资料来源 : Webster's Revised Unabridged Dictionary (1913)

Soundless \Sound"less\, a.
   Having no sound; noiseless; silent. -- {Sound"less*ly}, adv.
   -- {Sound"less*ness}, n.

资料来源 : WordNet®

soundlessness
     n : the property of making no sound [syn: {quietness}]