The Collaborative International Dictionary of English v.0.48:

Sonometer \So*nom"e*ter\, n. [L. sonus a sound + -meter.]
   1. (Physiol.) An instrument for exhibiting the transverse
      vibrations of cords, and ascertaining the relations
      between musical notes. It consists of a cord stretched by
      weight along a box, and divided into different lengths at
      pleasure by a bridge, the place of which is determined by
      a scale on the face of the box.
      [1913 Webster]

   2. An instrument for testing the hearing capacity.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sonometer
    n 1: an instrument used to measure the sensitivity of hearing
         [syn: audiometer, sonometer]