无回声

[ adj ]
having or producing no echo
<adj.all>
the echoless darkness

Echoless \Ech"o*less\, a.
Without echo or response.

nonreflecting \nonreflecting\ nonreflective \nonreflective\adj.
1. not capable of total reflection of light; not polished or
shiny; as, It is complicated to create realistic shading
effects on nonreflecting surfaces. [Narrower terms:
{echoless ] {reflective}
[WordNet 1.5]