monotone

Adjective

• sounded or spoken in a tone unvarying in pitch
  • the owl's faint monotonous hooting
    

• of a sequence or function; consistently increasing and never decreasing or consistently decreasing and never increasing in value

Noun

• a single tone repeated with different words or different rhythms (especially in rendering liturgical texts)

• an unchanging intonation

Related words

• Synonyms: 
  flat , monotonic , monotonous , drone , droning