The Collaborative International Dictionary of English v.0.48:

Echometer \E*chom"e*ter\, n. [Gr. ?, ?, sound + -meter: cf. F.
   ['e]chom[`e]tre.] (Mus)
   A graduated scale for measuring the duration of sounds, and
   determining their different, and the relation of their
   intervals. --J. J. Rousseau.
   [1913 Webster]