The Collaborative International Dictionary of English v.0.48:

Equisonance \E*quis"o*nance\, n. [Equi- + L. sonans, p. pr. of
   sonare to sound: cf. F. ['e]quisonnance. See Sonant.]
   (Mus.)
   An equal sounding; the consonance of the unison and its
   octaves.
   [1913 Webster]