modulator

noun

1. A person who modulates.
2. A device or thing that modulates.

   “[Poetry] is a most musicall Modulator of all Intelligibles by her inventive Variations, undulling their Grossenesse, and subliming it into more refined Acceptablenesse to our own, or others understandings.”

3. A chart in the tonic sol-fa notation on which the modulations or changes from one scale to another are shown by the relative position of the notes.