monotone

Monotone is a droning, unchanging tone. Nothing can put you to sleep quite as effectively as a teacher talking in a monotone.

The Greek word for "one tone" is monotonia, which is the root for both monotone and the closely-related word monotonous, which means "dull and tedious." A continuous sound, especially someone's voice, that doesn't rise and fall in pitch, is a monotone. When someone speaks in a monotone, his voice is flat and boring — plus listeners don't know how the speaker is feeling when everything sounds the same.