aural illusion that a conversation is taking place in a cave; this effect might be accomplished by playing the recording of the conversation inside an echo chamber, with an accompanying microphone
to catch the echoes. With the advent of digital signal processing
and other digital audio
technologies, it became possible to duplicate this "echo chamber" effect by processing the digital signal, and so physical echo chambers fell into disuse.
Metaphorically, an echo chamber can refer to any situation in which some force or idea is amplified by transmission inside an enclosed space. For example, observers of journalism in the mass media have described the echo chamber effect: one purveyor of information will make a claim, which many like-minded people then repeat, overhear, and repeat again (often in an exaggerated or otherwise distorted form) until most people assume that some extreme variation of the story is true.