microcoustic

from The Century Dictionary.

• Serving to augment weak sounds; of or pertaining to an instrument for augmenting weak sounds.
• noun An aural instrument designed to collect and augment small sounds, for the purpose of assisting the partially deaf in hearing.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Pertaining, or suited, to the audition of small sounds; fitted to assist hearing.
• noun An instrument for making faint sounds audible, as to a partially deaf person.