1) Echo-free 2) Nonresonant 3) Sound-absorbent 4) Unreverberant 1) Nonresonant 2) Unreverberant 
The property of appearing echo-free or without echoes on a sonographic image; a clear cyst appears anechoic. ... See: transonic. ... Synonym: echo-free. ... Origin: G. An-priv. + echo + ic ... (05 Mar 2000) ... 
(an-ә-ko´ik) without echoes, such as a room used for hearing tests. sonolucent.
Type: Term Pronunciation: an′ĕ-kō′ik Definitions: 1. The property of being echo-free or appearing without echoes on a sonographic image; a cyst filled with clear fluid appears anechoic. Synonyms: echo-free See: transonic
[adj] - not having or producing echoes
anechoic Free from echoes; completely absorbing sound waves or radar signals.
Without echo. See also: Anechoic Chamber, Reverberant Sound Field.
adjective not having or producing echoes; sound-absorbent; `an anechoic chamber`
(of a recording chamber, television studio, or the like) characterized by an unusually low degree of reverberation; echo-free.