The Collaborative International Dictionary of English v.0.48:

Plangency \Plan"gen*cy\, n.
   The quality or state of being plangent; a beating sound. [R.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

plangency
    n 1: having the character of a loud deep sound; the quality of
         being resonant [syn: plangency, resonance,
         reverberance, ringing, sonorousness, sonority,
         vibrancy]