open diapason

noun

1. See under diapason(def 4).

noun Music.

1. a full, rich outpouring of melodious sound.
2. the compass of a voice or instrument.
3. a fixed standard of pitch.
4. either of two principal timbres or stops of a pipe organ, one of full, majestic tone (open diapason) and the other of strong, flutelike tone (stopped diapason).
5. any of several other organ stops.
6. a tuning fork.

noun music

1. either of two stops (open and stopped diapason) usually found throughout the compass of a pipe organ that give it its characteristic tone colour
2. the compass of an instrument or voice
3. (chiefly in French usage)
   1. a standard pitch used for tuning, esp the now largely obsolete one of A above middle C = 435 hertz, known as diapason normal (French (djapazɔ̃ nɔrmal)
   2. a tuning fork or pitch pipe
4. (in classical Greece) an octave