Source : Webster's Revised Unabridged Dictionary (1913)

Otacoustic \Ot`a*cous"tic\, a. [Oto- + acoustic: cf. F.
   otacoustique.]
   Assisting the sense of hearing; as, an otacoustic instrument.

Otacoustic \Ot`a*cous"tic\, Otacousticon \Ot`a*cous"ti*con\, n.
   An instrument to facilitate hearing, as an ear trumpet.