monotone
mo-no-tone
noun
A single, unvaried pitch or tone of voice or sound.
"His voice was a dull monotone."
adjective
Characterised by a single, unvaried pitch or tone.
"She spoke in a monotone voice."
Thesaurus
Synonyms: increasing monotonic, decreasing monotonic, monotonic, drone, droning, unmodulated, flat, monotonous
Antonyms: nonmonotonic