echo chamber

from WordNet (r) 3.0 (2006)

echo chamber
    n 1: an enclosed space for producing reverberation of a sound

from The Collaborative International Dictionary of English v.0.48

echo chamber \ech"o cham"ber\, n. (Music, broadcasting)
   A room with walls that resonate sound, producing audible
   echoes; it is used especially to create special sound effects
   in recording music.
   [PJC]