Webster’s Revised Unabridged Dictionary · 1913
Echometer
E·chom′e·ter (ē·kŏm′ē·tẽr), n. [Gr. ἦχος, ἠχώ, sound + -meter: cf. F. échomètre.] (Mus) A graduated scale for measuring the duration of sounds, and determining their different, and the relation of their intervals. J. J. Rousseau.