The Collaborative International Dictionary of English v.0.48:

Diacoustics \Di`a*cous"tics\, n. [Cf. F. diacoustique.]
   That branch of natural philosophy which treats of the
   properties of sound as affected by passing through different
   mediums; -- called also diaphonics. See the Note under
   Acoustics. Diacritic

The Collaborative International Dictionary of English v.0.48:

Diaphonics \Di`a*phon"ics\, n.
   The doctrine of refracted sound; diacoustics.
   [1913 Webster]