diapason

noun5

1. •
   The musical octave.

   • 2 to 1, which is a duple ratio, forms the [symphony] diapason — 1818
2. •
   A harmonious outpouring of sound.

   • he could hear nothing except the rattle of the crickets and the swelling diapason of the frogs […] — 1961
3. •
   (literary) The range or scope of something, especially of notes in a scale, or of a particular musical instrument.

   • the piano curving like a conch, corollas giving out diapasons of light […] — 1934
4. •
   A tonal grouping of the flue pipes of a pipe organ.
5. •
   One of the lowest strings on an archlute, theorbo, chitarrone, or similar bass lute-family instrument, usually unfrettable.